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Acids, Bases and Salts – CBSE Class 10 Science – NCERT Solutions

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?
The contents of each test tube would be identified by change in colour of red litmus paper. For example, when we wet the red litmus paper with the basic solution, it changes into blue colour. Put the changed blue litmus paper in the solution which turns the blue to red will be the acidic solution. The solution, which has no effect on any litmus paper, will be neutral and hence it will be distilled water.

Why should curd and sour substances not be kept in brass and copper vessels?
Curd and other sour foodstuffs contain acids, which can react with the metal of the vessel to form poisonous metal compounds which can cause food poisoning and damage our health.
Which gas is usually liberated when an acid reacts with a metal?
When an acid reacts with metal, a salt and hydrogen gas is formed. i.e

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.
The gas that extinguishes a burning candle is carbon dioxide, which is formed by the action of dilute hydrochloric acid on a metal carbonate and produces effervescence. Now, since one of the compounds formed is calcium chloride, it shows that the metal compound is calcium carbonate. Thus, the metal compound A is calcium carbonate (CaCO3). Calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, carbon dioxide and water. This can be written as:

Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
An acid is a substance, which dissociates on dissolving in water to produce hydrogen ions [H+(aq) ions]. The acids like HCl, H2SO4, HNO3 and CH3COOH, etc., show acidic character because they dissociate in aqueous solutions to produce hydrogen ions, H+(aq) ions.
The compounds such as glucose and alcohol also contain hydrogen but they do not show acidic character. The aqueous solutions of glucose and alcohol do not show acidic character because the hydrogen in them does not separate out as hydrogen ions [H+ (aq) ions] on dissolving in water.

Why does an aqueous solution of acid conduct electricity?
The aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.

Why does dry HCl gas not change the colour of the dry litmus paper?
Dry HCl gas does not contain any hydrogen ions in it, so it does not show acidic behaviour. In fact, dry HCl gas does not change the colour of dry litmus paper as it has no hydrogen ions [H+(aq) ions] in it.

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Diluting an acid should be done by adding concentrated acid to water gradually with stirring and not by adding water to concentrated acid.
The heat is evolved gradually when a concentrated acid is added to water for diluting an acid and the large amount of water is easily absorbed.
If, however, water is added to concentrated acid to dilute it, a large amount of heat is evolved at once. The heat generated may cause the mixture to splash the acid on our face or clothes and cause acid burns.

How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
When the concentrated solution of an acid is diluted by mixing water, the concentration of hydronium ions H3O+ per unit volume decreases.
How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in water?
When the solution of a base is diluted by mixing more water in it, the concentration of hydroxide ions (OH ions) per unit volume decreases.

You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8.
i. Which solution has more hydrogen ion concentration?
ii. Which of this is acidic and which one is basic?
The pH of a solution is inversely proportional to its hydrogen ion concentration. This means that the solution having lower pH will have more hydrogen ion concentration. In this case, solution A(having a lower pH of 6) will have more hydrogen ion concentration.Solution A is acidic and solution B is basic.

What effect does the concentration of H+(aq) ions have on the nature of the solution?
Acids produce hydrogen ions in water. So, when an acid is added to water, the concentration of hydrogen ions in water increases. The solution of acid thus formed will have more of hydrogen ions and it will be acidic in nature.

Do basic solutions also have H+(aq) ions? If yes, why are these basic?
No, the basic solution doesn’t have H+ ions as the solution has excess of hydroxide ions.

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)?
Most often the soil in the fields is too acidic. If the soil is too acidic (having low pH), it is treated with materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate). Thus, a farmer should add lime or slaked lime in his fields when the soil is too acidic.

What is the common name of the compound CaOCl2?
The common name of the compound CaOCl2 is bleaching powder.

Name the substance that on treatment with chlorine yields bleaching powder.
Calcium hydroxide is the substance that on treatment with chlorine yields bleaching powder.

Name the sodium compound, which is used, for softening hard water.
Sodium carbonate (washing soda) is used for softening hard water.

What will happen if a solution of sodium hydro carbonate is heated? Give the equation of the reaction involved.
Sodium carbonate and carbon dioxide are evolved when sodium hydro carbonate is heated.

Write an equation to show the reaction between plaster of Paris and water.
Plaster of Paris has a very remarkable property of setting into a hard mass on wetting with water. So, when water is added to plaster of Paris, it sets into a hard mass in about half an hour. The setting of plaster of Paris is due to the hydration crystals of gypsum, which set to form a hard, solid mass.
Why does distilled water not conduct electricity, whereas rainwater does?
Distilled water does not conduct electricity because it does not contain any ionic compound (like acids, bases or salts) dissolved in it. On the other hand, rain water conducts electricity. This can be explained as follows: Rain water, while falling to the earth through the atmosphere, dissolves an acidic gas carbon dioxide from the air and forms carbonic acid (H2CO3). Carbonic acid provides hydrogen ions, H+(aq), and carbonate ions, CO2-3 (aq), to rain water. So, due to the presence of carbonic acid (which provides ions to rain water), the rain water conducts electricity.

Why do acids not show acidic behaviour in the absence of water?
The acidic behaviour of acid is due to the presence of hydrogen ions. The acids will not show its acidic behaviour in the absence of water, this is because the acids produce hydrogen ions only in the presence of water.

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?
(i) Neutral?
(ii) Strongly alkaline?
(iii) Strongly acidic?
(iv) Weakly acidic?
(v) Weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.

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 test-tube will fizzing occur more vigorously and why?
Acetic acid (CH3COOH) is a weak acid whereas hydrochloric acid (HCl) is a strong acid. Fizzing occurs in the test tube due to the evolution of hydrogen gas by the action of acid on magnesium ribbon. Since hydrochloric acid is a strong acid a large amount of hydrogen gas is liberated in the test tube A. So fizzing occurs more vigorously in test tube A .
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain.
The pH will change to below 6, as lactic acid is formed when milk turns into curd.

Plaster of Paris should be stored in a moisture-proof container. Why?
The presence of moisture can affect the slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of Paris useless after some time.
What is a neutralization reaction? Give two examples.
The reaction of an acid and a base, giving rise to the corresponding salt and water is called neutralization reaction.
NaOH + HCl —> NaCl + H2O
Mg(OH)2 + H2CO3 —> MgCO3 + 2H2O

Give two important uses of washing soda and baking soda.
Washing soda
1. It is often used as an electrolyte.
2. Domestically it is used as a water softener during laundry.
Baking soda
1. It is used to test garden soil for acidity. If it develops bubbles, the soil is too acidic.
2. Washing a car with it can remove dead bug bodies without damaging the paint.

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