Acids, Bases and Salts • Topic 1 of 3

Acids, Bases & their Reactions

An acid is a substance that tastes sour and turns blue litmus red; a base tastes bitter, feels soapy and turns red litmus blue. The reason for this behaviour is hidden in water. When an acid dissolves in water it produces hydrogen ions, which exist as the hydronium ion H3O+; a base that dissolves in water (an alkali) produces hydroxide ions, OH−. So HCl → H+ + Cl− and NaOH → Na+ + OH−.

Why water is essential

The H+ ion cannot exist alone; it attaches to a water molecule to form H+ + H2O → H3O+. A dry acid such as glass-bottled HCl gas shows no acidic character — it only becomes acidic when dissolved in water, because only then are H+ ions released. Dissolving acids in water is highly exothermic, so the rule is always add acid to water slowly with stirring, never water to acid, or the mixture may spurt out and cause burns.

Reactions of acids

  • With metals: acid + metal → salt + hydrogen gas. Example: Zn + 2HCl → ZnCl2 + H2. The gas burns with a pop sound.
  • With metal carbonates and hydrogencarbonates: these give salt, water and carbon dioxide. Example: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. The CO2 turns lime water milky.
  • With bases (neutralisation): acid + base → salt + water, e.g. NaOH + HCl → NaCl + H2O.
  • With metal oxides: a metal oxide is basic, so it neutralises an acid: CuO + 2HCl → CuCl2 + H2O (a blue-green solution forms).

Reactions of bases

Reactive metals such as zinc and aluminium also react with strong alkalis to give hydrogen: 2NaOH + Zn → Na2ZnO2 + H2. A base reacting with a non-metal oxide (which is acidic) again gives salt and water, e.g. 2NaOH + CO2 → Na2CO3 + H2O.

Neutralisation and conduction

The essence of every neutralisation is H+ + OH− → H2O. Because acids and alkalis contain free moving ions, their aqueous solutions conduct electricity; a glucose or alcohol solution has no ions and does not conduct, which proves that it is the H+ and OH− ions, not the molecules, that carry the current.

Neutralisation: H+ from the acid combines with OH- from the base to form waterACIDgives H⁺BASEgives OH⁻reactNEUTRALISATIONH⁺ + OH⁻ → H₂OAcid + Base → Salt + Water
Solved Examples
1
Worked Example
Write the balanced equation for the reaction of zinc with dilute sulphuric acid and name the gas produced.
Solution
  1. An acid reacts with a metal to give a salt and hydrogen gas.
  2. Zinc forms zinc sulphate with H2SO4.
  3. Balance: Zn + H2SO4 → ZnSO4 + H2.

Answer: Zn + H2SO4 → ZnSO4 + H2; the gas is hydrogen, which burns with a pop sound.

2
Worked Example
When dilute HCl is added to sodium carbonate, a gas is evolved that turns lime water milky. Identify the gas and write the equation.
Solution
  1. Acid + metal carbonate → salt + water + carbon dioxide.
  2. The gas that turns lime water milky is CO2.
  3. Na2CO3 + 2HCl → 2NaCl + H2O + CO2.

Answer: The gas is carbon dioxide (CO2).

3
Worked Example
Why does dry HCl gas not change the colour of dry blue litmus paper, while its water solution does?
Solution
  1. Acidic behaviour is caused by H+ (H3O+) ions.
  2. Dry HCl gas does not ionise, so no H+ ions are present.
  3. When dissolved in water it ionises: HCl → H+ + Cl−, releasing H+ ions.

Answer: Acidity needs free H+ ions, which form only in water; dry HCl has none, so dry litmus does not change.

4
Worked Example
Write the neutralisation reaction between sodium hydroxide and hydrochloric acid and give the net ionic equation.
Solution
  1. Acid + base → salt + water.
  2. Molecular: NaOH + HCl → NaCl + H2O.
  3. Removing the spectator ions Na+ and Cl− leaves the net ionic form.

Answer: NaOH + HCl → NaCl + H2O; net ionic: H+ + OH− → H2O.

5
Worked Example
A glucose solution does not light a bulb in a conductivity test, but a dilute HCl solution does. Explain.
Solution
  1. Conduction needs free moving charged particles (ions).
  2. HCl dissolves to give H+ and Cl− ions, which carry the current, so the bulb glows.
  3. Glucose dissolves as neutral molecules and produces no ions, so no current flows.

Answer: Acids give ions in water and conduct electricity; glucose gives no ions, so it cannot conduct.

6
Worked Example
Why should water never be added to a concentrated acid?
Solution
  1. Mixing acid and water releases a large amount of heat (the process is highly exothermic).
  2. If water is added to acid, the small amount of water boils suddenly and the acid may splash out.
  3. Adding acid slowly to a large volume of water spreads the heat safely.

Answer: Acid is always added to water (not water to acid) to avoid violent boiling and acid splashing.

Key Points

  • Acids give H+ (as H3O+) ions in water; bases that dissolve (alkalis) give OH- ions; water is needed for acidic/basic behaviour.
  • Acid + metal gives salt + hydrogen; acid + metal carbonate/hydrogencarbonate gives salt + water + carbon dioxide.
  • Neutralisation: acid + base gives salt + water, with the net reaction H+ + OH- giving H2O.
  • Metal oxides are basic and non-metal oxides are acidic, so each neutralises the opposite type.
  • Acid and alkali solutions conduct electricity because they contain free moving ions; glucose/alcohol solutions do not.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Acids produce which ion when dissolved in water?
Explanation: Acids release hydrogen ions, which exist in water as the hydronium ion H3O+.
Q2.When dilute HCl reacts with a metal, the gas evolved is:
Explanation: Acid + metal gives salt + hydrogen; the gas burns with a characteristic pop sound.
Q3.The gas released when an acid reacts with a metal carbonate turns:
Explanation: The gas is CO2, which turns lime water milky by forming calcium carbonate.
Q4.Acidic and basic solutions conduct electricity because they contain:
Explanation: Free H+ and OH- ions carry the current; solutions without ions (like glucose) do not conduct.
Practice more MCQs → 📝 Assignment · 30 marks