NCERT Solutions For Class 10 Chemistry: Acids Bases and Salts

Intext Questions (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?

Answer-

First, use the red litmus paper to test all three test tubes. The test tube that turns the red litmus paper blue contains the basic solution because bases change the colour of the red litmus to blue. Next, use a drop of the resulting blue litmus solution to test the remaining two tubes. The tube that turns the blue litmus paper red contains the acidic solution. The last tube, which causes no change in the colour of either the red or blue litmus, contains the distilled water.

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Intext Questions (Page 22)

Question 1: Why should curd and sour substances not be kept in brass and copper vessels?

Answer-

Curd and sour substances are acidic in nature because they contain acids. These acidic substances should not be kept in brass and copper vessels because the acid can react with the metal in the vessels, forming a salt and hydrogen gas. This reaction might contaminate the food or form compounds that could be harmful.

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?

Answer-

The gas usually liberated when an acid reacts with a metal is Hydrogen gas. The reaction can be summarized as: Acid + Metal $\rightarrow$ Salt + Hydrogen gas.
Example: When dilute sulphuric acid reacts with zinc granules.
To test for the presence of the gas, the gas (evolved through soap solution, forming bubbles) is exposed to a burning candle. Hydrogen gas burns with a characteristic pop sound.

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.

Answer-

The gas evolved that causes effervescence and extinguishes a burning candle is carbon dioxide (CO$_{2}$). Since metal compound A reacts with an acid to produce carbon dioxide and a salt (calcium chloride), compound A must be Calcium carbonate (Limestone).
The balanced chemical equation is:
CaCO$_{3}$ + 2HCl(aq) $\rightarrow$ CaCl$_{2}$(aq) + CO$_{2}$(g) + H$_{2}$O(l)

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Intext Questions (Page 25)

Question 1: Why do HCl, HNO$_{3}$, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Answer-

HCl and HNO$_{3}$ show acidic character because they produce hydrogen ions (H$^{+}$(aq)) or hydronium ions (H$_{3}$O$^{+}$) when dissolved in an aqueous solution, and these ions are responsible for their acidic properties. Compounds like alcohol and glucose do not show acidic character because they do not produce H$^{+}$(aq) ions and consequently do not conduct electricity.

Question 2: Why does an aqueous solution of an acid conduct electricity?

Answer-

An aqueous solution of an acid conducts electricity because acids produce hydrogen ions (H$^{+}$(aq)) in water. The electric current is carried through the acidic solution by these ions.

Question 3: Why does dry HCl gas not change the colour of the dry litmus paper?

Answer-

Dry HCl gas does not change the colour of the dry litmus paper because the separation of the H$^{+}$ ion from the HCl molecule cannot occur in the absence of water. The acidic nature of a substance is due to the formation of H$^{+}$(aq) ions in solution, which are not produced by the dry gas.

Question 4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer-

The process of dissolving an acid in water is a highly exothermic one. If water is added to a concentrated acid, the large amount of heat generated may cause the mixture to splash out and cause burns. Additionally, the glass container may also break due to excessive local heating. Therefore, the acid must always be added slowly to water with constant stirring.

Question 5: How is the concentration of hydronium ions (H$_{3}$O$^{+}$) affected when a solution of an acid is diluted?

Answer-

When a solution of an acid is diluted (mixed with water), the concentration of hydronium ions (H$_{3}$O$^{+}$) per unit volume decreases.

Question 6: How is the concentration of hydroxide ions (OH$^{-}$) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer-

When excess base is dissolved in a solution of sodium hydroxide (making it more concentrated), the concentration of hydroxide ions (OH$^{-}$) is increased.

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Intext Questions (Page 29)

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?

Answer-

Solution A (pH 6) has more hydrogen ion concentration because the lower the pH value, the higher the hydronium ion (H$^{+}$(aq)) concentration. Solution A (pH less than 7) is acidic, and Solution B (pH more than 7) is basic.

Question 2: What effect does the concentration of H$^{+}$(aq) ions have on the nature of the solution?

Answer-

The concentration of H$^{+}$(aq) ions determines the acidity and strength of the solution. A higher concentration of H$^{+}$(aq) ions means a lower pH value and a more acidic solution. Acids that yield more H$^{+}$ ions are classified as strong acids.

Question 3: Do basic solutions also have H$^{+}$(aq) ions? If yes, then why are these basic?

Answer-

Yes, basic solutions also have H$^{+}$(aq) ions. They are considered basic because they generate hydroxide (OH$^{-}$) ions in water, and in a basic solution, the concentration of hydroxide ions (OH$^{-}$) is greater than the concentration of hydrogen ions (H$^{+}$(aq)).

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)?

Answer-

A farmer would treat the soil of his fields with these compounds (which are bases or metal carbonates) when the soil is acidic. Bases are used to neutralise the excess acid in the soil to bring it to the specific pH range required for the healthy growth of plants.

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Intext Questions (Page 33)

Question 1: What is the common name of the compound Ca(ClO)$_{2}$?

Answer-

The common name of the compound Ca(ClO)$_{2}$ is Bleaching powder.

Question 2: Name the substance which on treatment with chlorine yields bleaching powder.

Answer-

The substance which on treatment with chlorine yields bleaching powder is dry slaked lime [Ca(OH)$_{2}$].

Question 3: Name the sodium compound which is used for softening hard water.

Answer-

The sodium compound used for removing permanent hardness of water is Sodium carbonate (washing soda, Na$_{2}$CO$_{3}$.10H$_{2}$O).

Question 4: What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.

Answer-

If a solution of sodium hydrogencarbonate (baking soda) is heated, it decomposes to form sodium carbonate, water, and carbon dioxide.
Equation of the reaction:
2NaHCO$_{3}$ $\xrightarrow{Heat}$ Na$_{2}$CO$_{3}$ + H$_{2}$O + CO$_{2}$

Question 5: Write an equation to show the reaction between Plaster of Paris and water.

Answer-

Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form gypsum, giving a hard solid mass.
Equation of the reaction:
CaSO$_{4}$ . 1/2 H$_{2}$O + 1 1/2 H$_{2}$O $\rightarrow$ CaSO$_{4}$.2H$_{2}$O

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Exercise Questions

Question 1: A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10

Answer-

A solution that turns red litmus blue is basic. Basic solutions have a pH more than 7. The likely pH is (d) 10.

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-

Crushed egg-shells contain calcium carbonate (a metal carbonate). A substance that reacts with a metal carbonate to produce carbon dioxide (the gas that turns lime-water milky) is an acid. The solution contains (b) HCl (hydrochloric acid).

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 amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer-

Since 10 mL of NaOH requires 8 mL of HCl for neutralisation, doubling the volume of the base (20 mL of NaOH) requires doubling the volume of the acid. 2 x 8 mL = 16 mL. The amount of HCl required is (d) 16 mL.

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

Answer-

Indigestion is caused by the stomach producing excess acid. The medicine used to neutralise this excess acid is a base called an antacid. The correct medicine is (c) Antacid.

Question 5: Write word equations and then 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.

Answer-

(a) Word Equation: Sulphuric acid + Zinc $\rightarrow$ Zinc sulphate + Hydrogen gas
Balanced Equation: H$_{2}$SO$_{4}$(aq) + Zn(s) $\rightarrow$ ZnSO$_{4}$(aq) + H$_{2}$(g)

(b) Word Equation: Hydrochloric acid + Magnesium $\rightarrow$ Magnesium chloride + Hydrogen gas
Balanced Equation: 2HCl(aq) + Mg(s) $\rightarrow$ MgCl$_{2}$(aq) + H$_{2}$(g)

(c) Word Equation: Sulphuric acid + Aluminium $\rightarrow$ Aluminium sulphate + Hydrogen gas
Balanced Equation: 3H$_{2}$SO$_{4}$(aq) + 2Al(s) $\rightarrow$ Al$_{2}$(SO$_{4}$)$_{3}$(aq) + 3H$_{2}$(g)

(d) Word Equation: Hydrochloric acid + Iron $\rightarrow$ Iron(II) chloride + Hydrogen gas
Balanced Equation: 2HCl(aq) + Fe(s) $\rightarrow$ FeCl$_{2}$(aq) + H$_{2}$(g)

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

Answer-

This can be proved using an electrical conductivity test. Set up an apparatus connecting two nails (fixed on a cork) in a beaker to a 6 volt battery, a bulb, and a switch. First, pour dilute HCl (an acid) in the beaker; the bulb will glow because the acid produces H$^{+}$ ions which carry the current. Now, repeat the experiment with solutions of glucose and alcohol. The bulb will not glow in these cases. Since the bulb does not glow, it proves that glucose and alcohol solutions do not conduct electricity, meaning they do not produce the hydrogen ions (H$^{+}$(aq)) necessary for acidic properties.

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

Answer-

Distilled water is pure and contains negligible amounts of ions, so it does not conduct electricity. Rain water conducts electricity because, as it falls, it dissolves gases and impurities from the atmosphere, forming dissolved substances that produce hydrogen and hydroxide ions in the water. The presence of these ions allows the water to carry the electric current.

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

Answer-

Acids do not show acidic behaviour in the absence of water because the characteristic acidic nature is due to the formation of H$^{+}$(aq) ions in solution. The separation of the H$^{+}$ ion from the acid molecule (like HCl) cannot occur in the absence of water. Hydrogen ions must exist combined with water molecules as the hydronium ion (H$_{3}$O$^{+}$).

Question 9: 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
(a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.

Answer-

(a) Neutral: Solution D (pH 7).
(b) Strongly alkaline: Solution C (pH 11) (Highest pH value).
(c) Strongly acidic: Solution B (pH 1) (Lowest pH value).
(d) Weakly acidic: Solution A (pH 4) (Less acidic than pH 1, but below 7).
(e) Weakly alkaline: Solution E (pH 9) (Less alkaline than pH 11, but above 7).

Arrangement in increasing order of hydrogen-ion concentration:
(Increasing H$^{+}$ concentration corresponds to decreasing pH value):
C (pH 11) < E (pH 9) < D (pH 7) < A (pH 4) < B (pH 1).

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 (CH$_{3}$COOH) 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?

Answer-

The fizzing (evolution of hydrogen gas) will occur more vigorously in Test tube A (HCl). This is because HCl is a strong acid, meaning it gives rise to more H$^{+}$ ions in the solution compared to acetic acid (CH$_{3}$COOH), which is a weak acid. The higher concentration of H$^{+}$ ions in the strong acid causes the reaction with the metal to proceed faster, resulting in more vigorous fizzing.

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.

Answer-

As fresh milk turns into curd, the pH will decrease (become less than 6). This is because milk turns into curd due to the formation of lactic acid. Since the solution is becoming more acidic due the production of acid, the hydrogen ion concentration increases, which results in the pH value dropping.

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?

Answer-

(a) The milkman shifts the pH to slightly alkaline because baking soda (sodium hydrogencarbonate) is a mild non-corrosive basic salt. The slightly alkaline condition inhibits the development of acidity, thus preventing the milk from turning sour quickly and helping to preserve it for a longer duration.

(b) This milk takes a long time to set as curd because the lactic acid produced by the bacteria must first neutralise the added basic salt (baking soda) before the pH can drop below 6, which is the required acidic condition for the milk to coagulate and set into curd.

Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer-

Plaster of Paris (CaSO$_{4}$ . 1/2 H$_{2}$O) must be stored in a moisture-proof container because it is a white powder that reacts rapidly with water (moisture) to form gypsum (CaSO$_{4}$.2H$_{2}$O). Gypsum is a hard solid mass. If Plaster of Paris absorbs moisture from the atmosphere, it will set quickly and become unusable for its intended purpose.

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

Answer-

A neutralisation reaction is the reaction between an acid and a base to form a corresponding salt and water. In terms of ions, it is the reaction between $\text{H}^{+}(\text{aq})$ ions and $\text{OH}^{-}(\text{aq})$ ions to form water.

Example 1: Reaction between Sodium hydroxide and Hydrochloric acid:
NaOH(aq) + HCl(aq) $\rightarrow$ NaCl(aq) + H$_{2}$O(l)

Example 2: Reaction between a Metallic oxide (basic oxide) and an Acid (e.g., Copper oxide and HCl):
Metal oxide + Acid $\rightarrow$ Salt + Water

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

Answer-

Two important uses of Washing Soda (Na$_{2}$CO$_{3}$.10H$_{2}$O):

1. It is used in glass, soap and paper industries.
2. It is used for removing permanent hardness of water.

Two important uses of Baking Soda (Sodium hydrogencarbonate, NaHCO$_{3}$):

1. It is used as an ingredient in antacids to neutralise excess acid in the stomach and provide relief.
2. It is used for making baking powder, which produces carbon dioxide causing bread or cake to rise and become soft and spongy.