NCERT Solutions Class 10 Science Chapter 2

Q.1 (a) Sodium hydroxide is prepared by chlor-alkali process.

$2\text{NaCl}\left(\text{s}\right)+2{\text{H}}_2\text{O}\left(\mathrm{l}\right)\to {\text{\hspace{0.33em}Cl}}_2(\text{g})+{\text{H}}_2(\text{g})+2\text{NaOH}\left(\text{aq}\right)$

Why this process is called so?
(b) What happens when Plaster of Paris is wetted with water?

Ans- (a) It is called chlor-alkali process because of the products formed – chlor for chlorine and alkali for sodium hydroxide.
(b) When Plaster of Paris is wetted with water, it changes to gypsum.

$\underset{\text{Plaster\hspace{0.33em}of\hspace{0.33em}Paris}}{{\text{CaSO}}_4\cdot 1/2{\text{H}}_2\text{O}}+3/2{\text{H}}_2\text{O\hspace{0.33em}}\to \underset{\text{Gypsum}}{{\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}}$

Q.2 (a) How does a change in pH causes tooth decay?
(b) What are antacids?

Ans- (a) Tooth decay starts when pH of the mouth is lower than 5.5. Tooth enamel is made up of calcium phosphate and is insoluble in water but is corroded when pH in the mouth is below 5.5.
(b) Antacids are bases that neutralise the excess acid in our stomach to relieve indigestion and heartburn. An example of antacid is magnesium hydroxide.

Q.3 (a) Suppose you have an unknown solution (A).
Name the scale and its range which can we used to find out whether it is acidic or basic or neutral.
(b) Will the pH of orange juice be more than 7 or less than 7?

Ans- (a) pH scale is used to find if the solution is acidic or basic or neutral.
• If the pH is equal to 7, the solution is neutral.
• If the pH is less than 7, the solution is acidic.
• If the pH is more than 7, the solution is basic.
(b) Orange juice is sour in taste, i.e., it is acidic in nature. Therefore, its pH will be less than 7.

Q.4 Represent the following reactions by a balanced chemical equation:
(a) Iron reacts with sulphuric acid.
(b) Sodium carbonate reacts with acetic acid.
(c) Hydrochloric acid reacts with sodium hydroxide.

$\begin{array}{l}\text{<strong>Ans-</strong> (a) Fe}+{\text{H}}_2{\text{SO}}_4\to {\text{FeSO}}_4+{\text{H}}_2\\ {\text{(b) Na}}_2{\text{CO}}_3+2{\text{CH}}_3\text{COOH}\to 2{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2\\ \text{(c) HCl}+\text{NaOH}\to \text{NaCl}+{\text{H}}_2\text{O}\end{array}$

Q.5 (a) Name two edible acids.
(b) Classify aqueous solution of carbon dioxide as acid or base.
(c) Which acid will produce large number of H⁺ ions and why?
(i) aq. solution of acetic acid
(ii) aq. solution of hydrochloric acid

Ans- (a) Acetic acid and citric acid
(b) Aqueous solution of carbon dioxide is acidic (due to the presence of carbonic acid).
(c) Hydrochloric acid is a strong acid and acetic acid is a weak acid. Therefore, hydrochloric acid will produce large number of H⁺ ions.

Q.6 Tooth enamel is one of the hardest substances in our body. How does it damage due to eating cakes and sweets. What should we do to prevent it?

Ans- Tooth enamel is made up of calcium phosphate, Ca(PO₄)₂ . It gets corroded when the pH of the mouth becomes lower than 5.5. When we take cakes and sweets, some particles remain in the mouth after eating. Bacteria present in the mouth produce acid by degradation of sugar and food particles remaining in the mouth after eating. This lowers the pH in the mouth and results in tooth decay.

Tooth decay can be prevented by cleaning the mouth after eating sugary food. We can also use toothpastes which are alkaline to neutralise the excess acid and hence prevent the tooth decay.

Q.7 There are two test tubes – test tube (A) containing HCl and test tube (B) containing NaOH. What changes in colour will be observed when red cabbage indicator is added to them?

Ans- In acidic medium the colour of red cabbage indicator turns red. Therefore, when it is added to test tube (A) containing HCl, red colour will be observed. In basic medium the colour of red cabbage indicator turns green. Thus, when it is added to test tube (B) containing NaOH green colour will be observed.

Q.8 (a) Give an example each of a natural indicator and a synthetic indicator.
(b) What change in colour of red litmus paper is observed when it is dipped in a test tube filled with an acid?

Ans-

(a) Natural indicator: Litmus
Synthetic indicator: Phenolphthalein
(b) No change in colour is observed.

Q.9 What is a neutralisation reaction ?

Ans- A reaction between an acid and an alkali which results in the formation of salt and water is called neutralisation reaction.
Example :- NaOH + HCl → NaCl + H₂O

Q.10 What do you mean by universal indicator?

Ans- A natural indicator is a mixture of several indicators having different pH ranges. It shows many colour changes over a wide range of pH. Each colour corresponds to a certain pH value.

Q.11 What is a salt?

Ans- The substance formed other than water by the neutralisation reaction of an acid with a base is called salt.

$\text{NaOH}+\text{HCl\hspace{0.33em}}\to \text{\hspace{0.33em}NaCl}+{\text{H}}_2\text{O}$

Q.12 How does acetic acid (vinegar) help in the preservation of food?

Ans- The growth of micro-organisms responsible for food spoilage is prevented or they are rendered inactive by pickling in vinegar or acetic acid.

Q.13 Why should curd and sour substances not be kept in brass and copper vessels?

Ans- Curd and sour substances contain acids which can react with the metal of the vessel to form poisonous metal compounds. These compounds can cause food poisoning and damage our health.

Q.14 You have given three test tubes, one of them contain distilled water and the other two contain an acid 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- We put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus blue will be a basic solution. The blue litmus paper obtained here, is dipped in the remaining two test tubes, one by one.
The solution which turns the blue litmus paper to red will be the acidic solution and the solution which has no effect on any litmus paper will be neutral and hence is distilled water.

Q.15 Why a substance is acidic or basic in nature?

Ans- Acidic nature of a substance is due to the formation of H⁺ (aq) ions or H₃O⁺ (aq) ions in aqueous solution and basic nature of a substance is due to the furnishing of OH^– (aq) ions in solution.

Q.16 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?

Ans- Hydrogen gas is liberated when an acid reacts with a metal. For example, when zinc metal reacts with dil. HCl, hydrogen gas is evolved and zinc chloride is formed.

Zn (s) + 2HCl(aq) → ZnCl₂ (aq) + H₂ (g)

The presence of hydrogen gas can be tested by bringing a burning candle near the gas. The gas burns with a pop sound.

Q.17 Write a short note on pH scale.

Ans-

pH : The negative logarithm (base 10) of the hydrogen ion concentration in moles per litre is called pH.
pH = – log₁₀ [H⁺]
A scale for measuring hydrogen ion concentration in a solution is known as pH scale. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value.
The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the PH value increases from 7 to 14, it represents an increase in OH^– concentration that is, increase in the strength of alkali.

Q.18 What is baking soda? How is it produced? Write two uses of it.

Ans- Baking soda is sodium hydrogen carbonate (NaHCO₃). It is produced using sodium chloride.

$\underset{}{\text{NaCl}}+\underset{}{{\text{H}}_2\text{O}}+\underset{}{{\text{CO}}_2}+\underset{}{{\text{NH}}_3}\to {\text{\hspace{0.33em}NH}}_{\text{4}}{\text{Cl + NaHCO}}_{\text{3}}$

Uses of baking soda –

1. For making baking powder which is a mixture of baking soda and tartaric acid.
2. It is used as a mild antiseptic.

Q.19 A compound which is prepared from gypsum has a property of hardening when mixed with water. Identify the compound. Write the chemical equation of its preparation. Mention two uses of the compound.

Ans- The compound is plaster of Paris ( CaSO₄.(1/2) H₂O)
The chemical name of it, is calcium sulphate hemihydrate.

$\underset{\text{Gypsum\hspace{0.33em}}}{{\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}}\stackrel{\text{373K}}{\to }\underset{\text{Calcium sulphate}\mathrm{hemihydrate}}{{\text{CaSO}}_4\text{.}\frac{1}{2}{\text{H}}_2\text{O}}+1\frac{1}{2}{\text{H}}_2\text{O}$

Uses of plaster of Paris:

1. It is used for plastering the fractured bones in hospitals.
2. It is used for making toys and statues.

Q.20 What will happen if a solution of sodium hydrogen carbonate is heated?

Ans- A solution of sodium hydrogen carbonate on heating gives sodium carbonate and water, and carbon dioxide gas is liberated.

$2{\text{NaHCO}}_3\stackrel{∆}{\to }{\text{\hspace{0.33em}Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\text{↑}$

Q.21 What is an acid?

Ans- An acid is a substance which dissociates in aqueous solution to give hydrogen ion or hydronium ion.

Q.22 Name the substance which on treatment with chlorine yields bleaching powder.

Ans- Slaked lime [Ca(OH)₂]

$$\begin{gathered} \mathrm{Ca}{\left(\mathrm{OH}\right)}_2+{\mathrm{Cl}}_2\to {\mathrm{CaOCl}}_2+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Slaked}\mathrm{lime} \end{gathered}$$

Q.23 What is the common name of the compound with chemical formula CaOCl₂?

Ans- Bleaching powder

Q.24 What is the chemical name of washing soda?

Ans- The chemical name of washing soda is sodium carbonate decahydrate [Na₂CO₃.10H₂O].

Q.25 What is a base?

Ans- A base is a chemical substance which dissociates in aqueous solution to give hydroxyl ions (OH¯).

Q.26 What is an alkali?

Ans- A base which is soluble in water is called an alkali.

Q.27 Define pH.

Ans- pH is defined as the negative logarithm (base10) of the hydrogen ion concentration in moles per litre.
pH = – log10 [H+]

Q.28 What is an indicator?

Ans- An indicator is a ‘dye’ that changes colour when it is put in an acid or a base (alkali).

Q.29 Write an equation to show the reaction between Plaster of Paris and water.

Ans- CaSO4.12H2OPlaster of Paris+32H2O→CaSO4⋅2H2OGypsum

Q.30 Name two mineral acids.

Ans-

(i) Sulphuric acid (H2SO4)
(ii) Nitric acid (HNO3)

Q.31 Name the sodium compound which is used for softening hard water.

Ans- Washing soda [Na2CO3.10H2O]

Q.32 Identify the compound of calcium which is yellowish white powder and is used for disinfecting drinking water. Write its chemical name and formula. How is it manufactured?

Ans- The compound is bleaching powder.
Chemical name : Calcium oxychloride
Formula: CaOCl2
Bleaching powder is manufactured by the action of chlorine on dry slaked lime, Ca(OH)2.

Ca(OH)2Calcium hydroxide   (Slaked lime)+Cl2Chlorine → CaOCl2Calcium Oxychloride (Bleaching powder)+H2OWater

Q.33 A metal compound ‘X’ reacts with dilute hydrochloric acid to produce gas (Y). The gas (Y) evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Write names of X & Y.

Ans- Metal compound ‘X’ is calcium carbonate because it reacts with dilute hydrochloric acid to give calcium chloride, water and carbon dioxide gas. The carbon dioxide gas evolved extinguishes a burning candle. Hence, ‘Y’ is carbon dioxide.

CaCO3Calcium carbonate + 2HClHydrochloric acid → CaCl2Calcium chloride + CO2Carbondioxide + H2OWater

Q.34 How is concentration of hydroxide ions (OH-) affected when excess of base is dissolved in a solution of sodium hydroxide?

Ans- Concentration of hydroxide ions (OH-) becomes high when excess base is dissolved in a solution of sodium hydroxide.

Q.35 Why does an aqueous solution of an acid conduct electricity?

Ans- When acid is dissolved in water, hydrogen ions (H+) become free to move and hence they can conduct electricity.

Q.36 What are alkalis?

Ans- The bases which are soluble in water are known as alkalis. For example, NaOH, KOH.

Q.37 What are acid and base indicators?

Ans- Acid and base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. For example, phenolphthalein, methyl orange.

Q.38 What is bleaching powder? How is it prepared? Write chemical equation involved in the preparation of bleaching powder. Write two uses of bleaching powder.

Ans- Bleaching powder is calcium oxychloride ( CaOCl­2). It is prepared by passing chlorine gas over dry slaked lime.

Ca(OH)2Calcium Hydroxide     Slaked lime + Cl2Chlorine → CaOCl2Calcium oxy chloride Bleaching powder + H2OWater

Uses –

1. Bleaching powder is used for bleaching cotton and linen in the textile industry and for bleaching wood pulp in paper industry.
2. It is used for disinfecting drinking water to make it free of germs.

Q.39 Fresh milk has a pH 6. How do you think the pH will change as it becomes sour?

Ans- pH of the milk will decrease as it becomes sour. Sour milk contains lactic acid. So, the H+ concentration will increase.

Q.40 Why does distilled water not conduct electricity, whereas rainwater does?

Ans- Distilled water is the purest form of water free from all types of impurities while rainwater has impurities mixed into it which allows movement of ions thus results in the conduction of electricity. Hence, distilled water is a poor conductor of electricity whereas rainwater conducts electricity.

Q.41 Name the acid found in
(a) soft drinks
(b) vinegar

Ans-

(a) Soft drinks :- Carbonic acid
(b) Vinegar :- Acetic acid

Q.42 How does an acid taste?

Ans- An acid has a sour taste.