Acids, Bases and Salts
Subject: General Science
Sample Practice Questions
Question 1: What is the colour of hydrated copper sulphate crystals?
- White
- Blue
- Green
- Colourless
Explanation: Hydrated copper sulphate (CuSO4·5H2O) is blue in colour. The water of crystallisation gives it this characteristic blue colour.
Question 2: What happens when Plaster of Paris is mixed with water?
- It forms a hard solid mass
- It dissolves completely
- It evolves hydrogen gas
- It becomes a gas
Explanation: When Plaster of Paris is mixed with water, it changes to gypsum once again giving a hard solid mass. This is why it is used for supporting fractured bones.
Question 3: Sodium hydroxide reacts with zinc to produce hydrogen gas and which other compound?
- Sodium oxide
- Sodium zincate
- Zinc oxide
- Zinc hydroxide
Explanation: The reaction is: 2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g). Sodium zincate (Na2ZnO2) is formed along with hydrogen gas.
Question 4: Which of the following salts is formed from a strong acid and a weak base?
- Sodium chloride
- Potassium nitrate
- Aluminium chloride
- Sodium acetate
Explanation: Aluminium chloride is a salt of a strong acid (HCl) and a weak base (Al(OH)3). Such salts are acidic in nature with pH less than 7.
Question 5: Which substance is used for softening hard water?
- Baking soda
- Washing soda
- Bleaching powder
- Plaster of Paris
Explanation: Sodium carbonate (washing soda) is used for removing permanent hardness of water.
Question 6: What is the gas produced when zinc granules react with dilute sulphuric acid?
- Oxygen
- Hydrogen
- Carbon dioxide
- Chlorine
Explanation: When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved. The reaction is: Zn + H2SO4 → ZnSO4 + H2. This can be tested by bringing a burning candle near a soap bubble filled with the gas - it burns with a pop sound.
Question 7: When sodium carbonate reacts with dilute hydrochloric acid, which gas is produced?
- Hydrogen
- Oxygen
- Carbon dioxide
- Chlorine
Explanation: Metal carbonates and hydrogencarbonates react with acids to produce carbon dioxide gas. The reaction is: Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g).
Question 8: What is observed when carbon dioxide gas is passed through lime water?
- No change
- A white precipitate forms
- The solution turns blue
- Effervescence occurs
Explanation: Lime water (calcium hydroxide solution) turns milky when CO2 is passed through it due to the formation of a white precipitate of calcium carbonate: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l).
Question 9: Which mild base is used as an antacid to neutralise excess stomach acid?
- Sodium hydroxide
- Magnesium hydroxide
- Calcium hydroxide
- Potassium hydroxide
Explanation: Magnesium hydroxide (Milk of magnesia) is a mild base often used as an antacid to neutralise excess acid in the stomach.
Question 10: What is the chemical formula of bleaching powder?
- CaOCl2
- Ca(ClO)2
- NaClO
- CaCl2
Explanation: Bleaching powder is represented as Ca(ClO)2, though its actual composition is complex. It is produced by the action of chlorine on dry slaked lime.