Acids, Bases and Salts
In one line
Acids taste sour and turn blue litmus red, bases feel soapy and turn red litmus blue, and when they react together they make a salt and water.
Key points
- Acids give H⁺ ions in water. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), vinegar (acetic acid), lemon (citric acid).
- Bases give OH⁻ ions. Bases that dissolve in water are called alkalis, like sodium hydroxide (NaOH).
- Acid + base → salt + water. This is neutralisation.
- Acid + metal → salt + hydrogen gas. Hydrogen burns with a pop sound.
- Acid + metal carbonate → salt + water + carbon dioxide. CO₂ turns lime water milky.
- Always add acid to water, slowly, never water to acid. The mixing gives out a lot of heat.
Indicators
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue → red | Red → blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Stays yellow | Reddish brown |
The pH scale
Strong acidNeutral (7)Strong base
- pH below 7 is acidic, 7 is neutral (pure water), above 7 is basic.
- Tooth decay starts when the pH in the mouth falls below 5.5. Toothpaste is basic and neutralises the acid.
- Antacids such as milk of magnesia (a mild base) neutralise extra stomach acid.
Advertisement
Important salts
| Common name | Formula | Used for |
|---|---|---|
| Common salt | NaCl | Food; raw material for many chemicals |
| Baking soda | NaHCO₃ | Baking (releases CO₂ to make cakes rise), antacids |
| Washing soda | Na₂CO₃·10H₂O | Cleaning, removing hardness of water |
| Bleaching powder | CaOCl₂ | Disinfecting drinking water, bleaching cloth |
| Plaster of Paris | CaSO₄·½H₂O | Plaster for broken bones, statues |
Common mistakes
- "All bases are alkalis." Only bases that dissolve in water are alkalis.
- "Lower pH means weaker acid." It's the other way round. pH 1 is far more acidic than pH 5.
- "Salts are always neutral." Some salts give acidic or basic solutions, depending on the acid and base they come from.
Quick check
Revise in 30 seconds
| Acid | Sour, H⁺ ions, blue litmus → red, pH < 7 |
|---|---|
| Base | Soapy, OH⁻ ions, red litmus → blue, pH > 7 |
| Neutralisation | Acid + base → salt + water |
| Acid + metal | Salt + H₂ (pop test) |
| Acid + carbonate | Salt + water + CO₂ (lime water milky) |
| Salts | NaHCO₃ baking · Na₂CO₃·10H₂O washing · CaOCl₂ bleaching · CaSO₄·½H₂O PoP |