How acids and bases react

Acid + metal → salt + hydrogen. Drop zinc granules into dilute sulphuric acid and bubbles appear.

Zn + H₂SO₄ → ZnSO₄ + H₂

Bring a burning candle near the gas: it burns with a pop. In a question, "burns with a pop" is the hint that the gas is hydrogen.

Some metals react with bases too. Zinc with sodium hydroxide also gives hydrogen, and a salt called sodium zincate:

2NaOH + Zn → Na₂ZnO₂ + H₂

But not every metal does this.

Acid + metal carbonate (or hydrogencarbonate) → salt + water + carbon dioxide.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Pass the gas through lime water and it turns milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. "Turns lime water milky" is the hint for carbon dioxide. Keep passing gas for too long and the milkiness clears again, because excess CO₂ makes a salt that dissolves.

Acid + base → salt + water. This is neutralisation: the acid and the base cancel each other's effect.

NaOH + HCl → NaCl + H₂O

A few drops of phenolphthalein make sodium hydroxide pink. Add acid drop by drop and the pink disappears when the base is used up. Add a drop of base and the pink comes back.

Copper oxide is black. Stir it into dilute hydrochloric acid and the solution turns blue-green, because copper chloride forms:

CuO + 2HCl → CuCl₂ + H₂O

A metal oxide reacts like a base, so metal oxides are basic. Non-metal oxides, such as carbon dioxide, react with bases, so non-metal oxides are acidic.