Acids, Bases and Salts: 10th Science

Question: Write the equation: involved, When:
(i) chlorine is passed over dry slaked lime.
(ii) sodium bicarbonate reacts with dilute hydrochloric acid.
(iii) sodium bicarbonate is heated.

Answer: (i) Ca(OH)₂ + Cl₂ → CaOCl₂+ H₂O

(ii) NaHCO₃ + Dil.HCl → NaCl + CO₂ + H₂O

Question: (1) What is universal indicator?
(2) Write the chemical equation involved in the preparation of sodium hydroxide. Name the process.

Answer:

1. It is a mixture of several indicators, shows different colours at different concentrations of hydrogen ions in a solution.
2. Chlor-alkali process.
   2NaCl (aq) + 2H₂O (l) → 2NaOH(aq) + Cl₂ (g) + H₂ (g)

Question: Name the three products of ‘chlor – alkali process’. Write one commercially or industrially important material each that can be prepared from each of these products.

Answer: The three products are: Sodium hydroxide (NaOH), chlorine (Cl₂), and hydrogen (H₂).
NaOH – for soaps and detergents / paper making or artificial fibres.
Cl₂ – PVC, pesticides, CFCs.
H₂ – fuels, margarine, NH₃ for fertilisers.

Question: “Sodium hydrogen carbonate is a basic salt”. Justify the statement. How is it converted into washing soda? Explain.

Answer: It is a salt produced by the neutralisation reaction between a strong base (NaOH) and a weak acid (H₂CO₃), hence it is a basic salt.
It is heated strongly to produced sodium carbonate.

Sodium carbonate is recrystallised to produce washing soda.

Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O

Question: Write balanced chemical equations for the following:
(i) Bleaching powder is kept open in air.
(ii) Blue crystals of copper sulphate are heated.
(iii) Chlorine gas is passed through dry slaked lime.
(iv) Carbon dioxide gas is passed through lime water.
(v) NaOH solution is heated with zinc granules.

Answer:

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

Answer:

1. Vigorous reaction will be seen in test tube A.
2. It is because H₂SO₄ is stronger acid than H₂CO₃.
3. (i) Hydrogen gas is liberated in both the test tubes.
   (ii) 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 → MgC₃ + H₂
5. H₂SO₄ has lower pH value and H₂CO₃ has lower H⁺ concentration.

Question: How do metal carbonates and metal hydrogen carbonates react with acids? Give their chemical equations. Name the gas 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. (i) Metal carbonate: Na₂CO₃(s) + 2HCl(aq) → NaCl(aq) + H₂O (l) + CO₂(g)
   (ii) Metal hydrogen carbonate: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O (l) + CO₂(g)
3. Gas evolved is carbon dioxide (CO₂).
4. When this gas is passed through lime water, it turns milky.

Question: Write chemical equation when zinc granules react with:
(1) Sulphuric acid
(2) hydrochloric acid
(3) aluminium chloride
(4) sodium hydroxide
(5) nitric acid

Answer:

1. Zn + H₂SO₄ → ZnSO₄ + H₂
2. Zn + 2HCl → ZnCl₂ + H₂
3. Zn AlCl₃ → ZnCl₃ + Al
4. Zn + 2NaOH → Na₂ZnO₂ + H₂
5. 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₂ + 3H₂O

Question: The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B present in it is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to power C, it becomes blue again.
(a) Identify A, B, C, D.
(b) Write the chemical equations involved.
(c) Give an example of the salt which also shows above property.

Answer: (a) A: Hydrate copper sulphate
B: Water of crystallization
C: Anhydrous copper sulphate
D: Water

(c) One other salt which also shows the same property is gypsum (CaSO₄. 5H₂O).