Acids produce H+ ions in aqueous solution.
HCl  →  H⁺ + Cl⁻

Acid solutions have pH values less than 7. The lower the pH, the stronger the acid.

Strong Acids
A strong acid is one, which can completely dissociate its H+ ions in water. This means that it can give off the greatest number of hydrogen ions when placed in solution.
e.g. HCl, H₂SO₄

Weak Acids
A weak acid is one, which does not completely dissociate its H+ ions in water. This means that it does not give off many hydrogen ions when placed in solution.
e.g. ethanoic acid, citric acid


ACID REACTIONS

Acid + Metals
Acids react with a metal to form a salt and hydrogen gas. This is a redox reaction.

Acid + Metal  →  Salt + Hydrogen gas
E.g. 2HCl + Fe  →  FeCl₂ + H₂

Hydrogen’s oxidation number in HCl = +I
Hydrogen’s oxidation number in H₂ = 0
Since Hydrogen gains electrons, it is reduced.

Iron’s oxidation number in Fe = 0
Iron’s oxidation number in FeCl₂ = +2
Since Iron loses electrons, it is oxidised.

Since reduction and oxidation occurs at the same time, this reaction is a redox reaction.

Acid + Metal oxides
Acids react with a metal oxides to form a salt and water.
Acid + Metal Oxide  →  Salt + Water
E.g. 2HCl + CaO -> CaCl₂ + H₂O

Acid + Alkali
Acids react with alkali to form a salt and water.
Acid + Alkali  →  Salt + Water
E.g. HCl + NaOH -> NaCl + H₂O
This is a neutralisation reaction.

Acid + Carbonate
Acids react with carbontes to form a salt, water and carbon dioxide.
Acid + Carbonate  →  Salt + Water + Carbon dioxide
E.g. 2HCl + CaCO₃ -> CaCl₂ + H₂O + CO₂