NCERT Solutions Class 10 Science Chapter 2 In-text Questions — Acids, Bases and Salts
NCERT Class 10 Science — Acids, Bases and Salts, the five in-text QUESTIONS boxes on pages 18, 22, 25, 28 and 33. All 19 questions solved.
Also here. Solutions to the end-of-chapter questions for this chapter and the activities are on their own pages.
The five boxes track the chapter exactly: indicators first, then the reactions of acids, then what acids and bases actually are, then the pH scale, and finally the important salts. Three ideas carry all nineteen questions:
- An acid is a substance that gives $\text{H}^+$ (hydronium, $\text{H}_3\text{O}^+$) ions in water; a base gives $\text{OH}^-$ ions. Everything acidic or basic follows from that, including why dry hydrogen chloride is neither.
- pH measures the hydrogen-ion concentration — lower pH, more $\text{H}^+$; higher pH, more $\text{OH}^-$.
- Acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide. The gas identifies which reaction has happened: hydrogen pops with a flame, carbon dioxide puts it out.
Key insight. Almost every question in these boxes is answered by asking what ions are present. Glucose contains hydrogen but gives no $\text{H}^+$, so it is not an acid. Dry hydrogen chloride gives no $\text{H}^+$, so it does not redden litmus. A basic solution still contains some $\text{H}^+$ — it is basic because it contains more $\text{OH}^-$. The whole chapter is that one habit of thought.
Question, page 18. The chapter’s first box, holding a single question — which is why its banner reads QUESTION.
Page 18, 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?
Solution. Red litmus on its own can only detect one thing: it turns blue in a base, and is unchanged by an acid or by water. So a single test tells you which tube is basic, and leaves the other two indistinguishable. The trick is to make the indicator you are missing.
Step 1. Put a drop from each test tube on a separate strip of red litmus paper. The solution that turns red litmus blue is the base. The other two leave it red, so they are the acid and the distilled water, in some order.
Step 2. Now you have something you did not have before — a strip of blue litmus, made by the basic solution. Use it. Put a drop of each of the two remaining solutions on that blue strip.
The one that turns the blue litmus red is the acid. The one that leaves it blue is distilled water, which is neutral and changes neither colour.
First, test all three with red litmus: the one that turns it blue is the basic solution.
Then use that blue litmus strip on the other two: the one that turns it red is the acid, and the one that leaves it unchanged is distilled water.
Questions, page 22. The box that follows Section 2.1.6, after the reactions of acids with metals, carbonates and oxides.
Page 22, Question 1
Why should curd and sour substances not be kept in brass and copper vessels?
Solution. Curd and sour foods are sour because they contain acids — lactic acid in curd, citric acid in lemon, acetic acid in vinegar.
Brass is an alloy of copper and zinc, and both metals react with acids. So the acid in the food slowly attacks the metal of the vessel, forming copper and zinc salts and releasing hydrogen.
Those salts dissolve into the food. They are toxic, and they spoil the taste as well — so food kept this way can cause food poisoning. That is why sour foods are stored in glass, steel or earthenware instead.
Because curd and sour substances contain acids, which react with the copper and zinc of the vessel to form salts of those metals. These salts dissolve into the food, are toxic and spoil its taste, so such food can cause food poisoning.
Page 22, Question 2
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?
Solution. The general reaction is
$$\text{acid} + \text{metal} \longrightarrow \text{salt} + \text{hydrogen}$$
so the gas liberated is hydrogen. For example, zinc granules with dilute hydrochloric acid:
$$\text{Zn(s)} + 2\text{HCl(aq)} \longrightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$
The test. Pass the gas through soap solution so that it fills the bubbles, then bring a burning candle near them. The gas burns with a pop sound. That pop is the standard test for hydrogen, and it distinguishes it at once from carbon dioxide, which would put the flame out instead.
Hydrogen gas. For example,
$$\text{Zn(s)} + 2\text{HCl(aq)} \longrightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$
Test: bubble the gas through soap solution and bring a burning candle near the bubbles — hydrogen burns with a characteristic pop sound.
Page 22, Question 3
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.
Solution. Work through the three clues in turn.
The gas extinguishes a burning candle, so it is not hydrogen — hydrogen would burn with a pop. A gas that puts out a flame and is produced by an acid is carbon dioxide, and an acid gives carbon dioxide when it reacts with a carbonate or a hydrogencarbonate.
One product is calcium chloride, so the metal in compound A is calcium.
Put the two together: A is calcium carbonate, $\text{CaCO}_3$.
$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \longrightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$
The effervescence is the carbon dioxide bubbling off.
A is calcium carbonate, $\text{CaCO}_3$.
$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \longrightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$
The gas that extinguishes the candle is carbon dioxide, which is what an acid releases from a carbonate.
Questions, page 25. The box that closes Section 2.2, on what all acids and all bases have in common.
Page 25, Question 1
Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Solution. Acidic character does not come from containing hydrogen. It comes from releasing hydrogen ions in water.
$\text{HCl}$ and $\text{HNO}_3$ dissociate when dissolved:
$$\text{HCl} + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
Those hydronium ions are what turn blue litmus red, react with metals and neutralise bases.
Alcohol ($\text{C}_2\text{H}_5\text{OH}$) and glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) also contain hydrogen — but their hydrogen is bound in a way that does not come off as $\text{H}^+$ in water. Their solutions contain no hydrogen ions, so they show no acidic character, and the bulb in Activity 2.8 does not glow for them.
Because $\text{HCl}$ and $\text{HNO}_3$ dissociate in water to give $\text{H}^+$ (hydronium) ions, and it is those ions that give acidic character:
$$\text{HCl} + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
Alcohol and glucose contain hydrogen but do not ionise to release $\text{H}^+$ ions in water, so their solutions are not acidic.
Page 25, Question 2
Why does an aqueous solution of an acid conduct electricity?
Solution. A liquid conducts a current only if it contains charged particles free to move.
When an acid dissolves in water it dissociates, producing hydrogen (hydronium) ions and the corresponding negative ions — $\text{Cl}^-$ from hydrochloric acid, $\text{SO}_4^{2-}$ from sulphuric acid. These ions are free to move through the solution, and when a voltage is applied they drift towards the electrodes and carry the charge.
That is what makes the bulb glow in Activity 2.8 for acids, and not for glucose or alcohol, whose solutions contain no ions at all.
Because an acid dissociates in water to give ions — $\text{H}_3\text{O}^+$ and the corresponding negative ions — and these free ions move towards the electrodes and carry the current. A solution with no ions, such as glucose or alcohol, does not conduct.
Page 25, Question 3
Why does dry HCl gas not change the colour of the dry litmus paper?
Solution. Litmus responds to hydrogen ions, not to hydrogen chloride molecules.
Hydrogen chloride only produces those ions when water is present:
$$\text{HCl(g)} + \text{H}_2\text{O(l)} \longrightarrow \text{H}_3\text{O}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$$
With dry gas and dry paper there is no water anywhere, so the molecules do not dissociate, no $\text{H}^+$ ions are formed, and the litmus has nothing to respond to — the paper stays blue.
Moisten the litmus paper, and it turns red at once, because now the gas can dissolve and ionise.
Because litmus responds to $\text{H}^+$ ions, and hydrogen chloride releases them only in water. With both the gas and the paper dry, the $\text{HCl}$ molecules do not ionise, so there are no hydrogen ions and the colour does not change. Moist litmus paper turns red immediately.
Page 25, Question 4
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Solution. Mixing a concentrated acid with water is highly exothermic — a great deal of heat is released very quickly.
Adding acid to water spreads that heat through the whole, much larger, volume of water, which absorbs it safely. Adding it slowly and stirring constantly keeps the temperature under control.
Adding water to acid does the opposite: a small amount of water meets a large amount of concentrated acid, so all the heat is released into a tiny volume. The mixture can boil and splash out, throwing concentrated acid onto the skin and face, and the heat may crack the glass container.
It is a safety rule, and the reason is the direction in which the heat is diluted.
Because mixing a concentrated acid with water is highly exothermic. Adding acid slowly to water, with stirring, lets the large volume of water absorb the heat safely. Adding water to acid releases all that heat into a small volume: the mixture can boil and splash out, causing burns, and the container may crack.
Page 25, Question 5
How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Solution. Diluting means adding more water without adding any more acid. The number of hydronium ions in the solution stays much the same, but they are now spread through a larger volume.
Concentration is amount per unit volume, so a larger volume with the same number of ions means a smaller concentration. The hydronium ion concentration therefore decreases — and the pH, correspondingly, rises towards 7.
The concentration of hydronium ions decreases. The same number of $\text{H}_3\text{O}^+$ ions is spread through a larger volume of solution, so there are fewer per unit volume — and the pH rises towards 7.
Page 25, Question 6
How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Solution. This is the previous question run the other way. Sodium hydroxide dissolves to give $\text{Na}^+$ and $\text{OH}^-$ ions, so adding more of it puts more hydroxide ions into the same volume of solution.
More ions in the same volume means a greater concentration. The hydroxide ion concentration therefore increases, the solution becomes more strongly basic, and its pH rises.
The concentration of hydroxide ions increases. Dissolving more sodium hydroxide releases more $\text{OH}^-$ ions into the same volume, so the solution becomes more strongly basic and its pH rises.
Questions, page 28. The box that follows Section 2.3, on the pH scale and its importance in everyday life.
Page 28, Question 1
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?
Solution. The pH scale runs opposite to the hydrogen-ion concentration: the lower the pH, the higher the concentration of $\text{H}^+$. Solution A has the lower pH, 6, so A has the higher hydrogen ion concentration.
For the nature of each, compare with 7, the neutral value. A is at 6, below 7, so A is acidic. B is at 8, above 7, so B is basic.
Neither is far from neutral — A is only weakly acidic and B only weakly basic.
Solution A has the higher hydrogen-ion concentration, because a lower pH means more $\text{H}^+$.
A (pH 6) is acidic and B (pH 8) is basic, since acidic means below 7 and basic above 7.
Page 28, Question 2
What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
Solution. The hydrogen-ion concentration is what decides the nature of a solution, and the comparison is always with the hydroxide ion concentration.
- If $[\text{H}^+]$ is greater than $[\text{OH}^-]$, the solution is acidic, and its pH is below 7. The greater the excess, the more strongly acidic and the lower the pH.
- If $[\text{H}^+]$ equals $[\text{OH}^-]$, the solution is neutral, pH exactly 7.
- If $[\text{H}^+]$ is less than $[\text{OH}^-]$, the solution is basic, and its pH is above 7.
It decides the nature of the solution. A higher $[\text{H}^+]$ makes a solution more acidic and lowers its pH; a lower $[\text{H}^+]$ (that is, a higher $[\text{OH}^-]$) makes it more basic and raises the pH. When the two are equal the solution is neutral, at pH 7.
Page 28, Question 3
Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Solution. Yes. Water itself ionises very slightly, so every aqueous solution contains some hydrogen ions and some hydroxide ions — a basic solution included. There is no such thing as an aqueous solution with no $\text{H}^+$ at all.
What makes a solution basic is not the absence of hydrogen ions but the balance between the two. In a basic solution the concentration of $\text{OH}^-$ ions is greater than the concentration of $\text{H}^+$ ions, and it is that excess of hydroxide that gives the solution its basic character and its pH above 7.
Yes, they do — water itself ionises slightly, so every aqueous solution contains some $\text{H}^+$ ions. Such solutions are basic because the concentration of $\text{OH}^-$ ions is greater than that of $\text{H}^+$ ions; it is the excess of hydroxide, not the absence of hydrogen ions, that makes a solution basic.
Page 28, Question 4
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)?
Solution. Quick lime, slaked lime and chalk are all basic substances. A farmer would only add a base to the soil in order to neutralise an acid, so they are used when the soil is too acidic — when its pH has fallen below the range the crop needs.
Plants grow well only within a fairly narrow pH range, and acidic soil stunts them. Adding one of these bases neutralises the excess acid and brings the pH back up towards neutral, so the crop can grow properly.
(If the soil were too basic instead, the farmer would add organic matter such as compost, which is acidic — not lime, which would make matters worse.)
When the soil is too acidic — its pH too low for the crop. Quick lime, slaked lime and chalk are all bases, so they neutralise the excess acid and raise the soil’s pH back towards neutral, letting plants grow properly.
Questions, page 33. The last box, after Section 2.4 on salts, bleaching powder, baking soda, washing soda and plaster of Paris.
Page 33, Question 1
What is the common name of the compound Ca(ClO)₂?
Solution. $\text{Ca(ClO)}_2$ is calcium hypochlorite, and its common name is bleaching powder.
It is made by passing chlorine over dry slaked lime, and it is used to bleach cotton and linen in the textile industry and wood pulp in the paper industry, as an oxidising agent in chemical industries, and to disinfect drinking water.
(The chapter itself writes the formula of bleaching powder as $\text{CaOCl}_2$ a page earlier. Both formulae are used for the same commercial substance, which is a mixture rather than a single pure compound.)
Bleaching powder (calcium hypochlorite). It is used for bleaching cotton, linen and wood pulp, as an oxidising agent, and for disinfecting drinking water.
Page 33, Question 2
Name the substance which on treatment with chlorine yields bleaching powder.
Solution. Bleaching powder is manufactured by the action of chlorine on dry slaked lime, that is, calcium hydroxide, $\text{Ca(OH)}_2$:
$$\text{Ca(OH)}_2 + \text{Cl}_2 \longrightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$
The slaked lime must be dry; chlorine passed into a solution would give a different product.
Dry slaked lime — calcium hydroxide, $\text{Ca(OH)}_2$.
$$\text{Ca(OH)}_2 + \text{Cl}_2 \longrightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$
Page 33, Question 3
Name the sodium compound which is used for softening hard water.
Solution. The compound is washing soda, sodium carbonate decahydrate, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$.
Hard water is hard because it contains dissolved calcium and magnesium salts. The carbonate ions from washing soda react with those calcium and magnesium ions and precipitate them out as insoluble carbonates, leaving the water soft. It removes permanent hardness, which boiling cannot.
Washing soda, sodium carbonate decahydrate, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$. Its carbonate ions precipitate the dissolved calcium and magnesium salts that make water hard.
Page 33, Question 4
What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Solution. Sodium hydrogencarbonate — baking soda, $\text{NaHCO}_3$ — is decomposed by heat. It gives sodium carbonate, water and carbon dioxide:
$$2\text{NaHCO}_3\text{(s)} \xrightarrow{\ \text{heat}\ } \text{Na}_2\text{CO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$
This is exactly why baking soda is used in baking. The carbon dioxide released is trapped as bubbles in the dough, and they make the cake or bread rise and turn out soft and spongy.
It decomposes on heating to give sodium carbonate, water and carbon dioxide:
$$2\text{NaHCO}_3 \xrightarrow{\ \text{heat}\ } \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$
The carbon dioxide released is what makes baked goods rise.
Page 33, Question 5
Write an equation to show the reaction between Plaster of Paris and water.
Solution. Plaster of Paris is calcium sulphate hemihydrate — one molecule of water shared between two formula units of calcium sulphate, which is why it is written with a half.
On mixing with water it takes up the remaining $1\tfrac12$ molecules and sets into gypsum, a hard solid mass:
$$\text{CaSO}_4 \cdot \tfrac12\text{H}_2\text{O} + 1\tfrac12\text{H}_2\text{O} \longrightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
Counting: calcium 1 and sulphur 1 on each side; oxygen $4 + \tfrac12 + 1\tfrac12 = 6$ against $4 + 2 = 6$; hydrogen $1 + 3 = 4$ against 4. Balanced.
That setting is what makes plaster of Paris useful for supporting fractured bones, for toys and for decorative mouldings — and why it has to be stored away from moisture.
$$\text{CaSO}_4 \cdot \tfrac12\text{H}_2\text{O} + 1\tfrac12\text{H}_2\text{O} \longrightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
Plaster of Paris takes up water and sets into gypsum, a hard solid mass.
Common mistakes
- Giving up on the page-18 question because “red litmus cannot detect an acid”. It cannot on its own — but the basic solution turns a strip blue, and that blue strip is the second indicator you need. Making the missing indicator is the whole point.
- Saying in Question 3 of the page-28 box that basic solutions have no $\text{H}^+$ ions. They do; every aqueous solution does. They are basic because $[\text{OH}^-]$ exceeds $[\text{H}^+]$, not because hydrogen ions are absent.
- Answering “hydrogen” for the gas in Question 3 of the page-22 box. The gas extinguishes the candle, so it cannot be hydrogen, which would burn with a pop. A gas that puts out a flame points to a carbonate and carbon dioxide.
- Explaining the dilution rule as “acid is heavier than water”. The reason is the heat: adding water to concentrated acid releases a great deal of heat into a small volume, so the mixture can boil and splash.
- Saying dilution changes the number of hydronium ions. It does not add or remove ions; it spreads the same ions through a larger volume, which is what lowers the concentration.
- Confusing washing soda with baking soda in the last box. Washing soda, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$, softens hard water; baking soda, $\text{NaHCO}_3$, is the one that decomposes on heating.
Practise next
- The end-of-chapter Exercises for this chapter, where Questions 5, 9 and 10 apply the same ionisation and pH reasoning to harder cases, and Questions 13 to 15 return to these same salts.
- Chapter 1, Chemical Reactions and Equations, whose in-text questions cover the balancing and the reaction types this chapter’s equations rely on.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.