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

NCERT TEXTBOOK QUESTIONS

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 the three liquids in three test tubes. Dip red litmus paper strips separately in all the three. The tube in which red litmus strip turns blue, contains basic solution. Now remove the blue litmus paper and dip it one of the remaining test tubes. If the colour of the blue litmus paper changes to red, the tube contains acidic solution. In case, it remains blue then the tube contains distilled water.

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

Question 3. Which gas is usually liberated when an acid reacts with a metal ? Illustrate with an example. How will you test the presence of this gas ?
Answer: Metals are mostly reactive in nature. They react with dilute acids (HCl and H2SO4) to evolve hydrogen gas. For example,

The gas burns with a pop sound when a burning candle is brought near 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: Since the gas is evolved with effervescence and extinguishes fire, it is expected to be CO2 gas. As calcium chloride is formed as one of the products, this means that the substance A’ can be calcium carbonate. It reacts with dilute hydrochloric acid as :

Question 5. Aqueous solutions of HCl, HNO3 and H2SO4 etc. show acidic character while those of the compounds like ethyl alcohol (C2H5OH) and glucose (C6H12O6) fail to do so. Explain.
Answer: All the acids that are listed, have replaceable hydrogen atoms which they release in aqueous solution as H+ ions. Therefore, they show acidic character. However, both ethyl alcohol (C2H5OH) and glucose (C6H12O6) do not have replaceable hydrogen atoms. They fail to evolve hydrogen gas and do not show any acidic character.

Question 6. Why does aqueous solution of an acid (HA) conduct electricity ?
Answer: Aqueous solution of an acid (HA) releases H+ ions or H3O+ ions and anions (A–) in solution. Since ions are the carrier of charge, the aqueous solution of an acid conducts electricity.

Question 7. Why does not dry HCl gas change the colour of the dry litmus paper ? (CBSE 2013)
Answer: Dry HCl gas fails to release any H+ ions which means that it is not acidic. It fails to change the colour of the dry litmus paper which has also no moisture present.

Question 8. While diluting an acid, why is it not recommended that acid should be added to water and not water to the acid ? (CBSE 2011)
Answer: Acids particularly the mineral acids like H2SO4, HNO3 and HCl etc., have strong affinity for water. The dilution process is highly exothermic in nature. The heat evolved may crack or break the container and may also convert the acid into fog which is likely to pollute the atmosphere. In order to control the heat evolved, it is advisable to add acid drop by drop to water. In case water is added to acid, then the entire acid will get itself involved in the exothermic process. It may not be possible to control the heat evolved.

Question 9. How is concentration of hydronium ions (H3O+) affected when solution of an acid is diluted with water ?
Answer: An acid dissociates into hydronium ions (H3O+) 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 H3O+ ions 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– ions and cations. Upon dissolving more of base in the solution, the concentration of OH– ions 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+ ions in solution. Lesser the pH of the solution, more will be the H+ ion concentration. The solution A with pH 6 has more H+ ion 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) ions have on acidic nature of absolution ?
Answer: The acidic nature of a solution is directly related to the concentration of H+ ions. As the concentration of H+ ions increases, the acidic nature of solution also increases.

Question 13. Do basic solutions also have H+(aq) ions ? If yes, then why are these basic ?
Answer: Yes, basic solutions have also H+(aq) ions present in them. Actually, these solutions are prepared in water. Being a weak electrolyte, it dissociates to give H+ and OH– ions. However, the number of H+ ions is very small as compared to the number of OH– ions which are released by the base and also by water. Therefore, the solutions as a whole are of basic nature.

Question 14. Under what soil conditions, do you think a farmer would spread or treat the soil of his fields 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, iron minerals or there is some rotting vegetable. 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 these are of basic nature.

Question 15. Name the substance which upon treating with chlorine gives bleaching powder. Write the chemical equation for the reaction. (CBSE 2011)
Answer: Slaked lime is the substance which reacts with chlorine to give bleaching powder

Question 16. Name the sodium compound used for softening hard water.
Answer: Washing soda or sodium carbonate. It is chemically sodium carbonate decahydrate (Na2CO3.10H2O). What will happen if the solution of sodium hydrogen carbonate is heated ?

Question 17. Write the chemical equation involved.
Answer: Carbon dioxide gas will evolve and sodium carbonate will be left.

Question 18. Write the chemical equation for the reaction between Plaster of Paris and water.
Answer:

NCERT EXERCISE

Question 1. A solution turns red litmus blue. Its pH is likely to be
(a) 2
(b) 4
(c) 7
(d) 10.
Answer: The solution is basic. Its pH is likely to be 10.
Therefore, (d) is the correct answer.

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: The crushed egg-shells consist of layer of calcium carbonate which reacts with dilute HCl to evolve CO2(g). The gas turns lime water milky.
Therefore (b) is the correct choice.

Question 3. 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 volume 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: 10 mL of NaOH will require HCl = 8 mL and 20 mL of NaOH will require HCl=16 mL.
Therefore, (d) is the correct answer.

Question 4. Which of the following types of medicines is used for treating indigestion ?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic.
Answer: Antacid is used for treating indigestion.
The correct answer is (c).

Question 5. Write the word equations 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 fillings.
(d) dilute hydrochloric acid reacts with iron fillings.
Answer:

Question 6. Compounds like alcohol and glucose also contain hydrogen but are not characterised as acids. Describe an activity to prove it.
The chemical formula of ethyl alcohol is C2H5OH which is an alcohol and of glucose is C6H12O6. Both are organic compounds and contain hydrogen atoms. However, they do not behave as acids. (CBSE 2011, 2013)
Answer: This can be shown by the following activity :
In a glass beaker, take a dilute solution of glucose (C6H12O6). 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, this shows that in solution, glucose has not given any H+ ions.

Now repeat the same experiment with ethyl alcohol (C2H5OH). The bulb will not glow in this case also. This shows that both of them do not behave as acids although they contain hydrogen atoms in their molecules.

Question 7. Why does not distilled water conduct electricity whereas rain water does ?
Answer: Pure water (or distilled water) is a very weak electrolyte and does not dissociate into ions. Therefore, it does not conduct electricity. However, rain water contains some dissolved acids like carbonic acid (H2CO3) and sulphurous acid (H2SO3). Actually air contains traces of both CO2 and SO2 gases which dissolve in rain water to produce corresponding acids. As a result, water becomes acidulated and gets ionised easily. Therefore, rain water conducts electricity.

Question 8. Why does not an acid show any acidic behaviour in the absence of water ?
Answer: An acid gets ionized only in aqueous solution i.e. in the presence of water. In other words, an acid releases H+ ions or shows acidic behaviour only in the presence of water.

Question 9. Five solutions A, B, C, D and E when tested with universal indicator show pH as 4, 2, 12, 7 and 9 respectively. Which solution is :
(a) neutral
(b) strongly alkaline
(c) strongly acidic
(d) weakly alkaline
(e) weakly acidic
(f) Arrange the pH in increasing order of H+ ion concentration.
Answer:
(a) Neutral : D with pH = 7
(b) Strongly alkaline : C with pH = 12
(c) Strongly acidic : B with pH =2
(d) Weakly alkaline : E with pH = 9
(e) Weakly acidic : A with pH = 4
(f) Increasing order of H+ ions 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 (CH3COOH) is added to test tube B. In which case, fizzing occurs more vigorously and why ?
Answer: Fizzing in the reaction is due to the evolution of hydrogen gas by the action of metal on the acid

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. (CBSE 2011)
Answer: When milk changes into curd, the pH decreases. Actually, lactose (carbohydrate) present in milk gets converted into lactic acid. As more of 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 this milk take a long time to set as curd ? (CBSE 2011)
Answer:
(a) We know that fesh milk is slightly acidic due to presence of lactic acid and its pH is 6. Upon
standing, its pH slowly decreases and it becomes sour since more of acid to released. The purpose of adding baking soda or sodium hydrogen carbonate (NaHCO3) is to make medium slighly alkaline. The base released will neutralise the effect of lactic acid present in milk. This will check the milk from getting sour.
(b) When milk sets as curd, it becomes more acidic and pH decreases. In the alkaline medium, it takes longer time to achieve acidic medium back so that milk may set as curd.

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.

It can be 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 acid and base dissolved in aqueous solution to form salt and water.

Question 15. Both NaCl and KNO3 are neutral in nature. They neither change blue litmus red nor red litmus blue. That is why the reaction is called neutralisation reaction.
Give two important uses of washing soda and baking soda. (CBSE 2011)
Answer:
Uses of washing soda:
1. In the manufacture of glass, soap, paper and chemicals like caustic soda (NaOH) and borax (Na2B4O7) etc.
2. As a cleansing agent for domestic purposes.
Uses of baking soda:
1. In baking powder used for preparing cakes.
2. In antacids to reduce acidity in the stomach.