NCERT Solutions For Class 10 Science Chapter 2 Acids, Bases And Salts

NCERT Solutions For Class 10 Science Chapter 2 Acids, Bases And Salts Long Question And Answers

Question 1.

1. Mention the pH range within which our body works. Explain how antacids give relief from acidity. Write the name of one such antacid.
2. Fresh milk has a pH of 6. How does the pH will change as it turns to curd? Explain your answer.
3. A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?
4. Mention the nature of toothpaste. How do they prevent tooth decay? 

Answer:

1. Our stomach has a pH equal to 2. Antacids neutralise excess acid in our body and give relief from hyperacidity. Sodium hydrogen carbonate is one such antacid.
2. pH will decrease as it turns to curd because curd is acidic due to the presence of lactic acid.
3. It takes a longer time to set as curd as bacteria do not work well in the presence of sodium hydrogen carbonate, i.e., fermentation will take place slowly.
4. Toothpastes are basic. They neutralise the acid formed in the mouth which causes tooth decay.

Question 2.

1. State the chemical properties on which the following uses of baking soda are based

1. As an antacid
2. As soda-acid fire extinguisher
3. To make bread and cake soft and spongy.

2. How is washing soda obtained from baking soda? Write a balanced chemical equation.

Answer:

1.

1. It is weakly basic and neutralises hyperacidity.
2. It liberates CO₂ with H₂SO₄, which extinguishes fire.
3. It liberates CO₂ on heating which makes bread and cake soft and spongy

2. 2NaHCO₃→ (Heat) → Na₂CO₃ + CO₂ + H₂O

Baking soda on heating gives sodium carbonate which on crystallisation from an aqueous solution gives washing soda,

Example:

Na₂CO₃ + 10H₂O → Na₂CO₃ .10H₂O

Question 3. State the reason for the following statements:

1. Tap water conducts electricity whereas distilled water does not.
2. Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.
3. During the summer season, a milkman usually adds a very small amount of baking soda to fresh milk.
4. For a dilution of acid, acid is added into water and not water into acid.
5. Ammonia is a base but does not contain a hydroxyl group.

Answer:

1. Tap water contains ions which conduct electricity, distilled water does not contain ions.
2. Dry HCl does not form ions but HCl gives H⁺ and Cl^–.
3. Baking soda does not allow milk to change to lactic acid which makes milk sour.
4. Adding water to acid is highly exothermic. Therefore, water is added to the acid very slowly with cooling.
5. Ammonia dissolves in water and forms OH^–. Therefore, it is basic.

NH₃ + H₂O→  NH⁺ + OH^–

Question 4. Equal lengths of magnesium ribbon are taken in two test tubes A and B. H₂SO₄ is added to test tube A and H₂CO₃ in test tube B in equal amounts.

1. Identify the test tube showing a vigorous reaction.
2. Give reasons to support your answer.
3. Name the gas liberated in both test tubes. How will you prove its liberation?
4. Write chemical equations for both reactions.
5. Out of the two acids taken above, which one will have a lower value and lower H⁺ concentration respectively?

Answer:

1. Vigorous reaction will be seen in test tube A.

2. It is because H₂SO₄ is a stronger acid than H₂CO₃

3.

1. Hydrogen gas is liberated in both the test tubes.
2. When we bring a burning candle near the evolving gas, it burns with a pop sound. This proves that the produced gas is hydrogen gas.

4.

Mg + H₂SO₄→ MgSO₄ + H₂

Mg + H₂CO₃→ MgCO₃ + H₂

5.

‘A’ (H₂SO₄) will have a lower pH.

‘B’ (H₂CO₃) will have a lower concentration of H⁺

Question 5. Write balanced chemical equations for the following:

1. Bleaching powder is kept open in the air.
2. Blue crystals of copper sulphate are heated.
3. Chlorine gas is passed through dry slaked lime.
4. Carbon dioxide gas is passed through lime water.
5. NaOH solution is heated with zinc granules

Answer:

1. \(\underset{\text { Bleaching powder }}{\mathrm{CaOCl}_2(s)}+\underset{\text { Carbon dioxide }}{\mathrm{CO}_2(g)} \longrightarrow \underset{\text { Calcium carbonate }}{\mathrm{CaCO}_3(s)}+\underset{\text { Chlorine }}{\mathrm{Cl}_2(s)}\)

2. (Blue) CuSO₄.5H₂O → (Heat) → CuSO₄ + 5H₂O

3.  \(\underset{\text { Dry slaked lime }}{\mathrm{Ca}(\mathrm{OH})_2}+\underset{\text { Chlorine }}{\mathrm{Cl}_2} \longrightarrow \underset{\text { Bleaching powder }}{\mathrm{CaOCl}_2}+\mathrm{H}_2 \mathrm{O}\)

4. \(\underset{\text { Lime water }}{\mathrm{Ca}(\mathrm{OH})_2}+\mathrm{CO}_2 \longrightarrow \underset{\text { Calcium carbonate(Milky ppts) }}{\mathrm{CaCO}_3}+\mathrm{H}_2 \mathrm{O}\)

5. 2NaOH + Zn→ (heat) →(Sodium zincate) Na₂ZnO₂ + (Hydrogen)H₂

Question 6.

1. Identify the acid and the base whose combination forms the common salt that you use in your food. Write the formula and chemical name of this salt. Name the source from where it is obtained.
2. What is rock salt? Mention its colour and the reason why it has this colour.
3. What happens when electricity passes through brine? Write the chemical equation for it.

Answer:

1. Hydrochloric acid and sodium hydroxide.

NaCl, sodium chloride, ocean water.

2. solid salt deposits are found in several parts of the world. These large crystals are called rock salt.

Brown, due to impurities

3. 2NaCl + 2H₂O →(Electricity) → 2NaOH +Cl₂ +H₂

Question 7.

1. Write the chemical name and chemical formula of washing soda.
2. How is it obtained from sodium chloride? Give equations ofthe reactions.
3. Why is it called a basic salt? Give it anyone use.

Answer:

1. Chemical name—Sodium carbonate

Formula- Na₂CO₃.10H₂O

2. NaCl + H₂O + CO₂ + NH₃→ NH₄Cl + NaHCO₃

Na₂CO₃+ 10H₂O→ Na₂CO₃.10H₂O

3. It is a basic salt because it gives a strong base of NaOH when it dissolves in water.

1. In glass and paper industry.
2. As cleaning agent.
3. To remove permanent hardness, (anyone)

Question 8. A student dropped a few pieces of marble in dilute hydrochloric acid, contained in a test tube. The evolved gas was then passed through lime water. What change would be observed in lime water? What will happen if an excess of gas is passed through lime water? With the help of balanced chemical equations for all the changes explain the observations.

⇒ \(\underset{\text { Marble }}{\mathrm{CaCO}_3}+2 \mathrm{HCl}(\text { dil. }) \longrightarrow \underset{\text { Calcium chloride }}{\mathrm{CaCl}_2}+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\)

Lime water will turn milky.

Ca(OH)₂(aq) + CO₂(g) CaCO₃(s) + H₂O(l)

If excess CO₂ gas passes through lime water, milkiness will disappear due to the formation of Ca(HCO₃)(aq) which is soluble in water.

CaCO₃(S) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

Question 9.

1. Explain why is hydrochloric acid a strong acid and acetic acid, a weak acid. How can it be verified?

2. Explain why an aqueous solution and an acid conduct electricity

3. You have four solutions A, B, C, and D. The solution A is 6, B is 9, C is 12 and D is 7.

1. Identify the most acidic and most basic solutions.
2. Arrange the above four solutions in the increasing order of H+ ion concentration.
3. State the change in colour of pH paper on dipping in solutions C and D.

Answer:

1. HCl is completely ionised in aqueous solution, whereas acetic acid is partially ionised in aqueous solution. HCl gives a dark red colour with pH paper, whereas CH₃COOH gives an orange colour.

2. It is because acid ionises in aqueous solution.

3.

1. ‘A’ is most acidic and ‘C’ is most basic.
2. C (10^-12) < B (10^-9) < D (10^-7) < A (10^-6)
3. pH of paper will become blue in ‘C’ and green in ‘D’.

Question 10. Dry pellets ofa base ‘X’ when kept in open absorbs moisture and turns sticky. The compound is also formed by the chlor-alkali process. Write the chemical name and formula of X. Describe the chlor-alkali process with a balanced chemical equation. Name the type of reaction that occurs when X is treated with dilute hydrochloric acid. Write the chemical equation.

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

1. X is sodium hydroxide, NaOH.

When sodium chloride solution (brine solution) is electrolysed, sodium hydroxide solution is formed. H₂ and Cl₂ gases are liberated. This is a chlor-alkali process.

2NaCl + 2H₂O→ (Electrolysis) 2NaOH + H₂ + Cl₂

NaOH + HCl ↓  NaCl + H₂O

The above reaction is a neutralisation reaction.

2. It is because the process is highly exothermic. If water is added to acid, a bottle of acid will break.

Question 11. How do metal carbonates and metal hydrogen carbonates react with acids? Give their chemical equations. Name the gas that evolved during the reaction. What will happen when this gas is passed through lime water?
Answer:

1. Metal carbonate and hydrogen carbonate react with acids to give corresponding salt, carbon dioxide and water.

2.

Metal carbonate:

Na₂CO₃(s) + 2HCl(aq) →  2NaCl(aq) + H₂O(l) + CO₂(g)

Metal hydrogen carbonate:

NaHCO₃(s) + HCl(aq) – NaCl(aq) + H₂O(l) + CO₂(g)

3. The gas that evolved is carbon dioxide (CO₂).

4. When this gas is passed through lime water, it turns milky.

Question 12. 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:

When an acid reacts with a metal, hydrogen gas is usually liberated.

For example:

⇒ \(\underset{\text { Zinc }}{\mathrm{Zn}(s)}+\underset{\begin{array}{c}
\text { Sulphuric } \\
\text { acid }
\end{array}}{\mathrm{H}_2 \mathrm{SO}_4(a q)} \longrightarrow \underset{\begin{array}{c}
\text { Zinc } \\
\text { sulphate }
\end{array}}{\mathrm{ZnSO}_4(a q)}+\underset{\text { Hydrogen }}{\mathrm{H}_2(g)}\)

⇒ \(\underset{\text { Magnesium }}{\mathrm{Mg}(s)}+\underset{\text { Hydrochloric acid }}{2 \mathrm{HCl}(a q)} \longrightarrow \underset{\text { Magnesium chloride }}{\mathrm{MgCl}_2(a q)}+\underset{\text { Hydrogen }}{\mathrm{H}_2(g)}\)

Question 13. Write word equations and then balanced equations for the reaction taking place when:

1. Dilute sulphuric acid reacts with zinc granules.
2. Dilute hydrochloric acid reacts with magnesium ribbon.
3. Dilute sulphuric acid reacts with aluminium powder.
4. Dilute hydrochloric acid reacts with iron filings.

Answer:

1.  Zinc + dil. sulphuric acid→ Zinc sulphate + Hydrogen

Zn + H₂SO₄→ ZnSO₄ + H₂

2. Magnesium ribbon + dil. hydrochloric acid→Magnesium chloride + Hydrogen

Mg + 2HCl→MgCl₂ + H₂

3. Aluminium powder + dil. sulphuric acid→Aluminium sulphate + Hydrogen

2Al + 3H₂SO₄→ Al₂(SO₄)₃ + 3H₂

4. Dilute hydrochloric acid + Iron filings→Iron chloride + Hydrogen

2Fe + 6HCl→ 2FeCl₃ + 3H₂

Question 14. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Answer:

Take solutions of alcohol, and glucose in a beaker.

Take a cork and fix two nails on the cork up to the end. Keep this cork in the beaker. Connect the nails to two terminals ofa the 6-volt battery through a bulb and a switch. Switch on the current. You will see that the bulb does not glow. This means no ions or H+ ions are present in the solution. This shows that alcohols and glucose are not acids.

Question 15. What is a neutralisation reaction? Give two examples.
Answer:

The reaction between an acid and a base to give salt and water is known neutralization reaction.

Base+Acid → Salt + Water

Examples:

⇒ \(\underset{\text { Hydrochloric acid (Acid) }}{\mathrm{HCl}}+\underset{\text { Sodium Hydroxide (base) }}{\mathrm{NaOH}} \longrightarrow \underset{\text { Sodium chloride (salt) }}{\mathrm{NaCl}}+\underset{\text { Water }}{\mathrm{H}_2 \mathrm{O}}\)

⇒  \(\underset{\text { Acetic acid (Acid) }}{\mathrm{CH}_3 \mathrm{COOH}}+\underset{\text { Sodium hydroxide (Base)}}{\mathrm{NaOH}} \longrightarrow \underset{\text { Sodium acetate(salt) }}{\mathrm{CH}_3 \mathrm{COONa}}+\underset{\text { Water }}{\mathrm{H}_2 \mathrm{O}}\)

Question 16.

1. What will you observe when dilute hydrochloric acid is added to a small amount of copper oxide in a beaker?
2. An aqueous solution of HCl shows an acidic character. However, the aqueous solution of glucose fails to do so. Why?
3. Why curd and sour substances not be kept in brass and copper vessels?

Answer:

1. The colour of the solution becomes blue and the copper oxide dissolves

CuO +2HCl → CuCl₂ + H₂O

2. Aqueous solution of HCl can produce H+ ions. Therefore, they show an acidic character.

However the aqueous solution of glucose does not produce H+ ions, i.e., they fail to evolve hydrogen gas and do not show any acidic character.

3. Curd and sour substances have acids which react with copper present in brass and form compounds which are poisonous.

Question 17. Answer the following:

1. Why is Plaster of Paris written as CaSO₄.½H₂O? How is it possible to have half a water molecule attached to CaSO₄?
2. Why is sodium hydrogen carbonate an essential ingredient in antacids?
3. Three products are obtained when electricity is passed through an aqueous solution ofsodium chloride. Why is the process called chlor-alkali?

Answer:

1. It is written in this form because two formula units of CaS04 share one molecule of water.
2. Sodium hydrogen carbonate being alkaline, neutralises excess acid in the stomach and provides relief.
3. The process is called chlor-alkali because of the products formed—chlor for chlorine and alkali for sodium hydroxide.

Question 18. How would you distinguish between baking powder and washing soda by heating?
Answer:

The chemical formula of baking powder is sodium hydrogen carbonate (NaHCO₃). Whereas, that of washing soda is sodium carbonate (Na₂CO₃.10H₂O) Sodium hydrogen carbonate on heating gives CO₂ gas which will turn lime water milky whereas no such gas is obtained from sodium carbonate.

2NaHCO₃ → (Heat) →  Na₂CO₃ + H₃O + CO₂

Na₂CO₃.10H₂O →  (Heat) → Na₂CO₃+ 10H₂O

Question 19. Fill up the missing data in the following table

Answer: 

Question 20. For making a cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake,

1. How will it affect the taste ofthe cake and. why?
2. How can baking soda be converted into baking powder?
3. What is the role of tartaric acid added to baking soda?

Answer:

1. Baking soda is sodium hydrogen carbonate. On heating, it is converted into sodium carbonate which is bitter to taste

2NaHCO₃→ (Heat) →Na₂CO₃+ H₂O + CO₂

2. Baking soda can be converted into baking powder by the addition of an appropriate amount of tartaric acid to it.

3. The role of tartaric acid is to neutralise sodium carbonate and cake will not taste bitter.

Question 21. A metal carbonate X reacting with acid gives a gas which when passed through a solution Ygives the carbonate back. On the other hand, a gas G that is obtained at the anode during electrolysis ofbrine ispassed on dry Y, which gives a compound Z, used for disinfecting drinking water. Identity X, Y, G, and Z.

And

Hence, X = CaCO₃, Y = Ca(OH₂, G = Cl₂, Z = CaOCl₂.

Question 22. A dry pellet ofa common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by-product of the chloralkali process. Identify B. What type ofreaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution.
Answer:

The common base B which absorbs moisture and becomes sticky and is also n by-product of the chloroalkali process is sodium hydroxide (NaOH). It reacts with acidic oxides like

⇒ \(2 \mathrm{NaOH}+\mathrm{CO}_2 \longrightarrow \underset{\text { Sodium carbonate }}{\mathrm{Na}_2 \mathrm{CO}_3}+\mathrm{H}_2 \mathrm{O}\).

⇒  \(2 \mathrm{NaOH}+\mathrm{SO}_2 \longrightarrow \underset{\text { Sodium sulphite }}{\mathrm{Na}_2 \mathrm{SO}_3}+\mathrm{H}_2 \mathrm{O}\).

Question 23. A sulphate salt of Group 2 element ofthe Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in the open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why it shows such a behaviour. Give the reaction involved.
Answer:

The substance which is used for making different shapes is Plaster of Paris. Its chemical name is calcium sulphate hemihydrate (CaSO₄. ½H₂O). The two formula units of CaSO₄ share one molecule of water.

As a result, it is soft. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard solid mass.

⇒ \(\mathrm{CaSO}_4 \cdot \frac{1}{2} \mathrm{H}_2 \mathrm{O}+1 \frac{1}{2} \mathrm{H}_2 \mathrm{O}\)(Plaster of pairs )(Soft) (Sulphate salt) → \(\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}\) Gypsum(Hard mass)

Question 24. Identify the compound X based on the reactions given below. Also, write the name and chemical formulae ofA, B, and C.

Answer:

⇒ \(\underset{(\mathrm{X})}{2 \mathrm{NaOH}}+\mathrm{Zn} \longrightarrow \underset{(\mathrm{A})(Sodium zincate)}{\mathrm{Na}_2 \mathrm{ZnO}_2}+\mathrm{H}_2\)

⇒  \(\mathrm{NaOH}+\mathrm{HCl} \longrightarrow \underset{(\mathrm{B})(Sodium chloride)}{\mathrm{NaCl}} \quad+\mathrm{H}_2 \mathrm{O}\)

⇒  \(\mathrm{NaOH}+\mathrm{CH}_3 \mathrm{COOH} \longrightarrow \underset{\text { (C) Sodium acetate }}{\mathrm{CH}_3 \mathrm{COONa}}+\mathrm{H}_2 \mathrm{O}\)

Question 25. A metal carbonate (X) reacting with acid gives a gas which when passed through a solution (Y) gives the carbonate back. On the other hand, a gas (G) that is obtained at the anode during the electrolysis of brine is passed on dry substance (Y). It gives a compound (Z) used for disinfecting drinking water. Identify X, Y, G, and Z.
Answer:

Electrolysis of brine gives chlorine gas to the anode. It is gas G. This gas in reaction with slaked lime (Y) gives bleaching powder (Z) used in the disinfection of drinking water

Compound X is calcium carbonate which in reaction with acid gives calcium chloride and CO₂ gas. This CO₂ gas on reaction with Ca(OH)₂ gives compound (X) back

⇒ \(\underset{[\mathrm{X}]}{\mathrm{CaCO}_3}+\underset{\text { acid }}{2 \mathrm{HCl}} \longrightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \uparrow\)

⇒ \(\underset{[\text { Y] }}{\mathrm{Ca}(\mathrm{OH})_2}+\mathrm{CO}_2 \longrightarrow \underset{[\mathrm{X}]}{\mathrm{CaCO}_3}+\mathrm{H}_2 \mathrm{O}\)

⇒ \(\underset{[\mathrm{Y}]}{\mathrm{Ca}(\mathrm{OH})_2}+\underset{\text { obtained from brine solution }}{\mathrm{Cl}_2} \longrightarrow \mathrm{CaOCl}_2+\mathrm{H}_2 \mathrm{O}\)

Question 26. In the following schematic diagram for the preparation of hydrogen gas as shown in the figure, what would happen if the following changes are made?

1. In place of zinc granules, the same amount of zinc dust is taken in the test tube
2. Instead of dilute sulphuric acid, dilute hydrochloric acid is taken
3. In place of zinc, copper turnings are taken
4. Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.

Answer:

1. If zinc dust is taken, the reaction is faster because zinc dust has a larger surface area.
2. Both will react completely with the zinc to evolve the same amount of H₂ gas.
3. Copper does not react with dilute H₂SO₄ or dilute HCl. Hence, no reaction will occur.
4. Zinc also reacts with NaOH on heating to give out H₂ gas

Zn + 2NaOH → (Heat) →  Na₂ZnO₂ (Sodium zincate)+ H₂