NCERT Solutions for class 10th Science Chapter 2 Acid, Bases and Salts

NCERT Intex 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?

Sol. Dip a piece of red litmus paper separately in each of the given test-tubes.
Note the changes in colour of litmus paper.
(i)The test-tube in which the colour of red litmus paper turns purple indicates neutral nature i.e. it contains water.
(ii)The test-tube in which the colour of red litmus paper turns blue indicates basic nature i.e. it contains the basic solution.
(iii)The test-tube in which the colour ofred litmus paper remains unchanged indicates acidic nature i.e. it contains acidic solution.

Question 2. Why should curd and sour substances not be kept in brass and copper vessels?

Sol. Sour substances generally contain an acid and are thus acidic in nature. Curd also contains an acid. The acid present in curd and sour substances is likely
to react with copper or brass vessels and form certain salts which are poisonous in nature (i.e. toxic compounds). To avoid any such possibility, it is advisable not to store curd and sour substances in brass and copper vessels.

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?

Sol.: •The gas usually liberated by the action of an acid with a metal is hydrogen (H2),

Test for the presence of gas: Take a few pieces of zinc granules and add 5 ml of dilute HzSO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas

We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

Question 4. 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

Sol.• A + HCl(dil)—->CaClz + gas
Since the gas is formed with effervescence and extinguishes a burning candle the gas is carbon dioxide.

•Balancing the equation, we get

Question 5. Why do HCl, HNO3 etc. show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Sol. HCl and HNO3 etc. dissociate in the presence of water to form hydrogen ions (H+). These hydrogen ions combine with H2O to form hydronium ions (H3O). The reaction can be given as follows:

Because of this property, HCl and HNO3 show an acidic character in aqueous solutions. On the other hand, alcohol and glucose cannot dissociate in water to form hydrogen ions. Hence, they do not show an acidic character.

Question 6. Why does an aqueous solution of an acid conduct electricity?

Sol. In an aqueous solution, an acid dissociates to produce H+ ions or H3o+ ions. Due to the presence of ions, the aqueous solution of an acid conducts electricity, e.g. HCL

Question 7. Why does dry HCl gas not change the colour of dry litmus paper?

Sol. The change in colour of litmus is observed only in the presence of H+ions (acidic nature) or OH-(basic nature).
HCl is acidic only when moist or aqueous. Since dry HCl gas fails to produce H+ions, so the colour of dry litmus paper does not change when it comes in contact with dry HCl gas.

Note: Acids produce H+ ions in aqueous solution

Question 8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Sol. The process of dissolving an acid or base in water is highly exothermic. The acids particularly mineral acids (HCl, HNO3, H2SO4 etc.) must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause bums. The glass container may also break due to excessive local heating.

Question 9. How is the concentration of hydronium ions (H30+) affected when a solution of an acid is diluted?

Sol. The concentration of hydronium ions (H3O+) decreases when solution of an acid is diluted with water.

Note: Mixing of an acid or base with water results in a decrease in the concentration of ions (H30+/OH-) per unit volume. Such a process is called dilution and acid or base is
said to be diluted.

Question 10. How is concentration of hydroxyl ions (OH) affected when excess of base is dissolved in a solution of sodium hydroxide?

Sol. Sodium hydroxide (NaOH) is a strong base (i.e. it is completely ionised in solution). When NaOH is dissolved, more OH- will be produced and thus there will be an increase in the concentration of OH ions.

Question 11. 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 them is acidic and which one is basic?

Sol. The pH of a solution is inversely proportional to its H+ ion concentration (i.e. lesser the pH more will be H+ ion concentration). Thus, the solution A with pH 6 has more H+ion concentration than the solution B with pH 8.
For an acidic solution, the pH lies between Oto 7 and for a basic solution the pH range is 7-14. Thus, A with pH 6 is acidic and B with pH 8 is basic.

Question 12. What effect does concentration of H+(aq) ions have on the nature of a solution?

Sol. The acidic nature of a solution is directly related to the concentration of H+(aq) ions in solution. More the concentration of H+ (aq) ions in solution more acidic will be the solution.
Thus, if the concentration of H+(aq) ions increases in a solution, the solution becomes more acidic in nature and if the concentration of H+ (aq) ions in a solution decreases it becomes less acidic in nature.

Question 13. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?

Sol. • Yes, basic solution also have H+ (aq) ions present in them.
•Actually these solutions are prepared in water. Water is a weak electrolyte (slightly ionised) and produces H+ and OH- ions.

However, the number of H+ ions released by water is small in comparison to OH- ions released by a base and also by water. Because of this (number of OH-> number of H+) the solution is basic in nature.

Question 14. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Sol. The farmer will treat the soil of his fields with quick lime (a basic oxide) slaked lime (basic hydroxide) or chalk (a metal carbonate) if the soil is acidic. An acidic soil will react with this basic substance and a neutralisation reaction will occur. In this way, the acidic nature of the soil will be neutralised.

Question 15. What is the common name of the compound CaOCl2?

Sol. The common name of this compound is bleaching powder.

Note: CaOCl2 or Calcium hypochlorite is an inorganic compound. It is specifically a mixture of lime and calcium chloride and the common name for the compound is bleaching powder or chlorine powder which finds extensive use in water treatment plants.

Question 16. Name the substance which on treating with chlorine yields bleaching powder.

Sol. When slaked lime or calcium hydroxide [Ca(OH)2] is treated with chlorine gas, it yields bleaching powder. It is shown in the given chemical equation:

Question 17. Name the sodium compound which is used for softening hard water.

Sol. Washing Soda (Na2CO3.l0H2O) is hydrated sodium carbonate which is used for softening hard water.

Question 18. What will happen if the solution of sodium hydrogen carbonate is heated? Write the chemical equation involved.

Sol. A solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide will be evolved.

Question 19. Write the chemical equation for the reaction between the plaster of paris and water.

Sol. When plaster of paris reacts with water, it produces gypsum as shown in the following chemical reaction:

Excersice:

Question 1. A solution turns red litmus blue. Its pH is likely to be
(a) 2 (b) 4 (c)5 (d) 10

Sol. (d) The red litmus solution turns blue in basic solution. The pH of the basic solution is more than 7. Hence it is likely to be 10.

Question2. A solution reacts with crushed eggshells to give a gas that turns lime water milky. The solution contains.
(a)NaCl (b)HCI (c)LiCl (d)KCI

Sol.(b)Since the gas produced turns lime water milky the gas is CO2. The egg­shell is made of CaCO3.CaCO3 reacts with an acid (dil. HCl) to produce CO2.

Question 3. 10 ml of a solution of NaOH is found to be completely neutralised by 8 ml of a given solution of HCI. If we take 20 ml of the same solution ofNaOH, the amount ofHCI solution (the same solution as before) required to neutralise will be
(a) 4 ml (b) 8 ml (c) 12 ml (d) 16 ml

Question 4. Which of the following type of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Sol. (c) Due to over-eating, our stomach produce more acid, which generally
results in indigestion. Thus, to neutralise excess acid produced by stomach, antacid is taken as medicine

Question 5. Write word equations and the 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.

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

Sol. From their chemical formula it is evident that both alcohol (C2H5OH) and glucose (C6H12O6) contain hydrogen. But in their aqueous solution they do not produce any H+ ion so they are not categorised as acids.
Materials required: Beaker, cork, nails (iron), battery (6V), connecting wires, bulb, switch, ethyl alcohol or glucose.
Procedure:
(i) Set up the apparatus as shown in the figure.

(ii) Take ethyl alcohol/glucose solution in the beaker such that the upper ends of the nails remain slightly above the level of the liquid.
(iii) Switch on and observe if the bulb glows (If bulb glows then the solution is conducting i.e., it contains H+ ion otherwise not).
(iv) Repeat it with the other solution.
Conclusion: In both the cases we find that the bulb does not glow. It proves that there are no H+ ion in solution so the substance is not acidic

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

Sol.•Distilled water is pure water and it contains no H+ ions. Due to absence of H+ ion (does not ionise), it does not conduct electricity.
Rain water contains small amount of some dissolved acids like carbonic acid (H2CO3) and sulphurous acid (H2SO3). Because of presence of these acids, the rain water becomes acidic and ionises easily to produce ions (H+ ion). Thus, rain water conducts electricity.

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

Sol. The ionisation ofan acid occurs only in an aqueous solution (i.e., in presence of water). Thus, an acid releases H+ ions only in presence of water, so it shows acidic behaviour in presence of water.

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 acidic?
(e) weakly acidic?
Arrange the pH in increasing order of hydrogen-ion concentration.

Sol. (a) Neutral: Solution D
The pH of a neutral solution is 7. Thus, the neutral solution is D with pH=7.
(b) Strongly alkaline: Solution C
The pH value of greater than 7 indicates the basic solution. Higher pH value represents strong alkaline solution. Thus, strong alkaline solution is C with pH= 11.
(c) Strongly acidic: Solution B
The pH value ofless than 7 indicates the acidic solution. Lower pH value represents a strong acidic solution. Thus, a strong acidic solution is B with pH= 1.
(d) Weakly acidic: Solution A
Solution A is considered as weakly acidic because it has pH= 4.
(e) Weakly alkaline: Solution E
Solution E is considered as weakly alkaline because it has pH= 9.
•pH in ascending order o H+ ion concentration: pH scale quantifies the concentration of hydrogen ion in a solution. The pH value increases with a decrease in hydrogen ion concentration. The solution having pH 11 will have the minimum hydrogen ion concentration than the solution having pH 1. Thus, the arrangement of solutions in the increasing order of their hydrogen ion concentration as follows:
C<E<D<A<B
(pH= 11)<(pH= 9)<(pH= 7)<(pH =4)<(pH= 1)

Question 10. Equal lengths of magnesium ribbons are taken in test tube A and B.
Hydrochloric acid (HCI) is added totest tube Awhile acetic acid (CH3COOH) is added to testtube B. In which test tube will the fizzing occur more vigorously
and why?

Sol. In both cases, hydrogen gas evolved. When a metal reacts with acid, it forms salt and liberates hydrogen gas.

Fizzing will occur more vigorously in test tube A containing hydrochloric acid. This is because hydrochloric acid is stronger acid than acetic acid so reaction between magnesium ribbon and HCl is faster than the reaction between Mg and acetic acid

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

Sol. Fresh milk has a pH 6. During curdling, the lactobacillus bacteria keeps growing and it breaks down the lactose (the natural sugar in milk) into lactic acid which acidifies the milk even further. Therefore, the pH of the milk will decrease. More acidic the substance, lower is the pH, so it becomes more acidic

Question 12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?

Sol. (a)We know that the pH of fresh milk is 6, thus it is slightly acidic due to the presence of lactic acid in it. Upon standing, its pH decreases slowly and it becomes more acidic because more lactic acid is released. Milkman adds a very small amount of baking soda (NaHC03) to fresh milk to make the medium slightly alkaline. The base released by baking soda will neutralise the effect of lactic acid present in milk and helps to check the milk getting sour.
(b) When milk sets as curd, the pH decreases so it becomes more acidic. In
case of alkaline medium due to addition of NaHC03 it will take 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?

Sol. In the presence of moisture, plaster of paris gets hydrated and changes to gypsum which is a hard mass.

Plaster of paris converted to gypsum can no longer be used for making moulds and statues. To avoid the above reaction from occurring, the plaster of paris is stored in moisture-proof containers.

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

Sol. A reaction which occurs between an acid and a base to produce salt and water is known as neutralisation reaction.

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

Sol. Uses of washing soda
(i) Used as a cleansing agent for domestic purposes.
(ii) Used in manufacture of soap, paper, chemicals such as NaOH, Borax etc.
Uses of baking soda
(i) Used in baking powder.
(ii) Used in antacids

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