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

Topic: Introduction Page no-18

Question 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?

Answer:

Litmus is a common indicator that is used to identify acid and base. The colour of litmus paper turns to blue indicates that the chemical is a base. If there is no change in colour, then the chemical is either acidic or neutral.
First marks the three test tubes as A, B, and C.
Now,

(i) First of all, put the red litmus paper in each test tube one by one. The test tube in which red litmus paper will turn blue indicates the solution of that test tube is basic/Alkaline in nature.

(ii) Now remaining two are either acidic or neutral. Now mix a drop of the basic solution 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 colour of the red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic. This is because acidic and basic solutions neutralize each other.

Topic 2.1 - Understanding the chemical properties of acids and bases. Page no-18

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

Answer:

Curd and sour substances not be kept in brass and copper vessels because curd and other sour substances contain acids.
Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby may spoil the food.

Question.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?

Answer:

Hydrogen gas is usually liberated when an acid reacts with a metal.
When zinc granules react with dilute sulphuric acid, then the bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas and a zinc sulphate solution is formed.

$\mathrm{Zn}+2 \mathrm{H}_2 \mathrm{SO}_4 \Rightarrow \mathrm{Zn}\left(\mathrm{SO}_4\right)_2+2 \mathrm{H}_2$

The evolved hydrogen gas can be tested by taking a burning candle near a soap solution containing hydrogen gas. If the candle burns with a pop sound, it confirms the evolution of hydrogen gas.

Question.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.

Answer:

During the chemical reaction, the evolved gas extinguishes a burning candle which means the gas is carbon dioxide.

Since, one of the compounds formed during this reaction is calcium chloride and carbon dioxide is formed when dilute hydrochloric acid reacts with a metal carbonate (or metal hydrogen carbonate) producing effervescence it means the metal compound is calcium carbonate (It cannot be calcium hydrogen carbonate because calcium hydrogen carbonate is found only in solution, it is also unstable to exist as a solid). Hence, the compound A is calcium carbonate.

The chemical reaction is written as follows:

$\mathrm{CaCO}_3+2 \mathrm{HCl}_{(a q)} \rightarrow \mathrm{CaCl}_{2(a q)}+\mathrm{CO}_{2(g)}+\mathrm{H}_2 \mathrm{O}_{(l)}$

Topic 2.2 What do all acids and all bases have in common? Page no- 25

Question.1 Why do HCl, HNO ₃ , etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Answer:

Acids form hydrogen ions [ $H^{+}(\mathrm{aq})$ ions] in its aqueous solution which is responsible for their acidic characters. HCl and $\mathrm{HNO}_3$ form hydrogen ions in aqueous solution hence, they show acidic characters in aqueous solution.

The reaction of HCL is as follows:

$
\begin{aligned}
& \mathrm{HCl}_{(a q)} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-} \\
& \mathrm{H}^{+}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}
\end{aligned}
$

While solutions of compounds like alcohol and glucose do not form hydrogen ions in the solution, hence, they do not show acidic character.

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

Answer:

Acids dissociate in aqueous solutions to form ions. Like hydrogen ions ( $H^{+}$) or hydronium ions ( $\mathrm{H}_3 \mathrm{O}^{+}$) and the presence of these ions in an aqueous solution of acid are responsible for conducting electricity.

Question.3 Why does dry HCl gas not change the colour of the dry litmus paper?

Answer:

The colour of litmus paper changes only in the presence of ions like hydrogen ( $\mathrm{H}^{+}$) or hydronium ( $\mathrm{H}_3 \mathrm{O}^{+}$) ions. Dry HCl does not contain ( $\mathrm{H}^{+}$) or ( $\mathrm{H}_3 \mathrm{O}^{+}$) ions. It can produce these ions only in the form of an aqueous solution.

Hence, because of that dry HCl gas does not change the colour of dry litmus paper.

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

Answer:

The process of dissolving an acid in water is highly exothermic in which a large amount of heat is evolved. It is always recommended that acid should slowly be added to water because evolved heat is easily absorbed by a large amount of water.

If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns,

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

Answer:

When the solution of an acid is diluted, the concentration of hydronium ions $\left(\mathrm{H}_3 \mathrm{O}^{+}\right)$per unit volume decreases.

That means, the strength of the acid decreases.

Question.6 How is the concentration of hydroxide ions $\left(\mathrm{OH}^{-}\right)$ affected when excess base is dissolved in a solution of sodium hydroxide?

Answer:

The concentration of hydroxide ions $\left(\mathrm{OH}^{-}\right)$ would increase when excess base is dissolved in a solution of sodium hydroxide .

Topic 2.3 How strong are acid or base solutions Page no-28

Question.1 You have two solutions, A and B. The pH of solution A is 6 and the pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Answer:

The pH value of less than 7 indicates an acidic solution while greater than 7 indicates a basic solution. The pH value of a neutral solution is 7. And the acidic solution has more hydrogen ion concentration.

Therefore, solution A with pH = 6 is acidic and has more hydrogen ion concentration than solution B with pH = 8 which is basic.

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

Answer:

Concentration of $H^{+}(\mathrm{aq})$ can have a varied effect on the nature of the solution. When we increase the concentration of $H^{+}$ions in the solution, the solution becomes more acidic, while when we decrease the concentration of $H^{+}$ions in the solution, the basicity of the solution increases.

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

Answer:

Yes, the basic solution also has $\mathrm{H}+(\mathrm{aq})$ ions. However, they are basic because the concentration of hydrogen ions in them is less as compared to the concentration of hydroxide ions $\left[\mathrm{OH}^{-}\right.$ions $]$that makes the solution basic.

Question.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)?

Answer:

Quicklime, slaked lime and chalk are basic in nature. When they react with acids, the neutralization reaction takes place.

Therefore, If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quicklime or slaked lime or chalk

Topic 2.4 More about salts Page no- 33

Question.1 What is the common name of the compound $\mathrm{CaOCl}_2$ ?

Answer:

The common name of the compound $\mathrm{CaOCl}_2$ is bleaching powder.

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

Answer:

Calcium hydroxide $\left[\mathrm{Ca}(\mathrm{OH})_2\right]$ on treatment with chlorine yields bleaching powder.

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

Answer:

Washing soda $\left(\mathrm{Na}_2 \mathrm{CO}_3 \cdot 10 \mathrm{H}_2 0\right)$ is used for softening hard water.

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

Answer:

When a solution of sodium hydrogen carbonate (sodium hydrogen carbonate) is heated, it undergoes decomposition reaction to form sodium carbonate and water, and carbon dioxide gas is liberated.

The equation of the reaction involved is:

$2 \mathrm{NaHCO}_3 \xrightarrow{\text { heat }} \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2$

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

Answer :

The chemical equation for the reaction of Plaster of Paris and water can be represented as $\mathrm{CaSO}_4 \cdot \frac{1}{2} \mathrm{H}_2 \mathrm{O}+1 \frac{1}{2} \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$

Question.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

Answer:

(b) The solution contains HCI reacts with crushed egg-shells to give a gas that turns lime-water milky.

Question.3 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralize it will be

(a) 4 mL

(b) 8 mL

(c) 12 mL

(d) 16 mL

Answer:

It is given that
10 ml of NaOH neutralizes 8 ml Solution of HCl $\therefore 1 \mathrm{ml}$ of NaOH neutralizes $=\frac{8}{10}$

$=\frac{4}{5} \mathrm{ml}$ Solution of HCl

Therefore, 20 ml of NaOH will neutralise $=\frac{4}{5} \times 20=16 \mathrm{ml}$ solution of HCL

Therefore, 16 ml of HCl solution will be required to neutralize the 20 ml of NaOH.

The correct answer is (d)

Question.4 Which one of the following types of medicines are used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Antiseptic

Answer:

(c) The antacid is used for treating indigestion.

Question.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 aluminum powder.

(d) dilute hydrochloric acid reacts with iron filings.

Answer:

Balanced equations are:-
(a) $\mathrm{H}_2 \mathrm{SO}_{4(a q)}+\mathrm{Zn}_{(s)} \rightarrow \mathrm{ZnSO}_{4(a q)}+\mathrm{H}_{2(g)}$

(b) $2 \mathrm{HCl}_{(a q)}+\mathrm{Mg}_{(s)} \rightarrow \mathrm{MgCl}_{2(a q)}+\mathrm{H}_{2(g)}$

(c) $3 \mathrm{H}_2 \mathrm{SO}_{4(a q)}+2 \mathrm{Al}_{(s)} \rightarrow \mathrm{Al}_2\left(\mathrm{SO}_4\right)_{3(a q)}+3 \mathrm{H}_{2(g)}$

(d) $6 \mathrm{HCl}_{(a q)}+2 \mathrm{Fe}_{(s)} \rightarrow 2 \mathrm{FeCl}_{3(a q)}+3 \mathrm{H}_{2(g)}$

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

Answer:

Take the solution of dilute hydrochloric solution and glucose. Two nails are fitted on a cork and are kept it in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Dilute HCl is poured in the beaker and current is switched on. Now, the same experiment is performed with a glucose solution and an alcohol solution.

We will observe that the bulb glows in the HCl solution and does not glow in the glucose solution.

Because: HCl dissociate into H⁺ (aq) and Cl^-(aq) ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.

Conclusion:
From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids. That is why, though compounds like alcohols and glucose contain hydrogen, they are not categorized as acids.

Question.7 Why does distilled water not conduct electricity, whereas rainwater does?

Answer:

Distilled water is a pure form of water that does not contain ions. Therefore, it does not conduct electricity. Rainwater on the other hand, being an impure form of water, contains many ionic compounds such as acids and therefore it conducts electricity.

Question.8 Why do acids not show acidic behaviour in the absence of water?

Answer:

Acids do not show acidic behaviour in the absence of water because acids form hydrogen ions [H ⁺ (aq) ions] in its aqueous solution which are responsible for their acidic characters and the dissociation of hydrogen ions from an acid occurs in the presence of water only.
Therefore in the absence of water, acids do not produce hydrogen ions and hence do not show acidic behaviour.

Question.9 (a) 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

• Neutral?

Answer:

Solution with pH equal to 7 is a neutral solution. Therefore, a neutral solution is D

Question.9 (b) 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

• Strongly alkaline?

Answer:

The solution with the highest pH is strongly alkaline. Therefore, a strongly alkaline solution is C with a pH equal to 11

Question.9 (c) 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

• Strongly acidic?

Answer:

The solution with the lowest pH value is strongly acidic. Therefore, a strongly acidic solution is B with pH equal to 1

Question.9 (d) 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

• Weakly acidic?

Answer:

A solution with a pH less than 7 but close to 7 is weakly acidic. Therefore, the weakly acidic solution is A with a pH equal to 4

Question.9 (e) 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

e) Weakly alkaline?

Answer:

The solution with a pH greater than 7 but close to 7 is weakly alkaline. Therefore, a weakly alkaline solution is E with pH equal to 9

Question.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 $\left(\mathrm{CH}_3 \mathrm{COOH}\right)$ is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Answer:

When an acid reacts with magnesium metal, hydrogen gas is produced which causes fizzing. Stronger acids having a greater number of hydrogen ions in it. Hence, fizzing will occur strongly in test tube A, in which hydrochloric acid (HCI) is added.
This is because HCI is a stronger acid as compared to $\left(\mathrm{CH}_3 \mathrm{COOH}\right)$ and therefore, during the chemical reaction with magnesium metal, HCl will produce more hydrogen gas due to which fizzing will be more vigorous in test-tube A.

Question.11 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer:

The pH of fresh milk is 6.
As milk turns into curd, lactic acid is formed due to which it becomes more acidic. Therefore, the pH of milk will decrease.

Question.12 (a) 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?

Answer:

Fresh milk is slightly acidic in nature, The milkman shifts the pH of the fresh milk from 6 to slightly alkaline by adding little baking soda.
Because this will prevent the milk from getting sour due to lactic acid formation.

Question.12 (b) A milkman adds a very small amount of baking soda to fresh milk.

(b) Why does this milk take a long time to set as curd?

Answer:

Since this milk is slightly basic than usual milk, the lactic acid, formed during curdling, needs to first neutralize the alkali present in the milk.
Therefore, it takes a longer time for the curd to set.

Question. 13 The plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer:

Plaster of Paris should be stored in a moisture-proof container because it absorbs moisture (or water) to form a hard substance called gypsum which makes Plaster of Paris useless.
$\mathrm{CaSO}_4 \cdot \frac{1}{2} \mathrm{H}_2 \mathrm{O}+1 \frac{1}{2} \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$

Question.14 What is a neutralization reaction? Give two examples.

Answer:

A reaction in which an acid and base react with each other to give a salt and water is known as neutralization reaction. That is: Acid + Base → Salt + Water

For example:

(i) Sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to form sodium chloride (NaCl) and water:

$
\mathrm{NaOH}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}
$

(ii) Magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water:

$
\mathrm{Mg}(\mathrm{OH})_2+2 \mathrm{HCl} \rightarrow \mathrm{MgCl}_2+2 \mathrm{H}_2 \mathrm{O}
$

Question.15 Give two important uses of washing soda and baking soda.

Answer:

Uses of Washing Soda:

(i) It is used for removing the permanent hardness of the water.

(ii) It is used in glass, soap, and paper industries.

Uses of Baking Soda:

(i) It is used in soda-acid fire extinguishers.

(ii) It is used in making baking powder. Baking powder is a mixture of baking soda and a mild edible acid called tartaric acid. When baking powder is heated

Or mixed in water, it releases CO₂ that makes cakes or bread fluffy.

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