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

In Text Questions Page No: 18

Q.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?

If the color of red litmus does not change then it is acid. If the color of red litmus changes to blue then it is base. If there is slight change in the color of red litmus (such as purple) then it is distilled water.
Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes color to blue, then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the color of red litmus turns blue, then the second solution is neutral and if there is no change in color , then the second solution is acidic.
This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

In Text Questions Page No: 22

Q.1. Why should curd and sour substances not be kept in brass and copper vessels?

Curd and the sour substances if kept in brass or copper vessels react with them and form hydrogen and other harmful substances due to presence of acid in them. These toxic substance can cause food poisoning or other damage to health. Due to this reason, curd and the sour substances should not be kept in brass and copper vessels.

Q.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?

Usually, hydrogen gas is liberated when an acid reacts with a metal. For example, let us take the reaction between zinc and sulphuric acid.

Reaction of zinc granules with dil. H2SO4 to liberate hydrogen gas which burns with a ‘pop’ sound
(i) Take 5 g of zinc granules in a test tube.
(ii) Set the apparatus as shown in the diagram.
(iii) Add 20 mL of dil. H2SO4 with the help of a thistle funnel.
(iv) Collect the gas evolved in a gas jar as shown in the figure.
(v) Observe the colour and odour of the gas.
(vi) Bring a burning matchstick near the gas jar and record your observations.
Observation: A colourless, odourless gas is evolved. It burns explosively with a ‘pop’ sound when a burning matchstick is brought near it, indicating the presence of hydrogen gas.
Chemical Reaction:
Zn(s) + H2SO4(dil) → ZnSO4(aq) + H2(g)

Q.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.

Calcium carbonate (A), when reacts with hydrochloric acid, produces carbon dioxide gas with effervescence. Carbon dioxide gas is used as a fire extinguisher. Therefore, it extinguishes a burning candle. Hence, the metal compound A is calcium carbonate.
CaCO3(s) + 2HCl(aq) → CaCl2(s)+ CO2 (g) + H2O(l)

In Text Questions Page No: 25

Q.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?

HCI, HNO3, etc. dissociate into their ions in the presence of water. They form hydrogen ions (H+). These hydrogen ions combine with H2O to form hydronium ions (H3O+). The reaction can be given as follows:
HCl → H+ + Cl
H+ + H2O → H3O+
Similarly, HNO3 → H+ + NO3
H+ + H2O → H3O+
Due to this property, HCl and HNO3 show acidic character in aqueous solutions. On the other hand, alcohol and glucose cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

Q.2. Why does an aqueous solution of an acid conduct electricity?

An aqueous solution of an acid conduct electricity because of the presence of charged particles called ions in it. When dissolved in water, acids dissociate to form ions. Example:
HCl + H2O → Cl + H3O
These ions are responsible for conducting electricity.

Q.3. Why does dry HCl gas not change the color of the dry litmus paper

We know that the colour of the litmus is changed by H+ ions of an acid. Dry HCI does not dissociate to give H+ ions. Acids dissociate to give ions only in the aqueous medium. Since in this case, neither HCI is in aqueous form nor is the litmus paper wet, so the colour of litmus paper does not change.

Q.4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

The process of dissolving an acid in water is a highly exothermic. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause severe burns. The glass container may also break due to excessive heating.

Q.5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Concentration of hydronium ions (H3O+) decreases when a solution of an acid is diluted.

Q.6. How is the concentration of hydroxide ions (OH) affected when excess base is dissolved in a solution of sodium hydroxide
Answer Concentration of hydroxide ions (OH) increases when excess base is dissolved in a solution of sodium hydroxide.

In Text Questions Page No: 28

Q.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?

Solution A has more H+ ion concentration. A pH value of less than 7 indicates the acidic nature while greater than 7 indicates the basic nature of a solution. So, solution A is acidic and solution B is basic.

Q.2. What effect does the concentration of H+(aq) ions have on the nature of the solution?

Concentration of H+(aq) can have a varied effect on the nature of the solution. With an increase in H+ ion concentration, the solution becomes more acidic, while a decrease of H+ ion causes an increase in the basicity of the solution.

Q.3. Do basic solutions also have H+(aq)  ions? If yes, then why are these basic?

Yes, basic solution also has H+(aq) ions. However, their concentration is less as compared to the concentration of OH ions that makes the solution basic.

Q.4. Under what soil conditions 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)?

If the farmer finds his soil to be more acidic, then to increase the basicity of the soil, he should treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

In Text Questions Page No: 33

Q.1. What is the common name of the compound CaOCl2?

Bleaching Powder.

Q.2. Name the substance which on treatment with chlorine yields bleaching powder?

Dry slaked lime [Ca(OH)2].

Q.3. Name the sodium compound which is used for softening hard water.

Washing soda (Na2CO3.10H2O)

Q.4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

When sodium hydrogen carbonate is heated then sodium carbonate and water are formed along with the evolution of carbon dioxide gas.

Q.5. Write an equation to show the reaction between Plaster of Paris and water.

Exercises

Q.1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10

(d)
pH = 10, bases turn red litmus blue and their pH is more than 7.

Q.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

(b)

Eggshells are made up of CaCO3 which reacts with HCl to form CO2 and this CO2 turns lime water milky. For example,
CaCO3(s) + 2HCl(dil)→ CaCl2(aq) + H2O(l) + CO2(g)
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

Q.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) 8mL
(c) 12 mL
(d) 16 mL

(d)

∴ 10 mL of NaOH will neutralise
= 8 mL of HCI.
= 20 mL of NaOH will neutralise
= 8/10 × 20 = 16 mL.

Q.4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

(c)
Antacids are used to neutralise hyperacidity in the stomach due to excess of HCI which causes indigestion. Antibiotics are used to fight infections. Analgesics are pain killer while antiseptics prevent growth of bacteria and other micro-organisms on wounds.

Q.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.

(a)

(b)

(c)

(d)

Q.6. Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

• Fix two iron nails on a cork and place this cork in a beaker.
Connect the nails to the two terminals of a 6 volt battery through a switch and a bulb as shown in figure.
Now add some dilute hydrochloric acid in beaker and switch on the current. Take the observation.
Repeat the experiment separately with alcohol and glucose solution.

Observation: You will observe that in case of dilute hydrochloric acid bulb glows but when glucose or alcohol solution is taken in beaker, the bulb does not glow.
Conclusion: The aqueous solution of hydrochloric acid conducts electricity due to presence of types of charged particles – hydrogen ions and chloride ions. Unlike acids glucose and ethanol do not ionise in aqueous solution, i.e. they do not give H+ ions, therefore they cannot conduct electricity. Thus, glucose and ethanol are not categorized as acids.

Q.7. Why do acids not show acidic behavior in the absence of water?

It is because acids do not dissociate into ions in absence of water. But when an acid is dissolved in water, it forms hydrogen ions and hence, shows acidic behavior,
Example:

Q.8. 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.

(a) Neutral → Solution D with pH 7
(b) Strongly alkaline → Solution C with pH 11
(c) Strongly acidic → Solution B with pH 1
(d) Weakly acidic → Solution A with pH 4
(e) Weakly alkaline → Solution E with pH 9
The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1.

Q.9. 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?

Fizzing occur more vigorously in hydrochloric acid (A) than in acetic acid (B) because HCl is a stronger acid than acetic acid. HCl dissociates completely into H+ and Cl ions completely whereas acetic acid being weak acid partially dissociates into its ions.

Q.10. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Milk contains a carbohydrate lactose. When milk sets into curd, lactose gets converted into an acid called lactic acid. Due to formation of lactic acid, pH of milk falls below 6.

Q.11. 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?

(a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because, in alkaline condition, milk does not set as curd easily.
(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.

Q.12. Plaster of Paris should be stored in a moisture-proof container. Explain why?

It will absorb water to form gypsum which set into hard solid mass. This will make Plaster of Paris useless after some time, e.g.

Q.13. What is a neutralization reaction? Give two examples.

A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction.
For Example:
NaOH(aq) + HCl(aq) → NaCl(aq) +H2O(l)
H2SO4(aq) + 2NH4OH(aq) → (NH4)2SO4(aq) + 2H2O

Q.14. Give two important uses of washing soda and baking soda.