# NCERT Solutions for Class 10 Science Chapter 2 Acids, Bases and Salts

## NCERT Intext Questions

Page Number 18
1. 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?
Ans. (i) Put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus to blue will be a basic solution. The blue litmus paper formed here can now be used to test the acidic solution. (ii) Put the blue litmus paper obtained above in the remaining two test-tubes, turn-byturn. The solution which turns the blue litmus paper to red will be the acidic solution. (iii) The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water.

Page Number 22

1. Why should curd and sour substances not be kept in brass and copper vessels?
Ans. Curd and sour substances should not be kept in brass and copper vessels because these and other sour food-stuffs contain acids which can react with the metal of the vessel to liberate H2 gas and other harmful products, thereby spoiling the food.
2. 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?
• (a) Hydrochloric acid
• (b) Sulphuric acid
• (c) Nitric acid
• (d) Aqua regia
Ans. (i) Acids react with metal and liberate Hydrogen (H2) gas.
(ii) Example: Set up the apparatus as shown in the given figure. Take some zinc granules in the test tube. Add about 5 mL dilute hydrochloric acid slowly. Soon the reaction between zinc and hydrochloric acid starts and hydrogen gas is evolved.
(iii) Test for H2 gas : H2 gas is not soluble in water. When passed through soap solution, it gets trapped into bubbles. Bring a burning candle near the soap bubble filled with gas. The soap bubble bursts and hydrogen gas burns with a pop sound.
3. Metal 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.
Ans. The gas evolved extinguishes a burning candle and the gas is CO2. As the end product formed is calcium chloride and carbon dioxide, the metal compound A can be drawn by equation.
$$\underset{\text{Calcium carbonate}}{CaCO_3(s)} + 2HCl (aq) → \underset{\text{Calcium chloride}}{CalCl_2 (aq)} + CO_2 (g) + H_2O (l)$$

Page Number 25

1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Ans. HCl, HNO3 get ionised in aqueous solution and due to presence of H+ ions they show acidic characters. While alcohol and glucose do not form any such ions so they do not show acidic characters.
Play Video about Chapter 2 Acids Bases and Salts
2. Why does an aqueous solution of an acid conduct electricity?
Ans. In aqueous solution acids produce ions (cation or anions) which have charge on it, and are responsible for the conduction of electricity.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Ans. Dry HCl gas does not change the colour of dry litmus paper, because it does not dissociate to give H+ ions, responsible for acidic character.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans. While diluting an acid it is recommended that the acid should be added to water and not water to the acid because if water is added to concentrated acid to dilute it, then a large amount of heat is evolved at once. This heat changes some of the water to steam explosively which can splash the face or clothes and cause acid burns.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans. The concentration of hydronium ions will decrease with dilution because of the reaction between the acid's ion and the water’s ion, which results in the formation of H2O. Thus making a solution less acidic with the decrease in the concentration of hydronium ion.
6. How is the concentration of hydroxide ions (OH) affected when excess base is dissolved in a solution of sodium hydroxide ?
Ans. Addition of an excess base to a solution of sodium hydroxide means an addition of more OH ions, because sodium hydroxide itself dissociates to give OH
$$NaOH (aq) → Na^+ (aq) + \underset{\text{ Hydroxide ions}}{OH^- (aq)}$$ Hence the concentration of OH ions in solution will increases.

Page Number 28

1. You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic ?
Ans. pH is defined as negative logarithm of hydrogen ion, so if the H+ ion is more, pH will be less, and if H+ ion is less, pH will be more.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution ?
Ans. The solution A is having pH = 6 and solution B is having pH = 8. From the defination of pH the solution of A will have more H+ ions compared to solution B and it is acidic in nature while solution B will be basic in nature as pH is below 7 and above 7 respectively. The acidity is directly proportional to the H+ ion concentration. The more the H+ ions, the higher the acidity. Less the H+ ion, lesser the acidity.
3. Do basic solutions also have H+ (aq) ions ? If yes, then why are these basic?
Ans. Basic solutions also have H+ (aq) ions. But H+ ions are far less in number than OH ions which is responsible for the basic nature of such solutions.
4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) ?
Ans. If the soil is too acidic (having low pH) then it is treated with materials like quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) in order to neutrialize the acidicity of soil and make it more fertile.

Page Number 33

1. What is the common name of the compound CaOCl2?
Ans. The common name of the compound CaOCl2 is Bleaching powder.
2. Name the substance which on treatment with chlorine yields bleaching powder.
Ans. Slaked lime Ca (OH)2 on treatment with chlorine yields bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
3. Name the sodium compound which is used for softening hard water.
Ans. Sodium carbonate is used for softening hard water.
4. What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the reaction involved?
Ans. Solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide gas is evolved.
$$\underset{\text{Sodium hydrogen carbonate}}{2NaHCO_3} \xrightarrow{Heat} \underset{\text{ Sodium carbonate}}{Na_2CO_3} + H_2O + CO_2$$
5. Write an equation to show the reaction between Plaster of Paris and water.
Ans. $$\underset{\text{Plaster of Paris}}{CaSO_4·\frac{1}{2}H_2O} + 1\frac{1}{2}H_2O \rightarrow \underset{\text{Gypsum}}{CaSO_4·2H_2O}$$

## NCERT Exercise Questions

1. A solution turns red litmus blue, its pH is likely to be:
• (a) 1
• (b) 4
• (c) 5
• (d) 10
Ans. (d) 10
Explanation : Red litmus turns into blue, when dipped in basic solution i.e., pH > 7.
2. A solution reacts with crushed-egg shells to give a gas that turns lime water milky. The solution contains:
• (a) NaCl
• (b) HCl
• (c) LiCl
• (d) KCl
Ans. (b) HCl
Explanation : The egg shells are made up of calcium carbonate and the gas which turns lime water milky is carbon dioxide. CO2 gas can be formed by the action of an acid on calcium barbonate, so the solution contains HCl.
3. 10 mL of a 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 it will be:
• (a) 4 mL
• (b) 8 mL
• (c) 12 mL
• (d) 16 mL
Ans.  (d) 16 mL

Explanation : HCl + NaOH → NaCl + H2O
From the above reaction 1 mole of HCl is neutrialised by 1 mole of NaOH if 8 ml of HCl is completely neutrilised 10 ml of NaOH then 20 ml of NaOH is neutralized by = $$\frac{20×8}{10} = \text{16 ml of HCl.}$$

4. Which one of the following types of medicines is used for treating indigestion?
• (a) Antibiotic
• (b) Analgesic
• (c) Antacid
• (d) Antiseptic
Ans.  (c) Antacid
Explanation : Antacids act as neutralizing agents. These agents cures stomach acidity by neutralizing gastric HCl.
5. Write word equations and then balanced equations for the reaction taking place when:
• (a) dilute sulphuric acid reacts with zinc granules.
• (b) dilute hydrochloric acid reacts with magnesium ribbon.
• (c) dilute sulphuric acid reacts with aluminium powder.
• (d) dilute hydrochloric acid reacts with iron filings.
Ans. (a) Zinc + dilute sulphuric acid → Zinc sulphate + Hydrogen gas
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
(b) Magnesium ribbon + dil. Hydrochloricacid → Magnesium chloride + Hydrogen gas
Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
(c) Aluminium powder + dil. Sulphuric acid → Aluminium sulphate + Hydrogen gas
2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)
(d) Iron filings + Dilute hydrochloric acid → Ferric chloride + Hydrogen gas
2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)
6. Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Ans. Though compounds like alcohol and glucose contain hydrogen but they do not ionise in the solution to produce H+ ions on passing current through them.
(i) Take solutions of alcohols and glucose.
(ii) Fix two nails on a cork, and place the cork in 100 mL beaker.
(iii) Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in the given figure.
(iv) Now pour alcohol in the beaker and switch on the current.
(v) The bulb does not glow.
(vi) Repeat the experiment with glucose. The bulb does not glow in this case also.
(vii) This means no ions or H+ ions are present in the solution. This shows that alcohols and glucose are not acidic in nature.
7. Why does distilled water not conduct electricity, whereas rainwater does ?
Ans. Distilled water cannot conduct electricity because it does not contain ions while rain water contains dissolved salts and acids which dissociates in ions and conducts electricity.
8. Why do acids not show acidic behaviour in the absence of water ?
Ans. The acidic behaviour of acid is due to presence of H+ ions as pH is inversly proportional to H+ ion. In the absence of water, acids do not dissociate to produce H+ ions, hence do not show acidic behaviour.
9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Which solution is:
• (a) Neutral
• (b) Strongly alkaline
• (c) Strongly acidic
• (d) Weakly acidic
• (e) Weakly alkaline
Arrange the pH in increasing order of hydrogen ion concentration.
• Ans. (a) D
• (b) C
• (c) B
• (d) A
• (e) E
Increasing order of hydrogen ion concentration 11 < 9 < 7 < 4 < 1. i.e., C < E < D < A < B
10. 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 test tube will the fizzing occur more vigorously and why ?
Ans. HCl is a strong acid. It will produce more H+ ions which reacts with magnesium ribbon. Acetic acid is a weak acid. It will produce less H+ ions which reacts with magnesium ribbon. Therefore fizzing will occur more vigorously in test tube A containing HCl.
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd ? Explain.
Ans. Milk is converted into curd by lactic acid bacteria which convert lactose sugar present in milk into lactic acid. Thus the curd is more acidic than milk and pH of curd is less than the pH of milk.
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline ?
(b) Why does this milk take a long time to set as curd ?
Ans. (a) Milk is made slightly alkaline so that it may not get sour easily due to the formation of lactic acid in it.
(b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.
13. Plaster of Paris should be stored in a moisture proof container. Explain why?
Ans. Plaster of Paris is calcium sulphate and addition of water into it leads to the formation of gypsum which is a hard material. This will make plaster of Paris useless.
$$\underset{\text{Plaster of Paris}}{CaSO_4 \frac{1}{2}H_2O} + \frac{3}{2}H_2O \rightarrow \underset{\text{Gypsum}}{CaSO_4.2H_2O}$$

That is why it is always stored in a moisture proof container.

14. What is a neutralisation reaction ? Give two examples.
Ans. The reaction between an acid and a base to form salt and water is called a neutralisation reaction.

$$\text{(i)} \underset{\text{Sodium hydroxide (Base)}}{NaOH (aq)} + \underset{\text{ Hydrochloric acid (Acid)}}{HCl (aq)} → \underset{\text{Sodium chloride (Salt)}}{NaCl (aq)} +\underset{\text{Water }} {H_2O (l)} \\ \text{(ii)} \space \underset{\text{Acetic acid (Acid)}}{CH_3 COOH (aq)} + \underset{\text{Sodium hydroxide (Base)}}{NaOH (aq)} \rightarrow \underset{\text{Sodium acetate (Salt)}}{CH_3COONa} + \underset{\text{Water}}{H_2O (l)}$$

15. Give two important uses of washing soda and baking soda.
Ans. The uses of washing soda and baking soda are:
Uses of washing soda:
(i) Washing soda is used in glass, soap and paper industries.
(ii) It is used for removing permanent hardness of water.
(iii) It is used in the manufacturing of sodium compounds such as borax.
(iv) It is used in fire extinguishers.
Uses of baking soda:
(i) Baking soda is used as an antacid in medicines to remove acidity of the stomach.
(ii) Baking soda is used for making baking powder (used in making cakes, bread, etc.).