Properties of acids and bases

Which gas comes off?

The book lists three typical reactions of acids. First, with alkalis or metal oxides they make a salt and water: nitric acid and calcium oxide give calcium nitrate and water.

Second, with reactive metals such as magnesium and zinc they make a salt and give off hydrogen gas: Mg + 2HCl gives MgCl₂ + H₂. Less reactive metals (copper, silver, gold, platinum) give no hydrogen. Bases do not react with metals this way.

Third, acids break down carbonates and hydrogen carbonates, giving off carbon dioxide: CaCO₃ + 2HCl gives CaCl₂ + H₂O + CO₂.

Alkalis neutralize acids to give a salt and water, and they also react with ammonium salts to release ammonia gas: Ca(OH)₂ + 2NH₄Cl gives CaCl₂ + 2H₂O + 2NH₃. Not every base dissolves: Ca(OH)₂ is only sparingly soluble and Cu(OH)₂ is insoluble.

Metal plus acid gives hydrogen; carbonate plus acid gives carbon dioxide; ammonium salt plus alkali gives ammonia.

Key terms

Reactive metal

Testing acids.

Carbonate

Fizzing test.

Ammonium salt

Testing bases.

Sparingly soluble

Describing Ca(OH)₂.

Short questions with model answers

1. Q1. How does sulphuric acid react with MgO and with MgCO₃? (Descriptive question iii, p. 104)

   • MgO is a metal oxide
   • MgCO₃ is a carbonate

   Both give magnesium sulphate and water; only MgCO₃ also fizzes with carbon dioxide.

2. Q2. A gas that turns red litmus blue is produced. Which reaction was it? (Exercise x, p. 103)

   • Basic gas
   • Source of ammonia

   An alkali reacting with an ammonium salt.

Common mistakes

• ✗ Every metal gives hydrogen with an acid.

  ✓ Cu, Ag, Au and Pt give no hydrogen (p. 100).

• ✗ Acid with a carbonate gives hydrogen.

  ✓ It gives carbon dioxide (p. 101).

• ✗ Neutralization always gives off a gas.

  ✓ Acid + alkali gives only a salt and water.

MCQs

1. 1. Main characteristics of metals reacting with acids: (Exercise iii, p. 103)

   1. (a) Metals dissolve
   2. (b) Metals become salts
   3. (c) Hydrogen is evolved
   4. (d) All of these
   Show answer

   (d) p. 100.

2. 2. While baking, which gas raises the bread and makes it soft? (Exercise ii, p. 103)

   1. (a) Oxygen
   2. (b) Carbon dioxide
   3. (c) Nitrogen
   4. (d) Carbon monoxide
   Show answer

   (b) Hydrogen carbonates give CO₂ (p. 101).

3. 3. Which metal does NOT give hydrogen with dilute acid?

   1. (a) Mg
   2. (b) Zn
   3. (c) Cu
   4. (d) None of these
   Show answer

   (c) p. 100.

4. 4. How many KOH react with one H₃PO₄ for full neutralization? (Exercise v, p. 103)

   1. (a) 1
   2. (b) 2
   3. (c) 3
   4. (d) 4
   Show answer

   (c) H₃PO₄ has three H⁺; each OH⁻ cancels one.

5. 5. NaOH + NH₄NO₃ gives NaNO₃, water and:

   1. (a) H₂
   2. (b) CO₂
   3. (c) NH₃
   4. (d) O₂
   Show answer

   (c) p. 101.

Quick revision

• Acid + alkali or metal oxide gives salt + water.
• Acid + reactive metal gives salt + hydrogen; Cu, Ag, Au, Pt do not.
• Acid + carbonate or hydrogen carbonate gives salt, water and CO₂.
• Alkali + ammonium salt gives ammonia gas.