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10th Class - Science

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10th Class - Science
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NCERT Solutions for Class 10 Science Chapter 2 Acids Bases and Salts

Question1.
You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Answer:Take a small volume of all three liquids in three different test tubes. Dip red litmus paper strips separately in all the three test tubes. The tube in which red litmus strip turns blue, contains a basic solution. Now, we use the blue litmus paper as testing paper and dip it into the remaining two solutions. The solution which changes the colour of the blue litmus paper into red is acidic and the other which does not affect it, is neutral, i.e. distilled water

Question2.
Why should curd and other sour substances not be kept in containers made up of brass or copper?

Answer:Both curd and other sour substances contain some acids in them. They react with copper or brass vessels to form certain salts which are not good for health. Therefore, it is not advisable to keep them in brass or copper containers

Question3.
Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer:Metals are mostly reactive in nature. They react with dilute acids (HCl and H2SO4) to evolve hydrogen gas. For example,

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

The gas burns with a pop sound when a burning candle is brought near to it

Question4.
A metallic compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride?

Answer:The gas evolved with effervescence and extinguishes a lit candle is CO2. If one of the compounds formed is CaCl2, the reaction would be

CaCO3(s) + HCl(aq) → CaCl2(aq) + CO2 + H2O(l)

Question5.
Aqueous solutions of HCl, HNO3 and H2SO4, etc. show acidic character while those of the compounds like ethyl alcohol (C2H5OH) and glucose (C6H12O6) fail to do so. Explain?

Answer:All the listed acids have replaceable hydrogen atoms which they release in aqueous solution as hydrogen ions. Therefore, they show acidic character. However, both ethyl alcohol (C2H5OH) and glucose (C6H12O6) do not undergo dissociation in aqueous solution. That why they do not conduct electricity in aqueous solution

Question6.
Why does an aqueous solution of an acid conduct electricity?

Answer:Aqueous solution of an acid releases H+ and H3O+ in solution. Since ions are carriers of charge, therefore they are responsible for conducting electricity

Question7.
Why does dry HCl gas not change the colour of dry litmus paper?

Answer:A dry HCl gas has no H+ so it does not show any acidic character therefore, no change in colour takes place until we moisten the litmus paper

Question8.
While diluting an acid, why is it recommended to add acid to water and not water to the acid?

Answer:Mineral acids such as, H2SO4, HNO3, HCl, etc. have strong affinity for water, so dilution of acid is highly exothermic in nature. This heat may cause jumping of solution or cracking of apparatus. In order to avoid it, acid is added drop by drop to water which dilutes the heat and prevent accident

Question9.
How is concentration of Hydronium ions (H3O+) affected when solution of an acid is diluted?

Answer:An acid dissociates into hydronium ions (H3O+) and anions when dissolved in water. Upon dilution, the volume of the solution increases and the number of ions per unit volume decreases. Therefore, the concentration of H3O+ per unit volume decreases

Question10.
How is concentration of hydroxyl (OH-) ions affected when excess of base is dissolved in solution of sodium hydroxide?

Answer:Sodium hydroxide (NaOH) is a strong base. It immediately dissociates in solution to give OH- and cat ions. Upon dissolving more of the base in the solution, the concentration of OH- further increases

Question11.
You have two solutions A and B. The pH of solution A is 6 and that of solution B is 8. Which solution has more hydrogen ion concentration? Which of these is acidic and which one is basic?

Answer:The pH of a solution is inversely proportional to the concentration of H+ in solution. Lesser the pH of the solution, more will be the H+ concentration. The solution A with pH 6 has more H+ concentration than the solution with pH equal to 8. The solution A is acidic because its pH is less than 7 and the solution B is basic because its pH is more than 7

Question12.
What effect does concentration of H+(aq) have on acidic nature of a solution?

Answer:The acidic nature of a solution is directly related to the concentration of H+. As the concentration of H+ increases, the acidic nature of the solution also increases

Question13.
Do basic solutions also have H+(aq)? If yes, then why are these basic?

Answer:Yes, basic solutions also have H+. As the solutions are prepared in water and water being a weak electrolyte, it dissociates into H+ and OH- but the number of H+ are very small as compared to OH- ions

Question14.
Under what soil conditions, do you think a farmer would spread or treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer:A soil usually becomes acidic when there is either a high peat content or, iron minerals or there are some rotten vegetables in the soil. In order to reduce the acidic strength, liming of soil is usually done. For this, any of the substances that have been mentioned are added to the soil since they are of basic nature

Question15.
Name the substance which upon treating with chlorine gives bleaching powder. Write the chemical equation for the reaction?

Answer:Slaked lime is the substance which reacts with chlorine to give bleaching powder

Question16.
Name the sodium compound used for softening hard water?

Answer:Washing soda or sodium carbonate decahydrate

(Na2CO3.10H2O)

Question17.
What will happen if the solution of sodium hydrogen carbonate is heated? Write the chemical equation involved?

Answer:Carbon dioxide gas will evolve and sodium car-bonate will be formed.

2NaHCO3 → Na2CO3 + CO2 + H2O

Question18.
Write the chemical equation for the reaction between Plaster of Paris and water?

Answer:

Question1.
What is the common name of the compound CaOCl2?

Answer:The common name of the compound CaOCl2 is bleaching powder

Chapter End Questions

Question1.
A solution turns red litmus blue. Its pH is likely to be?
  • 2
  • 4
  • 7
  • 10

Answer:

  • 10
Question2.
A solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains?
  • NaCl
  • HCl
  • LiCl
  • KCl

Answer:

  • HCl
Question3.
10 mL of solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise will be?
  • 4 mL
  • 8 mL
  • 12 mL
  • 16 mL

Answer:

  • 16 mL
Question4.
Which of the following types of medicines is used for treating indigestion?
  • Antibiotic
  • Analgesic
  • Antacid
  • Antiseptic

Answer:

  • Antacid
Question5.
Write the word equation and the balanced equations for the reactions when?
  • dilute sulphuric acid reacts with zinc granules
  • dilute hydrochloric acid reacts with magnesium ribbon
  • dilute sulphuric acid reacts with aluminium powder
  • dilute hydrochloric acid reacts with iron filings

Answer:

  • Word equation: Zinc + Sulphuric add →Zinc sulphate + Hydrogen Balanced equation: Zn(s) + H2SO4(dil.) → ZnSO4(aq) + H2(g)
  • Word equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen Balanced equation: Mg(s) + 2HCl (dil.) → MgCl2(aq) + H2(g)
  • Word equation: Aluminium + Sulphuric acid →Aluminium sulphate + Hydrogen Balanced equation: 2Al(s) + 3H2SO4(dil.) → Al2(SO4)3(aq) + 3H2(g)
  • Word equation: Iron + Hydrochloric acid → Iron chloride + Hydrogen Balanced equation: Fe(s) + 2HCl(dil.) → FeCl2(aq) + H2(g)
Question6.
Compounds such as alcohol and glucose also contain hydrogen but are not characterised as acids. Describe an activity to prove it?

Answer:Compounds such as alcohol and glucose also contain hydrogen but do not behave like an acid. Both are organic compounds with the formulae C2H5OH and C6H12O6, respectively. This can be proved by the following activity: In a glass beaker, take a dilute solution of glucose (C6H12O6). Fix two small nails of iron in a rubber cork and place the cork in the beaker as shown in the figure. Connect the nails to the terminals of a 6 volt battery through a bulb. Switch on the current. The bulb will not glow. This shows that the electric current has not passed through the glucose solution. As the current is carried by the movement of ions, it shows that the solution of glucose has not given any H+. Now repeat the same experiment with ethyl alcohol (C2H5OH). The bulb will again not glow. This shows that both of them do not behave as acids although they contain hydrogen atoms in their molecules

Question7.
Why does distilled water not conduct electricity whereas rain water does?

Answer:Pure or distilled water has no ions as it is a very weak electrolyte. So, no conduction of electricity takes place but rain water contains dissolved acids and so rain water is a good conductor of electricity

Question8.
Why does an acid not show any acidic behaviour in the absence of water?

Answer:The acidic behaviour of a substance is due to the presence of H+(aq) ions. As acids do not dissociate to produce H+(aq) ions in the absence of water so they do not show acidic behavior

Question9.
Five solutions A, B, C, D and E when tested with universal indicator show pH as 4, 1, 11, 7 and 9, respectively. Which solution is?
  • neutral
  • strongly alkaline
  • strongly acidic
  • weakly alkaline
  • weakly acidic

Arrange the pH in increasing order of H+ concentration.

Answer:

  • Neutral: D with pH = 7
  • Strongly alkaline: C with pH = 11
  • Strongly acidic: B with pH = 1
  • Weakly alkaline: E with pH = 9
  • Weakly acidic: A with pH = 4

Increasing order of H+ concentration:

C<E<D<A<B

Question10.
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A while acetic acid (CH3COOH) is added to test tube B. In which case, fizzing occurs more vigorously and why? Fizzing in the reaction is due to the evolution of hydrogen gas by the action of metal on the acid?
  • Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
  • Mg(s) + 2CH3COOH(aq) → (CH3COO)2Mg(aq) + H2(g)

Since hydrochloric acid is a stronger acid than acetic acid, fizzing occurs more readily in tube A than in tube B. Actually hydrogen gas will evolve at more brisk speed in test tube A.

Question11.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer?

Answer:When milk changes into curd, the pH decreases. Lactose (carbohydrate) present in milk gets converted into lactic acid. As more acid is formed, pH of the medium decreases

Question12.
A milkman adds a very small amount of baking soda to fresh milk?
  • Why does he shift the pH of the fresh milk from 6 to slightly alkaline
  • Why does milk take a long time to set as curd

Answer:

  • Fresh milk is slightly acidic due to the presence of lactic acid. The presence of bacteria decreases the pH of milk and makes it sour. To prevent it, baking soda (NaHCO3) is added to neutralise the acidic nature making it slightly alkaline
  • When milk changes to curd, it becomes more acidic but adding baking soda neutralises it and checks curdling
Question13.
Plaster of Paris should be stored in a moisture-proof container. Why?

Answer:The presence of moisture can affect the slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of Paris useless after some time.

Question14.
What is neutralisation reaction? Give two examples?

Answer:The reaction of an acid and a base, giving rise to the corresponding salt and water is called neutralization reaction. Examples:
NaOH + HCl → NaCl + H2O
Mg(OH)2 + H2CO3 → MgCO3 + 2H2O

Question15.
Give two important uses of washing soda and baking soda?

Answer:Washing soda
1. It is often used as an electrolyte.
2. Domestically it is used as a water softener during laundry.
Baking soda
1. It is used to test garden soil for acidity. If it develops bubbles, the soil is too acidic.
2. Washing a car with it can remove dead bug bodies without damaging the paint.

NCERT Solutions for Class 10 Science Chapter 2 (MCQs) [1 Mark each]

Question 1.
Equal pieces of zinc granules are dropped in four test tubes. Following substances are poured in all the four test tubes. The reaction will be vigorous with [CCE 2014]
(a) CH3COOH
(b) HCl
(c) sodium bicarbonate solution
(d) lemon juice
Answer:
(b) Strong adds like HCl react vigorously with active metals like Zn and form metal salt and evolve H2 gas.
Question 2.
Which of the following statements shows the property of an acid? [CCE 2014]
(a) It turns blue litmus to red
(b) It is sour in taste
(c) It has no effect on red litmus
(d) All of the above
Answer:
(d) An acid turns blue litmus red. Thus, it has no effect on red litmus and acids are sour in taste.
Question 3.
A drop of a liquid sample was put on the pH paper. It was observed that the colour of the pH paper turned blue. The liquid sample is [CCE 2014]
(a) lemon juice
(b) sodium bicarbonate solution
(c) distilled water
(d) hydrochloric acid
Answer:
(b) The liquid sample is of sodium bicarbonate (NaHC3) solution. It is a basic solution. And we know that a basic solution turns pH paper blue.
Question 4.
Two solutions X and Y were found to have pH value of 4 and 10 respectively. The inference that can be drawn is [CCE 2014]
(a) X is a base and Y is an acid
(b) Both X and Y are acidic solutions
(c) X is an acid and Yis a base
(d) Both X and Y are bases

Answer:
(c) Any solution having pH > 7 will be a base while the solution having pH < 7 will surely be an acid. Hence, it can be concluded that X is an acid (pH=4, i.e. < 7) and Yis a base (pH =10, i.e. > 7).

Question 5.
A student was asked to collect apparatus from lab store, for doing experiment of pH of given sample. Identify the article which he is not supposed to pick. [CCE 2014]
(a) pH paper
(b) Dropper
(c) Litmus paper
(d) Petri dish
Answer:
(d) Petri dish is not required for doing experiment of pH.
Question 6.
Which one of the following would you need to identify the gas that evolve when you heat NaOH solution with zinc metal? [CCE 2014]
(a) Red litmus solution
(b) Blue litmus solution
(c) A burning splinter / matchstick
(d) Lime water
Answer:
(c) When a base like NaOH is treated with any active metal like Zn, it produces H2 gas. And the presence of the hydrogen gas can be tested by bringing a burning splinter/ matchstick near the gas produced. The gas will burn with a pop sound confirming the presence of hydrogen gas.

Important Question

Important Questions of Acids Bases and Salts Class 10 Science Chapter 2

Question 1.
With the help of an example explain what happens when a base reacts with a non- metallic oxide. What do you infer about the nature of non-metal oxide?
Year of Question :(2017)

Answer: Oxides of non-metals react with bases to form salt and water. For example, the reaction between carbon dioxide and calcium hydroxide. Calcium hydroxide, which is a base, reacts with carbon dioxide to produce salt and water. Hence, oxides of non-metals are acidic in nature

Question 2.
What is observed when carbon dioxide gas is passed through lime water?
Year of Question :(2013)
  • (i) for a short duration
  • (ii) for a long duration? Also write the chemical equations for the reactions involved. (Board Term I, 2016)

Answer:

  • (i) When CO2 is passed through lime water for short interval of time, it turns milky due to the formation of insoluble calcium carbonate
  • (ii) If CO2 is passed for long duration through lime water, the white precipitate formed dissolves due to the formation of soluble calcium hydrogen carbonate and the solution becomes clear
Question 3.
2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the content are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid?
Year of Question :(2013)

Answer: It is observed that active metals like zinc react with strong bases like NaOH, KOH etc. to liberate hydrogen gas and corresponding salt. The evolution of gas is confirmed by the bubble formation in soap solution. Test to detect H2 gas: When burning matchstick is kept on the mouth of this test tube, pop sound is heard which confirms the presence of H2 gas. When Zn metal reacts with dilute solution of strong acid, H2 gas is evolved.

Question 4.
Write the names of the product formed when zinc reacts with NaOH. Also write the balanced chemical equation for the reaction involved. Write a test to confirm the presence of the gas evolved during this reaction?
Year of Question :(2015)

Answer:The evolution of gas is confirmed by the bubble formation in soap solution. Test to detect H2 gas: When burning matchstick is kept on the mouth of this test tube, pop sound is heard which confirms the presence of H2 gas. When Zn metal reacts with dilute solution of strong acid, H2 gas is evolved

Question 5.
To. a solution of sodium hydroxide in a test tube, two drops of phenolphthalein are added?
Year of Question :(Board Term I, 2013)
  • (i) State the colour change observed
  • (ii) If dil HCl is added dropwise to the solution, what will be the colour change
  • (iii) On adding few drops of NaOH solution to the above mixture the colour of the solution reappears. Why?

Answer:

  • (i) On adding phenolphthalein to NaOH solution, the colour becomes pink
  • (ii) On adding dilute HCl solution dropwise to the same test tube, the pink colour disappears and the solution again becomes colourless
  • (iii) On again adding NaOH to the above mixture, pink colour reappears because the medium becomes basic again
Question 6.
A clothstrip dipped in onion juice is used for testing a liquid X. The liquid X changes its odour. Which type of an indicator is onion juice? The liquid X turns blue litmus red. List the observations the liquid X will show on reacting with the following?
Year of Question :(2013)
  • (a) Zinc granules
  • (b) Solid sodium carbonate Write the chemical equations for the reactions involved

Answer: Onion juice is an olfactory indicator. Olfactory indicators give one type of odour in acidic medium and a different odour in basic medium. As the liquid X turns blue litmus red, hence it is an acidic solution

  • (a) Acids react with active metals such as zinc, magnesium etc. and evolve hydrogen gas, for example, Zn(s) dil.H2SO4(aq) → ZnSO4H2(g)
  • (b) Acids react with metal carbonates to give carbon dioxide with brisk effervescence. For example, Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O
Question 7
  • (a) Write the chemical name and formula of marble
  • (b) It has been found that marbles of Taj are getting corroded due to development of industrial areas around it. Explain this fact giving a chemical equation
  • (c) (i) What happens when CO2 is passed through lime water
  • (ii) What happens when CO2 is passed in excess through lime? (Board Term I, 2013)

Answer:

  • (a) The chemical formula of marble (lime stone) is CaCO3. Its chemical name is calcium carbonate
  • (b) Taj Mahal, one of the seven wonders of the world situated at Agra, is continuously losing its luster day by day due to rapid industrialisation which causes acid rain. The sulphuric acid present in the acid rain causes the marble (CaCO3) to be washed off as calcium sulphate (CaSO4), leading to the deterioration of such a splendid piece of architecture. CaCO3(s) + H2SO4(aq) → CaSO4(aq) + H2Ol + CO2(g)
  • (c) When CO2 is passed through lime water for short interval of time, it turns milky due to the formation of insoluble calcium carbonat
Question 8.
On diluting an acid, it is advised to add acid to water and not water to acid. Explain why it is so advised?
Year of Question :(2014)
  • Draw a labelled diagram to show the preparation of hydrogen chloride gas in laboratory
  • (ii) Test the gas evolved first with dry and then with wet litmus paper. In which of the two cases, does the litmus paper show change in colour
  • (iii) State the reason of exhibiting acidic character by dry HCl gas/HCl solution

Answer: Diluting a concentrated acid with water is a highly exothermic process. So, when water is added to concentrated acid, large amounts of heat is liberated which changes some water to steam explosively which can splash the acid and even the glass apparatus may break due to excessive heating

Question 9
  • (i) Draw a labelled diagram to show the preparation of hydrogen chloride gas in laboratory
  • (ii) Test the gas evolved first with dry and then with wet litmus paper. In which of the two cases, does the litmus paper show change in colour
  • (iii) State the reason of exhibiting acidic character by dry HCl gas/HCl solution. (2020)

Answer:

  • (ii) There is no change in the colour of dry blue litmus paper but moist blue litmus paper turns red if brought near the mouth of the test tube. This shows that HCl gas does not show acidic behaviour in absence of water but it shows acidic behaviour in presence of water
  • (iii) When HC1 gas dissolves in water, forms hydrochloric acid solution i.e., HCl(aq) which then produces H+(aq) or H3O+(aq) ions. HCl + H2O → H3O+ + Cl- Due to the presence of H+ or H3O+ it shows acidic behaviour
Question 10.
Complete and balance the following chemical equations?
Year of Question :(2017)
  • (i) NaOH(aq) + Zn(s) →
  • (ii) CaCO3(s) + H2O(l) + CO2(g)→
  • (iii) HCl(aq) + H2O(l) →

Answer:

  • (i) 2NaOH(aq) + Zn(s) → Na2ZnO2(aq) + H2(g)
  • (ii) CaCO3(s) + CO2(g) + H2Ol → Ca(HCO3)2(aq)
  • (iii) HCl(aq) + H2Ol > H3O+ Cl-(aq)
Question 11.
How the following substances will dissociate to produce ions in their solutions?
Year of Question :(Board Term 1, 2017)
  • (i) Hydrochloric acid
  • (ii) Nitric acid
  • (iii) Sulphuric acid
  • (iv) Sodium hydroxide
  • (v) Potassium hydroxide
  • (vi) Magnesium hydroxide

Answer: Dissociation of various substances to produce ions in their solutions are

  • (i) Hydrochloric acid (HCl): HCl(aq) → H+(aq) + Cl-(aq)
  • (ii) Nitric acid (HNO3 HNO3(aq) → + H+aq + NO-3(aq)
  • (iii) Sulphuric acid (H2SO4): H2SO4(aq) → 2H+(aq) + SO2-4(aq)
  • (iv) Sodium hydroxide (NaOH): NaOH(aq) → Na+(aq) + OH-(aq)
  • (v) Potassium hydroxide (KOH) : KOH(aq)→ K+(aq) + OH-(aq)
  • (vi) Magnesium hydroxide [Mg(OH)2] : Mg(OH)2(aq) → Mg2++(aq) + 2OH- (aq)
Question 12.
Sugandha prepares HCl gas in her school laboratory using certain chemicals. She puts both dry and wet blue litmus papers in contact with the gas?
Year of Question :(2013)
  • (i) Name the reagents used by Sugandha to prepare HCl gas
  • (ii) State the colour changes observed with the dry and wet blue litmus papers
  • (iii) Show the formation of ions when HCl gas combines with water. (Board Term I, 2013)

Answer:

  • (i) Dense white fumes of hydrogen chloride gas are evolved on heating solid sodium chloride with concentrated sulphuric acid
  • (ii) There is no change in the colour of dry blue litmus paper but moist blue litmus paper turns red if brought near the mouth of the test tube. This shows that HCl gas does not show acidic behaviour in absence of water but it shows acidic behaviour in presence of water
  • (iii) When HC1 gas dissolves in water, forms hydrochloric acid solution i.e., HCl(aq) which then produces H+(aq) or H3O+(aq) ions. HCl + H2O → H3O+ + Cl- Due to the presence of H+ or H3O+ it shows acidic behaviour
Question 13
  • (a) Illustrate an activity to investigate whether all compounds containing hydrogen are acidic
  • (b) What happens when hydrochloric acid and sodium hydroxide are dissolved in water. Explain by giving equation of each. (Board Term 1, 2016)

Answer:

  • (a) Take two beakers, one containing HCl acid and other containing alcohol which is not an acid but contains hydrogen. Now, fix two iron nails on a rubber cork and insert in a beaker and connect the nail to the two terminal of 6V battery through a switch and a bulb. Pour some dilute HCl solution in beaker and switch on the current. The bulb starts glowing. This shows that acids get dissociated as H+ and Cl- ions and these ions are responsible for conducting electricity
  • Let us now take alcohol solution in the beaker and switch on the current. The bulb does not glow in this case. This shows that alcohol does not conduct electricity
  • So, all acids have hydrogen but all hydrogen containing compounds are not acid
  • (b) HCl dissociates in aqueous solution to give hydrogen ions (or hydronium ions) and chloride ions. NaOH when dissolved in water produces sodium ions and hydroxide ions in the solution
Question 14.
An aqueous solution A turns phenolphthalein solution pink. On addition of an aqueous solution B to A the pink colour disappears. The following statement is true for solution A and B?
Year of Question :(2013)
  • (a) A is strongly basic and B is a weak base
  • (b) A is strongly acidic and B is a weak acid
  • (c) A has pH greater than 7 and B has pH less than 7
  • (d) A has pH less than 7 and B has pH greater than 7. (2020)

Answer:

  • (c) As the aqueous solution of A turns phenolphthalein solution pink, hence A is basic in nature. On adding an acidic solution, the pink colour will disappear. Hence, B is an acid
Question 15.
Out of HCl and CH3COOH, which one is a weak acid and why? Explain with the help of an example?
Year of Question :(2019)

Answer: Out of HCl and CH3COOH, CH3COOH is a weak acid because it dissociates partially in the solution. This can be proved with the help of following example. If 1 M HCl and 1 M CH3COOH are taken in the beaker as shown in the figure, greater deflection is observed in case of HCl which shows that more ions are produced by HCl in solution which produce more current.

Question 16.
Explain how an antacid works?
Year of Question :(2017)

Answer: The acidity produced due to excess hydrochloric acid in the stomach which cause indigestion, produce pain and irritation. Milk of magnesia (chemically magnesium hydroxide) is used as an antacid. Since, it is basic in nature, reacts with the excess hydrochloric acid present in the stomach and neutralises it.

Question 17.
  • (a) Three acidic solutions A, B and C have pH = 0, 3 and 5 respectively
  • (i) Which solution has highest concentration of H+ ions
  • (ii) Which solution has the lowest concentration of H+ ions
  • (b) How concentrated sulphuric acid can be diluted? Describe the process. (Board Term I, 2014)

Answer:

  • (a) (i) The solution having lower pH will have more hydrogen ion concentration. Hence, solution A will have highest H+ ion concentration
  • (ii) Solution CC i.e., pH = 5 has the lowest concentration of H+ ions
  • (b) Mixing of an acid with water is called dilution. This process is highly exothermic and therefore, acid is always added to the water not water to acid. The process for diluting concentrated sulphuric acid is
  • (i) Take about 10 mL of water in a beaker
  • (ii) Add concentrated sulphuric acid dropwise to water and swirl the beaker slowly
Question 18.
A compound P forms the enamel of teeth. It is the hardest substance of the body. It doesnt dissolve in water but gets corroded when the pH is lowered below 5.5?
Year of Question :(2013)
  • (a) Identify the compound P
  • (b) How does it undergo damage due to eating chocolate and sweets? What should we do to prevent tooth decay? (Board Term I, 2014, 2013)

Answer:

  • (a) The compound P is calcium phosphate,
  • (b) Eating chocolates and sweets produce large amount of acid in the mouth which is not completely neutralised by the saliva produced in the mouth. Excess acid attacks the enamel and tooth decay starts as pH of the mouth falls below 5.5. The best way to prevent tooth decay is to clean the teeth by using toothpastes after eating food. Toothpastes which are generally basic neutralise the excess acid in the mouth
Question 19. Baking soda is a mixture of?
Year of Question :(2013)
  • (a) sodium carbonate and acetic acid
  • (b) sodium carbonate and tartaric acid
  • (c) sodium hydrogen carbonate and tartaric acid
  • (d) sodium hydrogen carbonate and acetic acid

Answer:

  • (c) : Baking soda is a mixture of sodium hydrogen carbonate and a mild edible acid like tartaric acid or citric acid
Question 20.
The chemical formula for plaster of Paris is?
Year of Question :(2013)
  • (a) CaSO4.2H2O
  • (b) CaSO4.2H4O
  • (c) CaSO4.frac { 1 }{ 2 }H2O
  • (d) 2CaSO4.2H2O

Answer: (c, d) : Plaster of Paris is calcium sulphate hemihydrate which can be represented as, CaSO4. frac { 1 }{ 2 } H2O and 2CaSO4.H20

Question 21.
"Sodium hydrogen carbonate is a basic salt". Justify this statement. How is it converted into washing soda?
Year of Question :(2013)

Answer: Sodium hydrogen carbonate (NaHCO3) is basic in nature as on hydrolysis it gives a mixture of strong base (NaOH) and weak acid (H2CO3). Sodium hydrogen carbonate is converted to washing soda in the following way

  • (i) Thermal decomposition of NaHCO3
  • (ii) Recrystallisation of sodium carbonate
Question 22.
Write the chemical formula of Bleaching powder. How is bleaching powder prepared? For what purpose is it used in drinking water?
Year of Question :(2016)

Answer: The chemical formula of bleaching powder is CaOCl2. It is prepared by the action of chlorine gas on dry slaked lime Ca(OH)2. The chlorine used in the above reaction is the by-product produced during the electrolysis of brine. It is used in disinfecting drinking water as chlorine liberated by it, kills the germs

Question 23.
A student collected common names and formulae of some substances but he forgot to note which formula is for which compound. Help him to match the correct formula. ?
Year of Question :(2013)
(i) Caustic soda NaHCO3 (ii) Slaked lime CaO (iii) Baking soda NaOH (iv) Lime Ca(OH)2

Answer:

  • (i) Caustic soda → NaOH
  • (ii) Slaked lime → Ca(OH)2
  • (iii) Baking soda → NaHCO3
  • (iv) Lime → CaO
    • Question 24.
    List the important products of the Chlor-alkali process. Write one important use of each?
    Year of Question :(2020)

    Answer: Sodium hydroxide is prepared by electrolysis of an aqueous solution of sodium chloride (brine). The complete reaction can be represented as: The process of electrolysis of sodium chloride solution is called chlor-alkali process because of the products formed : chlor for chlorine and alkali for sodium hydroxide. The three very useful products obtained by the electrolysis of sodium chloride solution are sodium hydroxide, chlorine and hydrogen

    At anode : Cl2 gas is liberated At cathode : H2 gas is liberated. Uses of sodium hydroxide: In the manufacture of soaps and detergents. Uses of chlorine : As a germicide and disinfectant for sterilisation of drinking water and for water of swimming pools. Uses of hydrogen: In the manufacture of ammonia which is used for the preparation of various fertilizers like urea, ammonium sulphate etc

    Question 25.
    How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?
    Year of Question :(2020)

    Answer: Washing soda is prepared by recrystallisation of sodium carbonate: It is used to remove the permanent hardness of water. Hard water is treated with a calculated amount of washing soda when chlorides and sulphates of calcium and magnesium present in hard water get precipitated as insoluble calcium and magnesium carbonates which can be easily filtered off. The water thus becomes soft. CaCl2 + Na2CO3 → CaCO3→ + 2NaCl MgSO4 + Na2CO3 → MgCO3→ + Na2SO4

    Question 26.
    Give reasons for the following?
    Year of Question :(2013)
    • (i) Only one half of water molecule is shown in the formula of plaster of Paris
    • (ii) Sodium hydrogen carbonate is used as an antacid
    • (iii) On strong heating, blue coloured copper sulphate crystals turn white. (2020)

    Answer:

    • (i) Only one half of water molecule is shown in the formula of plaster of Paris (CaSO4. frac { 1 }{ 2 } H2O) as one molecule of water is being shared by two molecules of calcium sulphate (CaSO4). So the effective water of crystallisation for one CaSO4 unit comes to half molecule of water
    • (ii) Acidity can be neutralised by a base. Sodium hydrogen carbonate can be used as an antacid solution because it is a weak base and will react with excess acid produced in the stomach due to hyperacidity and will neutralise it
    • (iii) Blue coloured copper sulphate crystals are hydrated copper sulphate, CuSO4.5H2O. On heating blue copper sulphate crystals looses its water of crystallisation and turns into anhydrous copper sulphate which is white in colour.
    Question 27.
    During electrolysis of brine, a gas G is liberated at anode. When this gas G is passed through slaked lime, a compound C is formed, which is used for disinfecting drinking water?
    Year of Question :(2013)
    • (i) Write formula of G and C
    • (ii) State the chemical equations involved
    • (iii) What is common name of compound C ? Give its chemical name. (2020)

    Answer:

    • (i) During electrolysis of brine, chlorine is obtained at anode. When chlorine is passed through slaked lime, bleaching powder is formed which is used for disinfecting drinking water. Hence, G is Cl2 and C is CaOCl2
    • (iii) Common name of C is bleaching powder. Its chemical name is calcium hypochlorite
    Question 28.
    Identify the acid and the base from which sodium chloride is obtained. Which type of salt is it? When is it called rock salt? How is rock salt formed?
    Year of Question :(2019)

    Answer: Sodium chloride is obtained by the neutralisation of sodium hydroxide (base) with hydrochloric acid (acid). It is a neutral salt. Common salt found in the form of solid deposits is often brown in colour due to presence of impurities which is called rock salt. Rock salt is formed by evaporation of salty water of inland lakes

    Question 29.
    A white powder is added while baking cakes to make it soft and spongy. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaction taking place when the powder is heated during baking?
    Year of Question :(2013)

    Answer: The white powder added while baking cakes to make it soft and spongy is baking powder. Its main ingredients are sodium hydrogen carbonate and a mild edible acid like tartaric acid or citric acid. NaHCO3 decomposes to give out CO2 which causes the cake to rise and makes it soft and spongy. The function of tartaric acid or citric acid is to neutralise sodium carbonate formed during heating which can otherwise make the cake bitter. Reaction taking place when the powder is heated:

    Question 30.
    The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses?
    Year of Question :(2018)

    Answer: Salt used to make tasty and crispy pakoras is sodium bicarbonate (NaHCO3), pH = 9. On large scale, sodium bicarbonate is prepared a: Two uses of sodium bicarbonate are as follows

    • (i) It is used as an antacid in medicines
    • (ii) It is used as an additive in food and drinks. Note : In the question paper, the given pH is 14 which should be 9
    Question 31.
    Write one point of difference between each of the following?
    Year of Question :(2017)
    • (i) A hydrated salt and an anhydrous salt
    • (ii) Washing soda and soda ash.
    • (iii) Baking soda and baking powder

    Answer:

    • (i)
      • Hydrated salt Anhydrous salt A salt with one or more chemically combined water molecule is called hydrated salt, e.g., washing soda, Na2CO3.10H2.O A salt in which all water molecules are removed, is called anhydrous salt, e.g., soda ash, Na2.CO3
    • (ii)
      • Washing soda Soda ash The hydrated salt of sodium carbonate containing 10 molecules of water of crystallisation, is known as washing soda i.e., Na2CO3.10H2.O The anhydrous sodium carbonate (Na2CO3.) which does not contain water of crystallisation, is known as soda ash
    • (iii)
      • Baking soda Baking powder Baking soda is sodiumhydrogen carbonate with the formula, NaHCO3 Baking powder is a mixture of NaHCO3. and tartaric acid or citric acid
    Question 32.
    Complete the following table?
    Sample Solution Red litmus solution Blue litmus solution Phenolp-hthalein sol Acetic acid # # # Sodium hydroxide # # # Baking soda # # #

    Answer: Sample Solution Red litmus solution Blue litmus solution Phenolp-hthalein sol Acetic acid(CH3.COOH) No effect litmus hthalein Sodium hydroxide Blue solution solution Baking soda(NaHCO3.) Blue Red Colourless

    Question 33.
    A white coloured powder is used by doctors for supporting fractured bones?
    Year of Question :(Board Term I, 2016)
    • (a) Write chemical name and formula of the powder
    • (b) When this white powder is mixed with water a hard solid mass is obtained. Write balanced chemical equation for this change.

    Answer:

    • (a) Chemical name of the powder is calcium sulphate hemihydrate. Chemical formula of the powder is CaSO4. frac { 1 }{ 2 } ApH2O
    • (b) When water is added to plaster of Paris, it sets into a hard mass in about half an hour. The setting of plaster of Paris is due to its hydration to form crystals of gypsum which set to form a hard, solid mass.
    Question 34
    • (a) Define an acid-base indicator. Mention one synthetic acid-base indicator
    • (b) If someone in the family is suffering from a problem of acidity after overeating, which of the following substances would you suggest as a remedy? Lemon juice, vinegar or baking soda solution. Mention the property on the basis of which you will choose the remedy

Answer:

  • (a) Acid - base indicators : The indicators which show different colours in acidic and basic medium are called acid-base indicators. Phenolphthalein is a synthetic indicator
  • (b) Acidity can be neutralised by a base. Hence, we should choose baking soda solution because it is a weak base and will react with excess acid produced in the stomach due to hyperacidity and will neutralise it
Question 35.
Define water of crystallisation. Give the chemical formula for two compounds as examples. How can it be proved that the water of crystallisation makes a difference in the state and colour of the compounds?
Year of Question :(2020)

Answer: Water of crystallisation : It is the fixed number of water molecules present in one formula unit of a salt, e.g., Gypsum (CaSO4.2H2O) has two molecules of water of crystallisation. In hydrated copper sulphate (CuSO4.5H2O), there are five molecules of water of crystallisation. Activity: - Take few crystals of copper sulphate in a dry boiling tube. These are blue in colour. - Heat the boiling tube by holding it with a test tube holder on the flame of the burner. Observations : You will observe that the colour of copper sulphate crystals after heating becomes white. You may also notice water droplets on the mouth side of the boiling tube which are obtained from water of crystallisation.After adding 2-3 drops of water on the white sample of copper sulphate (obtained after heating) you will observe that the blue colour of copper sulphate crystals is restored.

Question 36.
  • (a) A student dropped a few pieces of marble in dilute hydrochloric acid contained in a test tube. The evolved gas was passed through lime water. What change would be observed in lime water? Write balanced chemical equations for both the changes observed
  • (b) State the chemical property in each case on which the following uses of baking soda are based
  • (i) as an antacid
  • (ii) as a constituent of baking powder

Answer:

  • (a) When marble reacts with dilute HCl carbon dioxide gas is liberated. When CO2 gas is passed through lime water, insoluble calcium carbonate is formed which appears milky
  • (b) (i) The excess acid formed in the stomach due to various reasons (one being overeating) is neutralised by sodium hydrogen carbonate. Hence, it is used as an ingredient of antacid
  • (ii) Baking soda (sodium hydrogen carbonate) is a constituent of baking power. On heating it gives out CO2 which causes the cake to rise and make it soft and spongy
Question 37
  • (a) What are anhydrous and hydrated salts? Explain with a suitable example of each]
  • (b) How is plaster of Paris prepared? What reaction takes place when it sets to a hard mass

Answer:

  • (a) (i) A hydrated salt and an anhydrous salt
  • (b) It is prepared from gypsum which is calcium sulphate dihydrate (CaSO4.2H2O). Gypsum is heated in a kiln to a temperature of 100°C (373 K). At this temperature, it loses three-fourth of its water of crystallisation forming plaster of Paris. When water is added to plaster of Paris, it sets into a hard mass in about half an hour. The setting of plaster of Paris is due to its hydration to form crystals of gypsum which set to form a hard, solid mass
Question 38.
  • (a) Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity illustrate how these two are interconvertible
  • (b) Write chemical names and formulae of plaster of Paris and gypsum. (Board Term 1, 2016)

Answer:

  • (a) The chemical formula of hydrated copper sulphate is CuSO4.5H2O(s) and anhydrous copper sulphate is CuSO4(s). For activity refer to answer 35
  • (b) Plaster of Paris is calcium sulphate hemihydrate; CaSO4.frac { 1 }{ 2 } H2O and Gypsum is calcium sulphate dihydrate; (CaSO4.2H2O)
Question 39.
How is sodium hydroxide produced? Write the balanced chemical equation also. Why is this process called as chlor-alkali process? In this process name the products given off at?
Year of Question :(2013)
  • (a) anode
  • (b) cathode Write one use of each of these products. (Board Term I, 2015)

Answer:

At anode : Cl2 gas is liberated At cathode : H2 gas is liberated. Uses of sodium hydroxide: In the manufacture of soaps and detergents. Uses of chlorine : As a germicide and disinfectant for sterilisation of drinking water and for water of swimming pools. Uses of hydrogen: In the manufacture of ammonia which is used for the preparation of various fertilizers like urea, ammonium sulphate etc

Question 40.
What is water of crystallization? Write the common name and chemical formula of a commercially important compound which has ten water molecules as water of crystallization. How is this compound obtained? Write the chemical equation also. List any two uses of this compound?
Year of Question :(2015)

Answer: Water of crystallization : Crystals of some salts contain certain amount of associated water. The water associated with the crystal (or molecule) of any salt is called water of crystallisation. The hydrated salt is known as washing soda which is sodium carbonate containing 10 molecules of water of crystallization, i.e., it is sodium carbonate decahydrate. Its molecular formula is Na2CO3.10H2O. It can be obtained by heating baking soda followed by recrystallisation from its aqueous solution Uses of sodium carbonate

  • (i) For the manufacture of glass, soap, papers and chemicals like caustic soda (NaOH), borax, etc
  • (ii) For washing purposes (laundry works)
Question 41.
  • (a) Name and describe giving chemical equation the process used for producing sodium hydroxide. Why is this process so named
  • (b) Give one use of each of any two products obtained in this process. (Board Term I, 2014)

Answer: The process of electrolysis of sodium chloride solution is called chlor-alkali process because of the products formed : chlor for chlorine and alkali for sodium hydroxide. The three very useful products obtained by the electrolysis of sodium chloride solution are sodium hydroxide, chlorine and hydrogen

Question 42.
  • (a) You have three solutions - A, B and C having a pH of 6, 2 and 9 respectively. Arrange these solutions in increasing order of hydrogen ion concentration. Which of the three is most acidic? What happens to the hydrogen ion concentration in A as it is diluted
  • (b) If someone is suffering from a stomach problem called acidity, why is a solution of baking soda offered as a remedy
  • (c) Write chemical name and formula of baking soda. (Board Term I, 2013)

Answer:

  • (a) The solution having lower pH will have more hydrogen ion concentration. Hence, solution B (i.e., pH = 2) will have more hydrogen ion concentration. Solution B is most acidic
    • Adding water to solution A, will reduce the concentration of hydrogen ions in the solution
  • (b) Acidity can be neutralised by a base. Sodium hydrogen carbonate can be used as an antacid solution because it is a weak base and will react with excess acid produced in the stomach due to hyperacidity and will neutralise it
  • (c) Baking soda and baking powder

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