10th

TEXTUAL QUESTIONS PAGE – 18

Q.1- You have been provided with three test tubes. One of them contained 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- Dip red litmus paper in each test tube. The tube in which red litmus paper turns purple, contains distilled water. The tube in which red litmus paper turns blue, contains basic solution. The tube in which red litmus paper remains red, contains acidic solution.

TEXTUAL QUESTIONS PAGE – 22

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

Ans- Curd and other sour substances contain acids which react with the metal surface of brass and copper vessels to produce toxic substances which are unfit for consumption and cause food poisoning.

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?

Ans- When an acid reacts with metal, hydrogen gas is produced. Eg.-

Zn + H₂SO₄ → ZnSO₄ + H₂

Pass this gas through soap solution. The soap bubbles filled with gas will rise up. If a burning candle is brought near the gas, the bubble will burn with a pop sound.

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 compound formed is calcium chloride.

Ans- As the end product is calcium chloride and the gas evolved is carbon dioxide, the metal compound A is calcium carbonate. The reaction is :

CaCO₃ (s) + 2HCl (Aq) → CaCl₂( Aq) + CO₂ (g) + H₂O (l)

CO₂ extinguishes a burning candle.

TEXTUAL QUESTIONS PAGE – 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?

Ans- HCl, HNO₃ etc. dissociate in water to give H⁺ ions and show acidic character. The compounds like alcohol and glucose do not dissociate in water to give H⁺ ions. Hence they don’t show acidic character.

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

Ans- The aqueous solution of an acid contains charged particles called ions, which conduct electricity.

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

Ans- This is because dry HCl does not dissociate to give H⁺ ions and therefore does not change the colour of the dry litmus paper.

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

Ans- This is because the process of adding water to acid is an exothermic process. When we add water to acid a large amount of heat is produced. The heat generated may cause the mixture to splash and cause acid burns. The glass container may also break due to excessive heating.

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

Ans- When a solution of an acid is diluted, the concentration of hydronium ions decreases.

Q.6- How is the concentration of Hydroxide ions  (OH^–) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans- When excess of base is dissolved in a solution of sodium hydroxide, concentration of  OH^– ions increases. This happens to some extent, after that the concentration becomes constant.

TEXTUAL EXERCISE PAGE – 33

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?

Ans- In solution A, [H⁺] =  10^-6 M

In solution B, [H⁺] = 10^-8 M

The solution A has more hydrogen ion concentration.

The pH value of less than 7 indicates acidic solution and pH value of more than 7 indicates basic solution. Hence 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?

Ans- The nature of a solution depends upon the concentration of H⁺ ions.

If [H⁺] < 10^-7 M, the solution is basic.

If [H⁺] > 10^-7 M, the solution is acidic.

If [H⁺] = 10^-7 M, the solution is neutral.

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

Ans- Basic solutions also contain H⁺(aq) ions. But in basic solutions, the concentration of OH^– ions is more as compare to H⁺ ions. That’s why they are basic in nature.

Q.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- The farmer will treat the soil with quick lime or slaked lime or chalk if the pH of the soil is less than 7, means when it is acidic.

TEXTUAL EXERCISE PAGE – 33

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

Ans- The common name of the compound CaOCl₂ is bleaching powder.

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

Ans- The substance which on treatment with chlorine yields bleaching powder is slaked lime or calcium hydroxide.

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

Ans- The sodium compound which is used for softening hard water is sodium carbonate (washing soda).

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

Ans- If the solution of sodium hydrocarbonate is heated, it decomposes to give sodium carbonate, water and carbon dioxide gas.

NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

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

Ans- CaSO₄.1/2H₂O + 3/2H₂O → CaSO₄.2H₂O

TEXTUAL EXERCISE PAGE – 34

Q.1- A solution turns red litmus blue, its pH is likely to be :

(a) 1

(b) 4

(c) 5

(d) 10

Ans- (d) 10

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

Ans- (b) HCl

Q.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 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

Q.4- Which of the following types of medicines is used for treating ingestion.

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Anticeptic

Ans- (c) Antacid

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 sulphuric acid reacts with iron fillings.

Ans-

(a) Zinc granules + Dilute Sulphuric acid → Zinc Sulphate + Hydrogen

Zn + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

(b) Magnesium ribbon + Hydrochloric acid → Magnesium Chloride + Hydrogen

Mg + 2HCl(aq) → MgCl₂(aq) + H₂(g)

(c) Aluminium powder + Dilute Sulphuric acid → Aluminium Sulphate + Hydrogen

2Al (s)+ 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Iron fillings + Dilute Sulphuric acid → Iron Sulphate + Hydrogen

3Fe (s) + 6HCl(aq) → 3FeCl₂(aq) + 3H₂(g)

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

Ans- Fix two nails on a cork and place it in a 100 ml of beaker. Connect these nails to a 6 volt battery, bulb and a wire connected to switch. Pour some aqueous solution of alcohol or aqueous solution of glucose in the beaker, so that the nails dip in it. Turn the switch on. The does not glow indicating that alcohol and glucose don’t dissociate in aqueous solution and hence does not produce H⁺ ions, though they contain hydrogen.

Q.7- Why does distilled water does not conduct electricity, whereas rain water does?

Ans- Distilled water does not conduct electricity because it has no ionic compounds dissolved in it. It lacks ions which are responsible for the conduction of electricity.

While falling, rain water dissolves acidic gas, carbon dioxide and form carbonic acid. The carbonic acid gives hydrogen and carbonate ions to rain water. Hence rainwater conducts electricity.

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

Ans- This is because in the absence of water, acids do not dissociate to give H⁺ ions. The acids produce H⁺ ions only in the presence of water.

Q.9- Five columns 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-

Solution	pH	Nature of solution
A	4	Weakly acidic
B	1	Strongly acidic
C	11	Strongly alkaline
D	7	Neutral
E	9	Weakly alkaline

The pH in increasing order of hydrogen ion concentration is :

(C) pH 11 < (E) pH 9 < (D) pH 7 < (A) pH 4 < (B) pH 1

Q.10- Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid is added to the test tube A, while acetic acid is added to the test tube B. In which test tube will the fizzing occur more vigorously and why?

Ans- Fizzing occurs more vigorously in test tube A. This is because the concentration of hydrogen ion is more in test tube A as compare to test tube B, as Hydrochloric acid is a strong acid and acetic acid is a weak acid.

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

Ans- The pH of milk will decrease due to the production of lactic acid.

Q.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) A milkman adds a very small amount of baking soda to fresh milk so that it may not get spoil easily due to the formation of lactic acid.

(b) The milk take a long time to set as a curd because the lactic acid produced during curdling of milk is neutralised by the base.

Q.13- Plaster of Paris should be stored in a moisture proof container. Explain why?

Ans- Plaster of Paris should be stored in a moisture proof container because it absorbs water from moisture and sets to give hard substance called Gypsum.

Q.14- What is neutralisation reaction? Give two examples.

Ans- The reaction between acid and a base to form salt and water is called neutralisation reaction. Examples :

(i) HNO₃ + KOH → KNO₃ + H₂O

(ii) NaOH + HCl → NaCl + H₂O

Q.15- Give two uses of washing soda and baking soda.

Ans- Uses of washing soda :

(i) It is used to soften the hard water.

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

Uses of baking soda :

(i) It is used for making baking powder.

(ii) It is used as an antacid in medicines.

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