What Is an Acid?
An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. The more hydrogen ions it produces, the stronger the acid.
Acids are found in everyday life:
- Citric acid — in lemons and oranges
- Acetic acid (ethanoic acid) — in vinegar
- Carbonic acid — in fizzy drinks
- Hydrochloric acid (HCl) — in your stomach (gastric acid)
- Sulfuric acid (H₂SO₄) — used in car batteries and industry
Physical Properties of Acids
- Often colourless liquids when dissolved in water
- Taste sour (though you should never taste chemicals in a lab)
- Turn blue litmus paper red
- Have a pH below 7
- Strong acids can be highly corrosive and dangerous
Strong acids such as sulfuric acid, hydrochloric acid, and nitric acid are corrosive — they can cause severe burns on skin and destroy materials. Always wear safety goggles and handle with care in a laboratory. Dilute solutions are safer but still require careful handling.
Chemical Reactions of Acids
1. Acids + Metals → Salt + Hydrogen
zinc + hydrochloric acid → zinc chloride + hydrogen Mg + H₂SO₄ → MgSO₄ + H₂
You can test for hydrogen gas: it produces a squeaky pop when a lit splint is held to the gas.
2. Acids + Metal Carbonates → Salt + Water + Carbon Dioxide
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Carbon dioxide turns limewater milky — this is the standard test for CO₂.
3. Acids + Metal Oxides → Salt + Water
copper oxide + sulfuric acid → copper sulfate + water
4. Acids + Alkalis (Bases) → Salt + Water (Neutralisation)
hydrochloric acid + sodium hydroxide → sodium chloride + water
The name of the salt produced depends on the acid used:
- Hydrochloric acid (HCl) → produces chloride salts (e.g. sodium chloride)
- Sulfuric acid (H₂SO₄) → produces sulfate salts (e.g. copper sulfate)
- Nitric acid (HNO₃) → produces nitrate salts (e.g. potassium nitrate)
Strong vs Weak Acids
Strong acids (e.g. hydrochloric acid, sulfuric acid, nitric acid) dissociate (split) completely in water, producing many H⁺ ions.
Weak acids (e.g. citric acid, ethanoic acid/vinegar) only partially dissociate — only some of the molecules produce H⁺ ions. They are less corrosive.
Lemon juice (pH ~2) is a weak acid but still tastes noticeably sour. Hydrochloric acid at the same concentration would be far more reactive and dangerous.
Real world: Stomach acid digesting food
Your stomach produces hydrochloric acid (HCl) at pH 1–2 — one of the strongest acids in your body. This acid kills bacteria in food, activates digestive enzymes, and begins breaking down proteins. The stomach lining produces a mucus layer to protect itself from its own acid. When this protection fails, stomach acid attacks the lining — causing the pain of a peptic ulcer. Antacid medicines contain bases that neutralise the excess acid.
Real world: Sulfuric acid in car batteries
A car's lead-acid battery contains concentrated sulfuric acid (H₂SO₄, about pH 1). When the car starts, a chemical reaction between the lead plates and the acid produces an electric current. The acid in old batteries can cause severe burns if it leaks — which is why garages handle old batteries carefully and wear PPE. The same sulfuric acid is also used in industrial chemical manufacturing on a massive scale.
Real world: Citric acid in food preservation
Citric acid (found naturally in lemons and limes, pH 2–3) is widely used as a food additive (E330) to preserve tinned fruits, flavour soft drinks, and prevent browning in fruit juices. The low pH it creates inhibits bacterial and mould growth. It is a weak acid — safe to consume in food quantities, unlike strong industrial acids — but it still reacts with carbonates: lemon juice on baking soda is a classic fizzing reaction (CO₂ is released).
Where do you see this in real life? Vinegar on chips (ethanoic acid), fizzy drinks that taste sour (carbonic acid + citric acid), antacids in a medicine cabinet (neutralising stomach HCl), car battery warning labels (sulfuric acid) — acids are everywhere, from your dinner to your engine.
Practice Problems
A gas is produced when dilute hydrochloric acid is added to a piece of zinc. How would you test to confirm the identity of the gas?
Show answer
Why do fizzy drinks taste slightly sour?