Class 10 Science Chapter 2 Acids Bases and Salts

NCERT Solutions For Class 10 Science Chapter 2 Acids Bases and Salts. Question Answer.

  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.
Take a small volume of all three liquids in three different test tubes. Dip red litmus paper strips separatelyin all the three test tubes. The tube in which red litmus strip turns blue, contains a basic solution. Now, we use the blue litmus paper as testing paper and dip it into the remaining two solutions. The solution which changes the colour of the blue litmus paper into red is acidic and the other which does not affect it, is neutral, i.e. distilled water.

Question 2.
Why should curd and other sour substances not be kept in containers made up of brass or copper?
Answer.
Both curd and other sour substances contain some acids in them. They react with copper or brass vessels to form certain salts which are not good for health. Therefore, it is not advisable to keep them in brass or copper containers.

Question 3.
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.
Metals are mostly reactive in nature. They react with dilute acids (HCl and H 2SO 4) to evolve hydrogen gas. For example,
Zn(s) + 2HCl(aq) ➝ ZnCl 2(aq) + H 2(g)
The gas burns with a pop sound when a burning candle is brought near to it.

Question 4.
A metallic 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 gas evolved with effervescence and extinguishes a lit candle is CO 2. If one of the compounds formed is CaCl 2, the reaction would be
CaCO 3(s) + HCl(aq) ➝ CaCl 2(aq) + CO 2 + H 2O(l)

Question 5.
Aqueous solutions of HCl, HNO 3 and H 2SO 4, etc. show acidic character while those of the compounds like ethyl alcohol (C 2H 5OH) and glucose (C 6H 12O 6) fail to do so. Explain.
Answer.
All the listed acids have replaceable hydrogen atoms which they release in aqueous solution as hydrogen ions. Therefore, they show acidic character. However, both ethyl alcohol (C 2H 5OH) and glucose (C 6H 12O 6) do not undergo dissociation in aqueous solution. That’s why they do not conduct electricity in aqueous solution.

Question 6.
Why does an aqueous solution of an acid conduct electricity?
Answer.
Aqueous solution of an acid releases H + and H 3O + in solution. Since ions are carriers of charge, therefore they are responsible for conducting electricity.

Question 7.
Why does dry HCl gas not change the colour of dry litmus paper?
Answer.
A dry HCl gas has no H + so it does not show any acidic character therefore, no change in colour takes place until we moisten the litmus paper.

Question 8.
While diluting an acid, why is it recommended to add acid to water and not water to the acid?
Answer.
Mineral acids such as, H 2SO 4, HNO 3, HCl, etc. have strong affinity for water, so dilution of acid is highly exothermic in nature. This heat may cause jumping of solution or cracking of apparatus. In order to avoid it, acid is added drop by drop to water which dilutes the heat and prevent accident.

Question 9.
How is concentration of hydronium ions (H 3O +) affected when solution of an acid is diluted?
Answer.
An acid dissociates into hydronium ions (H 3O+) and anions when dissolved in water. Upon dilution, the volume of the solution increases and the number of ions per unit volume decreases. Therefore, the concentration of H 3O + per unit volume decreases.

Question 10.
How is concentration of hydroxyl (OH ) ions affected when excess of base is dissolved in solution of sodium hydroxide?
Answer.
Sodium hydroxide (NaOH) is a strong base. It immediately dissociates in solution to give OH  and cations. Upon dissolving more of the base in the solution, the concentration of OH  further increases.

Question 11.
You have two solutions A and B. The pH of solution A is 6 and that of solution B is 8. Which solution has more hydrogen ion concentration? Which of these is acidic and which one is basic?
Answer.
The pH of a solution is inversely proportional to the concentration of H + in solution. Lesser the pH of the solution, more will be the H + concentration. The solution A with pH 6 has more H + concentration than the solution with pH equal to 8. The solution A is acidic because its pH is less than 7 and the solution B is basic because its pH is more than 7.

Question 12.
What effect does concentration of H +(aq) have on acidic nature of a solution?
Answer.
The acidic nature of a solution is directly related to the concentration of H +. As the concentration of H + increases, the acidic nature of the solution also increases.

Question 13.
Do basic solutions also have H +(aq)? If yes, then why are these basic?
Answer.
Yes, basic solutions also have H +. As the solutions are prepared in water and water being a weak electrolyte, it dissociates into H + and OH  but the number of H + are very small as compared to OH  ions.

Question 14.
Under what soil conditions, do you think a farmer would spread or treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Answer.
A soil usually becomes acidic when there is either a high peat content or, iron minerals or there are some rotten vegetables in the soil. In order to reduce the acidic strength, ‘liming of soil’ is usually done. For this, any of the substances that have been mentioned are added to the soil since they are of basic nature.

Question 15.
Name the substance which upon treating with chlorine gives bleaching powder. Write the chemical equation for the reaction.
Answer.
Slaked lime is the substance which reacts with chlorine to give bleaching powder.
NCERT Solutions for Class 10 Science Chapter 2 Acids Bases and Salts 1

Question 16.
Name the sodium compound used for softening hard water.
Answer.
Washing soda or sodium carbonate decahydrate
(Na 2CO 3.10H 2O)

Question 17.
What will happen if the solution of sodium hydrogen carbonate is heated? Write the chemical equation involved.
Answer.
Carbon dioxide gas will evolve and sodium car-bonate will be formed.
2NaHCO 3 ➝ Na 2CO 3 + CO 2 + H 2O

Question 18.
Write the chemical equation for the reaction between Plaster of Paris and water.
Answer.
NCERT Solutions for Class 10 Science Chapter 2 Acids Bases and Salts 2

Question 19.
What is the common name of the compound CaOCl 2?
Answer.
The common name of the compound CaOCl 2 is bleaching powder.

Chapter End Questions

Question 1.
A solution turns red litmus blue. Its pH is likely to be
(a) 2
(b) 4
(c) 7
(d) 10
Answer.
(d) 10

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) HCl

Question 3.
10 mL of 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 will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Answer.
(d) 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.
(c) Antacid

Question 5.
Write the word equation and the balanced equations for the reactions 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.
(a) Word equation:
Zinc + Sulphuric add ➝ Zinc sulphate + Hydrogen
Balanced equation:
Zn(s) + H 2SO 4(dil.) ➝ ZnSO 4(aq) + H 2(g)
(b) Word equation:
Magnesium + Hydrochloric acid ➝ Magnesium chloride + Hydrogen
Balanced equation:
Mg(s) + 2HCl (dil.) ➝ MgCl 2(aq) + H 2(g)
(c) Word equation:
Aluminium + Sulphuric acid ➝ Aluminium sulphate + Hydrogen
Balanced equation:
2Al(s) + 3H 2SO 4(dil.) ➝ Al 2(SO 4) 3(aq) + 3H 2(g)
(d) Word equation:
Iron + Hydrochloric acid ➝ Iron chloride + Hydrogen
Balanced equation:
Fe(s) + 2HCl(dil.) ➝ FeCl 2(aq) + H 2(g)

Question 6.
Compounds such as alcohol and glucose also contain hydrogen but are not characterised as
acids. Describe an activity to prove it.
Answer.
Compounds such as alcohol and glucose also contain hydrogen but do not behave like an acid. Both are organic compounds with the formulae C 2H 5OH and C 6H 12O 6, respectively. This can be proved by the following activity:
In a glass beaker, take a dilute solution of glucose (C 6H 12O 6). Fix two small nails of iron in a rubber cork and place the cork in the beaker as shown in the figure. Connect the nails to the terminals of a 6 volt battery through a bulb. Switch on the current. The bulb will not glow. This shows that the electric current has not passed through the glucose solution. As the current is carried by the movement of ions, it shows that the solution of

NCERT Solutions for Class 10 Science Chapter 2 Acids Bases and Salts 3

glucose has not given any H +. Now repeat the same experiment with ethyl alcohol (C 2H 5OH). The bulb will again not glow.
This shows that both of them do not behave as acids although they contain hydrogen atoms in their molecules.

Question 7.
Why does distilled water not conduct electricity whereas rain water does?
Answer.
Pure or distilled water has no ions as it is a very weak electrolyte. So, no conduction of electricity takes place but rain water contains dissolved acids and so rain water is a good conductor of electricity.

Question 8.
Why does an acid not show any acidic behaviour in the absence of water?
Answer.
The acidic behaviour of a substance is due to the presence of H +(aq) ions. As acids do not dissociate to produce H +(aq) ions in the absence of water so they do not show acidic behaviour.

Question 9.
Five solutions A, B, C, D and E when tested with universal indicator show pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
(a) neutral
(b) strongly alkaline
(c) strongly acidic
(d) weakly alkaline
(e) weakly acidic
Arrange the pH in increasing order of H + concentration.
Answer.
(a) Neutral: D with pH = 7
(b) Strongly alkaline: C with pH = 11
(c) Strongly acidic: B with pH = 1
(d) Weakly alkaline: E with pH = 9
(e) Weakly acidic: A with pH = 4
Increasing order of H + concentration:
C<E<D<A<B

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 (CH 3COOH) is added to test tube B. In which case, fizzing occurs more vigorously and why? Fizzing in the reaction is due to the evolution of hydrogen gas by the action of metal on the acid.
Mg(s) + 2HCl(aq) ➝ MgCl2(aq) + H 2(g)
(A)
Mg(s) + 2CH 3COOH(aq) ➝ (CH 3COO) 2Mg(aq) + H 2(g)
(B)
Since hydrochloric acid is a stronger acid than acetic acid, fizzing occurs more readily in tube A than in tube B. Actually hydrogen gas will evolve at more brisk speed 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.
When milk changes into curd, the pH decreases. Lactose (carbohydrate) present in milk gets converted into lactic acid. As more acid is formed, pH of the medium decreases.

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 milk take a long time to set as curd?
Answer.
(a) Fresh milk is slightly acidic due to the presence of lactic acid. The presence of bacteria decreases the pH of milk and makes it sour. To prevent it, baking soda (NaHCO 3) is added to neutralise the acidic nature making it slightly alkaline.
(b) When milk changes to curd, it becomes more acidic but adding baking soda neutralises it and checks curdling.

Question 13.
Why should Plaster of Paris be stored in a moisture-proof container?
Answer.
In the presence of moisture, Plaster of Paris gets hydrated and changes to gypsum which is a hard mass.
NCERT Solutions for Class 10 Science Chapter 2 Acids Bases and Salts 4

It can no longer be used for making moulds and statues. Therefore, Plaster of Paris is kept in moisture proof containers or bags.

Question 14.
What is neutralisation reaction? Give two examples.
Answer.
Neutralisation reaction is the reaction between an acid and a base dissolved in aqueous solution to form salt and water.
Acid + Base ➝ Salt + Water
HCl(aq) + NaOH(aq) ➝ NaCl(aq) + H 2O(l)
HNO 3(aq) + KOH(aq) ➝ KNO 3(aq) + H 2O(l)
Both NaCl and KNO 3 are neutral in nature. They neither change blue litmus red nor red litmus blue. That is why the reaction is called neutralisation reaction.

Question 15.
Give two important uses of washing soda and baking soda.
Answer.
•  Uses of Baking Soda: It is used:

  • as an antacid to neutralise excess acidity in the stomach.
  •  as an ingredient in certain food stuffs such as bread, cake, etc.
  •  in soda-acid fire extinguishers.
  •  in making aerated soft drinks.
  •  to produce carbon dioxide

•  Uses of Washing Soda: It is used:

  •  as a domestic cleaning agent.
  •  for removing permanent hardness of water.
  •  as a laboratory reagent.
  •  in the manufacture of sodium compounds such as borax.
  •  in the manufacture of glass, paper and soap.

NCERT Solutions