## General Science

### Chapter 2:- Acids, Bases, and Salts

Textual question:–

Q1.   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?

Ans:-  When we add a few drops of solution from all three test tubes on the red litmus paper separately. The solution which turns red litmus to blue contains a basic solution. The blue litmus so formed is now dipped in other two test tubes. The solution from the test tube which turns blue litmus paper to red will be the acidic solution and solution of the test tube which do not change either red or blue litmus paper contain water.

Q2.   Why should curd and sour substances not be kept in brass and copper vessels?

Ans:-  Curd and sour substances is acidic in nature which can react with copper vessels and brass to form toxic compounds.

Q3.   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 a metal, hydrogen gas is liberated.
E.g., Zn(s) + 2HCl(aq) → ZnCl2 (aq) + H2(g)↑
For proving the presence of H2 gas, bring a burning candle near the soap bubble filled with H2, it will burn with a pop sound.

Q4.   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.

Ans:- Metal compound A is calcium carbonate which reacts with HCl to give carbon dioxide and calcium chloride.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

Q5.   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:-  Solutions like HCl, HNO3 etc. get ionized in aqueous solutions and due to the presence of  $H_{3}O^{+}$ ions, they show acidic characters.

$HCl\;+H_{2}O\rightleftharpoons&space;\;H_{3}O^{+}\;+Cl^{-}$

$HNO_{3}\;+H_{2}O\;\rightleftharpoons&space;\;H_{3}O^{+}\;+\;NO^{-}$

While solutions of compounds like alcohol and glucose do not form any such ions so they do not show acidic characters.

Q6.   Why does an aqueous solution of acid conduct electricity?

Ans:- An aqueous solution of an acid ionises to produce $H^{+}$ ions, and due to the presence of these ions electricity is conducted through it.

Q7.   Why does dry HCl gas not change the colour of the dry litmus paper?

Ans:- Dry HCl gas does not release H+ ions and hence, the color of the dry litmus paper remain unchanged.

Q8.   While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans:-  When water is added to the acid, the reaction is highly exothermic, the acid may splash and the container may break down. To avoid this, acid is added to water slowly for dilution process.

Q9.   How is the concentration of hydronium ions (H30+) affected when a solution of an acid is diluted?

Ans:- when a solution of an acid is diluted, $H_{3}O^{+}$ ions per unit volume decreases, therefore concentration decreases.

Q10.   How is the concentration of hydroxide ions (OH– ) affected when the excess base is dissolved in a solution of sodium hydroxide?

Ans:- The concentration of hydroxide ions (OH– ) increase when the excess base is dissolved in a solution of sodium hydroxide.

Q11.   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:-  A with pH = 6 is acidic and so it has more hydrogen ion concentration, B with pH = 8 is basic.

Q12.   What effect does the concentration of H+ (aq) ions, have on the nature of the solution?

Ans:-  If a solution has a higher concentration of H+ ions it is more acidic in nature.

Q13.   Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Ans. Yes, basic solutions also have H+ ions, but they are basic in nature due to more number of OH– ions. Therefore the $p^{H}$ value is greater than 7.

Q14.   Under what soil condition do you think a farmer would treat the soil of his fields with quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Ans. When the soil is acidic in nature, the farmer would add quicklime (CaO) or slaked lime (Ca(OH)2) or chalk (CaCO3) to make it neutral.
Q15.   What is the common name of the compound CaOCl2?
Ans. The common name of CaOCl2 is bleaching powder.
Q16.   Name the substance which on treatment with chlorine yields bleaching powder.
Ans. Slacked lime or Calcium hydroxide /Ca(OH)2 when treated with chlorine yields bleaching powder.
$Ca(OH)_{2}&space;+&space;Cl_{2}&space;\rightarrow&space;CaOCl_{2}&space;+&space;H_{2}O$
Q17.   Name the sodium compound which is used for softening hard water.
Ans. Sodium carbonate⇒$NO_{2}CO_{3}$
Q18.   What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.
Ans. When sodium hydrogen carbonate is heated, sodium carbonate and carbon dioxide gas are obtained.
$2NaHCO_{3}\overset{Heat}{\rightarrow}Na_{2}CO_{3}+H_{2}O+CO_{2}$
Q19.   Write an equation to show the reaction between Plaster of Paris and water.
Ans:- $CaSO_{4}.\frac{1}{2}H_{2}O+1\frac{1}{2}H_{2}O\rightarrow&space;CaSO_{4}.2H_{2}O$
EXERCISES:-
Q1.   A solution turns red litmus blue, its pH is likely to be
(a) 1           (b) 4
(c) 5           (d) 10
Ans. (d) 10
Q2.   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
Q3.   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 HCl solution (the same solution as before) required to neutralize it will be
(a) 4 ml           (b) 8 ml
(c) 12 ml           (d) 16 ml
Ans. (d) 4 ml
Q4.   Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic           (b) Analgesic           (c) Antacid           (d) Antiseptic
Ans. (c) Antacid.
Q5.   Write word equations and then balance 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 aluminum powder,
(d) dilute hydrochloric acid reacts with ions filings.
Ans. (a) Zinc granule + dil. Hydrochloric acid → Zinc chloride + Hydrogen gas
Zn(s) + 2HC1 (aq) → ZnCl2(aq) + H2(g)
(b)  Magnesium+dil. Hydrochloric acid → Magnesium chloride + Hydrogen gas
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(c) Aluminium + dil. Sulphuric acid → Aluminium sulphate + Hydrogen gas
2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g)
(d)  Iron + dil. Hydrochloric acid → Iron chloride + Hydrogen
2Fe(s) + 3HCl(aq) → Fe2Cl3(aq) + 3H2(g)
Q6.   Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Ans. •  Take a cork with two nails fixed on it.
•  Keep this cork inside the beaker.
•  Connect the nails to the battery, bulb, and key as shown in figure 2.3(textbook page no-22).
•  Now add alcohol solution in it and record your observation, repeat the same set up for glucose and record your observations.
Observation: The bulb will not glow because the charge is not flowing through it.
Conclusion: The experiment shows that glucose and ethanol do not ionize, H+ ions are not released therefore, they are not categorized as acids.
Q7.   Why does distilled water not conduct electricity, whereas rain water does?
Ans. Distilled water is pure water and it does not form ions. So electricity is not conducted through it.
Whereas rainwater contains impurities in it like acid which contains ions and release them when dissolved in water. Hence electricity is conducted.
Q8.   Why do acids not show acidic behavior in the absence of water?
Ans:- Acids cannot release H+ ions in absence of water, only on dissolving in water acids release H+ ions. Therefore acids can not show acidic behavior in the absence of water because it is unable to ionize.
Q9.   Five solutions A, B, C, D, and E, when tested with a 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          (a) ‘D’ with pH = 7 is 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
(iii) The arrangement of pH in increasing order of hydrogen-ion concentration is 11<9<7<4<1.
Q10.   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. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Ans. In the test tube, A hydrochloric acid is present which is a strong acid as compared to acetic acid present in test tube B. hence the fizzing occurs more vigorously in test tube A.
Q11.   Fresh milk has pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans. pH of milk will decrease when it turns into curd due to the formation of lactic acid.
Q12.   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) pH is shifted to alkaline so that milk does not spoil by becoming sour by releasing lactic acid.
(b) Milk is made alkaline by adding baking soda and the lactic acid formed, will get neutralized and therefore will take a little more time to become acidic.
Q13.   Plaster of Paris should be stored in a moisture-proof container. Explain why?
Ans. Plaster of Paris reacts with moisture/water to form gypsum. therefore it should be stored in a moisture-proof container.
Q14.   What is a neutralization reaction? Give two examples.
Ans. The reaction in which acid reacts with the base to form salt and water is called neutralization reaction.
$HCl&space;+&space;NaOH&space;\rightarrow&space;NaCl&space;+&space;H_{2}O$
Q15.   Give two important uses of washing soda and baking soda.
Ans. Washing soda
(i) It is used in glass, soap and paper industries.
(ii) It is used to remove permanent hardness of water.
Baking Soda
(i) It is used as an antacid.
(ii) It is used in soda, acid fire extinguisher.