NCERT Solutions for Class 10 Science Chapter 2 Acid Bases And Salts

Q1 )

A solution turns red litmus blue, its pH is likely to be

• (a) 1
• (b) 4
• (c) 5
• (d) 10

Answer :

Red litmus paper turns blue under the condition of base or alkaline.
pH range of base: above 7
Threfore,
Ans: (d) 10

Q2 )

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 :

Egg shells contains calcium carbonate, which on reaction with HCl liberates CO2 gas which turn lime water to milky.
Therefore, Solution would be
Ans: (b) HCl

Q3 )

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

Answer :

Since, 10 ml of NaOH requires 8 ml of HCl for neutralization,20 ml of NaOH will require:
(8 divided by 10)×20 = 16
Therefore,
Ans: (d) 16 ml

Q4 )

Which one of the following types of medicines is used for treating indigestion ?

• (a) Antibiotic
• (b) Analgesic
• (c) Antacid
• (d) Antiseptic

Answer :

(C) Antacid

Q5 )

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 filing

Answer :

(a) $$Zinc + dilute sulphuric acid \rightarrow Zinc sulphate + Hydrogen $$

$$ Zn(s) + H_{2}SO_{4}(aq) $$

$$ \downarrow$$

$$ ZnSO_{4}(aq) + H_{2}(g) $$

$$ Mg(s) + 2HCl(aq) $$

$$ \downarrow$$

$$ MgCl_{2} + H_{2}(g) $$

$$ 2Al(s) + 3H_{2}SO_{4}(aq) $$

$$ \downarrow$$

$$Al_{2}(SO_{4})_{3} (aq) + 3H_{2}(g) $$

$$ 2Fe(s) + 6HCl(aq) $$

$$ \downarrow$$

$$2FeCl_{3}(aq) + 3H_{2}(g)$$

Q6 ) Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

Answer :

Alcohols and glucose does not disassociate in water to form hydrogen ions neither do they accept electrons.This is why they are not considered as Acid
An activity to prove it:
The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl.

Observations :
The bulb glows in HCl solution only. The reason is that HCl solution dissociates into H+ and Cl– ions which help to conduct electricity while glucose and alcohol solutions do not dissociate into ions and so, do not conduct electricity. That is why we find the bulb glowing in HCl solution only. Conclusions The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids.

Q1 ) You are given three test tubes. The three test tubes contain distilled water, acidic solution and the basic solution respectively. There is only red litmus paper available in order to identify what is there in each test tube. How will you find out what is in each of the test tubes?

Answer :

Red litmus paper can be used to identify the content in each test tube. Variation in color of Red Litmus Paper can help in identifying the content.
Red litmus paper has characteristic that it turns blue when is treated with base and turns purple when is treated with a neutral solution. Change of color of the red litmus paper can be observed in the following ways:
(i) If the red litmus paper changes to blue colour the solution is a basic solution.

(ii) If the red litmus paper experiences no change the solution is an acidic solution.

(iii) If the red litmus paper changes to purple colour the solution is distilled water.

Q2 ) Why should curd and sour substances not be kept in brass and copper vessels?

Answer :

Curd and sour substances are acidic in nature i.e, they have acids in them. Brass and copper vessels are metals.
When acids react with metals they form salt and hydrogen gas.
So,when the curd or sour substances are kept in brass and copper vessels ,they react with them and can corrode the vessel and form toxic substances which are very harmful for the human body . That's why the curd and sour substances are not kept in brass or copper vessels.

Q3 ) 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 :

When an acid reacts with a metal it liberates Hydrogen gas.
Metal + Acid \(\rightarrow \) Salt + Hydrogen gas.

For example:

\( Zn(s) + 2HCl(aq) → ZnCl_2(aq) + H_2(g) \)

Fig. Hydrogen production and testing the presence of it.

Q4 ) Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved to extinguish a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Answer :

The evolved gas which extinguishes a burning candle is carbon dioxide \(CO_2 \).
During chemical reaction,
Since, one of the compounds formed during this reaction is calcium chloride and as carbon dioxide is formed when dilute hydrochloric acid reacts with a metal carbonate producing effervescence , it means the metal compound is calcium carbonate \(CaCO_3 \). Hence, the compound A is calcium carbonate.

\( CaCO_3 (s) + 2HCl(aq) \)

\( ↓ \)

\( CaCl_2 (aq) + CO_2 (g) + H_2 O(l) \)

Q5 ) 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 :

The acidic character of a substance is due to the presence of hydrogen ions (H+) ions in its aqueous solution (water solution) and electron acceptance.
\(HCl \) and \(HNO_3 \)shows acidic properties because they produce hydrogen ions when dissolved in water.
The solutions of compounds like alcohol and glucose do not show acidic character because they do not ionise (or dissociate) in water to produce hydrogen ions.

Q6 ) Why does an aqueous solution of an acid conduct electricity?

Answer :

Charged particles are responsible for the conductance of electricity in an acid. Aqueous solutions of acids conduct electricity signifies the existence of ions in these solutions.
Acids dissociate in aqueous solution to form H+ ions. When electricity is passed through an aqueous solution of an acid, the H+ions reach the cathode and each H+ ion picks up one electron from the cathode to form \( H_2 \) gas. Because of this reaction, an aqueous solution of acid conducts electricity.

Q7 ) Why does dry \( HCl \) gas not change the colour of the dry litmus paper?

Answer :

Dry \( HCl \) does not change the colour of dry litmus paper because water is required for the formation of hydrogen ions which is responsible for the change in color of litmus paper. Since there is absence of water in this case, the formation of hydrogen ions won't take place and no colour change would be observed.

Q8 ) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer :

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, it releases a huge amount of heat which may result in explosion and can cause acid burns on face, clothes and body parts. Hence it is safe to add acid to water rather than water to acid.

Q9 ) How is the concentration of hydronium ions \( H_3 O^+ \) affected when a solution of an acid is diluted?

Answer :

The concentration of hydronium ions \( H_3 O^+ \) decreases when an acid is diluted because, on adding water the \( H^+ \) ions of the acid and hydroxyl ions of water react to form water molecules and this in-turn decreases Hydronium concentration in the solution.

Q10 ) How is the concentration of hydroxide ions \(OH^– \) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer :

The concentration of hydroxide ions increases when base is dissolved in Sodium Hydroxide solution till it reaches the saturation at some point. The concentration of hydroxide ions will not get affected if the base is added further after the saturation point is reached.

Q11 ) 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?

Answer :

We can use the rule to find the hydrogen ion concentration which states that, “The pH of any solution is inversely proportional to the hydrogen ion concentration” i.e, the solution that has a lower pH number will have a higher hydrogen ion concentration. Hence, solution A will have a higher hydrogen ion concentration than solution B.
Also the solution with pH below 7 shows acidic behaviour hence solution A is acidic and that for solution B it is greater than 7 so it’s a basic solution.

Q12 ) What effect does the concentration of \( H^+ (aq) \) ions have on the nature of the solution?

Answer :

Hydrogen ion \( H^+ (aq) \) concentration decides the nature of the solution. If Hydrogen ion concentration increases then solution turns acidic and similarly if the solution turns basic, the Hydrogen ion concentration decreases.

Q13 ) Do basic solutions also have \(H^+ (aq) \) ions? If yes, then why are these basic?

Answer :

Yes, basic solutions have \(H^+ \) ions. These type of solutions contain more hydroxide ions than \(H^+ \) ions. Hence the Hydroxide ions turn solution to basic.

Q14 ) 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 :

A farmer should treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) if the soil is acidic in nature i.e, pH below 7 which is not suitable for production of crops.

Q15 ) What is the common name of the compound \( CaOCl_2 \)?

Answer :

Common name of \( CaOCl_2 \) is bleaching powder.

Q16 ) Name the substance which on treatment with chlorine yields bleaching powder

Answer :

The substance which on treatment with chlorine yields bleaching powder is Calcium hydroxide.

\( Ca(OH)_2 (s) +Cl_2 (g) \)

\(↓ \)

\( CaOCl_2 (s) + H_2 O(l) \)

Q17 ) Name the sodium compound which is used for softening hard water.

Answer :

Sodium carbonate is the compound which is used for softening hard water.

Q18 ) What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Answer :

Heating sodium hydrocarbonate breaks down in sodium carbonate, carbon dioxide gas and water.The reaction is as follows:

Q19 ) 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 is

\(CaSO_4 .1/2 H_2 O + 3/2 H_2O \)

\(↓ \)

\(CaSO_4.2H_2 O \)