Intext Questions Solved

Page 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 :

  1. Add red litmus paper in each test tube. The one test tube changes to blue contains basic solution.
  2. Take the blue litmus obtained above and dip it in each test tube. If it changes to red, the test tube contains acidic solution.
  3. The third solution will not be affected by red litmus as well as blue litmus. It contains distilled water.

Page 22

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

Answer : It is because curd and sour substances contain acids which react with brass and copper vessel to form poisonous compounds.

Question 2 : Which gas is usually liberated when acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer : Hydrogen gas is liberated by reaction of acid with metal , e.g.,

Zn (s) + 2HCL (aq) ——–> ZnCl2 (aq) + H2 (g)

Test : Bring a matchstick near the gas. If it burns explosively with a ‘pop’ sound, the gas is hydrogen.

Question 3 : A 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 : 

The metal compound ‘A’ is CaCo3 (calcium carbonate).

CaCO3 + 2HCL (aq) ———–> CaCl2 + H2O + CO2 (g)

The gas evolved is carbon dioxide which extinguished a burning candle.

Page 25

Question 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?

Answer : It is because HCl, HNO3 form H+ and H3O+ in aqueous solution as their ionic compounds whereas alcohol and glucose are covalent compounds they do not form ions in aqueous solution, that is why, they do not show acidic properties in aqueous solution.

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

Answer : It is because aqueous solution of acid contains ions which can carry current.

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

Answer : It is because HCl (g) does not change into into in gaseous state. It changes into ion in aqueous solution.

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 : It is because reaction is highly exothermic and cannot be controlled if water is added to acid.

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

Answer : When acid is diluted, concentration of H3O+ decreases.

Question 6 : How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer : The concentration of OH- will increase.

Page 28

Question 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?

Answer : Solution ‘A’ has more concentration of (H+) . ‘A’ is acidic whereas ‘B’ is basic.

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

Answer : If concentration of solution is more than 10-7, the solution will be acidic. If it is less than 10-7, the solution will be basic. If it is 10-7, the solution will be neutral.

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

Answer : Basic solutions also have H+ ions. They are basic because [OH-] are more than [H+] concentration.

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 : If the soil is acidic, the farmer should treat the soil of his fields with quick lime, calcium hydroxide or chalk (CaCO3).

Page 33

Question 1 : What is the common name of the compound CaOCl2?

Answer : Bleaching powder.

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

Answer : Slaked lime [Ca(OH)2].

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

Answer : Sodium carbonate.

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

Answer : When sodium hydrogencarbonate is heated, sodium carbonate, carbon dioxide and water are formed.

2NaHCO3 (s) ——heat——> Na2CO3(s) + CO2 (g) + H2O (l)

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

Answer : 

Text Book Questions Solved

Question 1 : A solution turns red litmus blue, its pH is likely to be 

a) 1
b) 4
c) 5
d) 10

Answer : (d), because the solution is basic and bases have pH more than 7.

Question 2 : A solution reacts with crushed egg-shells to give a gas that turns limewater milky. The solution contains.

a) NaCl
b) HCl
c) LiCl
d) KCl

Answer : HCl reacts with CaCO3 present in egg-shells to liberate CO2 which turns limewater milky.

Question 3 : 10 ml 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 neutralised it will be

a) 4 ml
b) 8 ml
c) 12 ml
d) 16 ml

Answer : 10 ml of NaOH is neutralised by 8 ml.

20 ml of NaOH is neutralised by 8/10*20 = 16 ml.

Question 4 : Which of the following types of medicines is used for treating indigestion?

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

Answer : (a) Antacid is used to neutralise hyperacidity in stomach.

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 aluminium powder
d) dilute hydrochloric acid reacts with iron filings.

Answer : 

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

Answer : 

1. Take solution of glucose.

2. Fix two nails on a cork and place the cork in 100 ml beaker.





3. Connect the nails to the terminals of a battery through a bulb and a switch as shown in Figure given above.

4. Now switch on the current.

5. Repeat the experiment with solution of alcohol.

Observation : The bulb does not glow.

Conclusion : It shows that glucose and alcohol solutions do not conduct electricity in aqueous solutions, i.e., do not show acidic properties although they contain hydrogen.

Reasons : It is because they do not ionise in aqueous solution.

Question 7 : Why does distilled water not conduct electricity, whereas rain water does?

Answer : Distilled water does not ionise easily whereas rain water contains acids which help in ionisation, that is why it conducts electricity.

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

Answer : It is because they do not form ions in the absence of water.

Question 9 : 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.

Answer :

a) ‘D’ with pH is 7 neutral.
b) ‘C’ with pH 11 is strongly alkaline.
c) ‘B’ with pH 1 is strongly acidic.
d) ‘A’ with pH 4 is weakly acidic.
e) ‘E’ with pH 9 is weakly alkaline.

Question 10 : Equal lengths of magesium 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?

Answer : In test tube ‘A’ fizzling will occur more because HCl is stronger acid than acetic acid.

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 : pH will decrease when milk changes into curd.

Question 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?

Answer : a) It is because it cannot change into curd or does not get sour.
b) It is because enzyme which can change milk to curd does not work in alkaline pH.

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

Answer : It is because plaster of Paris will react with water and will become hard.

Question 14 : What is neutralisation reaction? Give two examples.

Answer : When an acid reacts with base to form salt and water, it is called neutralisation reaction, e.g.

NaOH + HCl ———–> NaCl + H2O

KOH + HNO3 ————-> KNO3 + H2O

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

Answer :

Washing soda
i) It is a component of washing poweders.
ii) It is used for softening hard water.

Baking soda
i) It is used in making biscuits, cakes, etc.
ii) It is used as antacid.