What are acids and bases?
Acids taste sour (lemon, tamarind, vinegar) and turn blue litmus red. Bases taste bitter and feel soapy; they turn red litmus blue. Never taste lab chemicals: we learn these properties from food only.
Everyday acids and bases
- Acids in nature: citric acid (lemon, orange), lactic acid (curd), acetic acid (vinegar), tartaric acid (tamarind), methanoic acid (ant and bee sting, nettle leaf), oxalic acid (tomato).
- Mineral acids used in labs: hydrochloric acid HCl, sulphuric acid H₂SO₄, nitric acid HNO₃.
- Bases: sodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)₂ (lime water), magnesium hydroxide Mg(OH)₂ (milk of magnesia), ammonium hydroxide NH₄OH.
A base that dissolves in water is called an alkali (NaOH, KOH, Ca(OH)₂, NH₄OH). All alkalis are bases, but not all bases are alkalis: copper oxide is a base that does not dissolve.
Indicators: natural, synthetic and olfactory
An indicator is a substance that changes colour (or smell) to tell us whether a solution is acidic or basic.
| Indicator | In acid | In base |
|---|---|---|
| Litmus (from lichen) | blue → red | red → blue |
| Phenolphthalein | colourless | pink |
| Methyl orange | red | yellow |
| Turmeric | stays yellow | reddish-brown |
| Red cabbage juice | red/pink | green |
Olfactory indicators
Some substances change their smell. Onion and clove oil lose their smell in a base; vanilla essence also loses its smell in a base but keeps it in an acid. These help students who cannot see colours.
A yellow turmeric curry stain turns reddish-brown when washed with soap, because soap is basic. Wash it again with plain water and the yellow slowly comes back.
Chemical properties of acids and bases
1. Acid + metal → salt + hydrogen gas
Zn + H₂SO₄ → ZnSO₄ + H₂↑. The gas burns with a pop sound near a burning splint: that is the test for hydrogen.
2. Base + metal (some metals only)
2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂↑. Not every metal does this.
3. Acid + metal carbonate / hydrogencarbonate → salt + water + carbon dioxide
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ and NaHCO₃ + HCl → NaCl + H₂O + CO₂↑. CO₂ turns lime water milky (CaCO₃ forms); too much CO₂ makes it clear again (soluble Ca(HCO₃)₂).
4. Acid + base → salt + water (neutralisation)
NaOH + HCl → NaCl + H₂O. The heat given out shows it is exothermic.
5. Acid + metal oxide → salt + water
CuO + 2HCl → CuCl₂ + H₂O. The black copper oxide dissolves and the solution turns blue-green. So metal oxides are basic.
6. Base + non-metal oxide → salt + water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. So non-metal oxides are acidic.
What do all acids and all bases have in common?
Glucose and alcohol also contain hydrogen, but they are not acids. The difference: an acid gives H⁺ ions in water. H⁺ cannot stay alone, so it joins a water molecule to form the hydronium ion H₃O⁺: HCl + H₂O → H₃O⁺ + Cl⁻.
A base gives OH⁻ ions in water: NaOH → Na⁺ + OH⁻.
Proof with a circuit: a bulb glows when dilute HCl or H₂SO₄ is placed between two electrodes, but not with glucose or alcohol solution. Ions carry the current.
Why water is needed: dry HCl gas does not change dry blue litmus, but moist litmus turns red, because H⁺ ions form only in water.
Diluting acids safely
Mixing an acid or base with water is strongly exothermic. Always add acid to water, slowly and with stirring, never water to acid: the heat can make the mixture splash and even crack the glass.
Neutralisation in terms of ions
H⁺ + OH⁻ → H₂O. The other ions (Na⁺, Cl⁻) stay behind as the salt.
Key formulas and definitions
- Acid in water: HCl + H₂O → H₃O⁺ + Cl⁻
- Base in water: NaOH → Na⁺ + OH⁻
- Acid + metal → salt + H₂
- Acid + carbonate/hydrogencarbonate → salt + H₂O + CO₂
- Acid + base → salt + water (neutralisation): H⁺ + OH⁻ → H₂O
- Metal oxide + acid → salt + water (metal oxides are basic)
- Non-metal oxide + base → salt + water (non-metal oxides are acidic)
Worked examples
1. Zinc granules are added to dilute sulphuric acid. What gas is formed and how will you test it?
Zn + H₂SO₄ → ZnSO₄ + H₂. The gas is hydrogen. Bring a burning splint near the mouth of the tube: the gas burns with a pop sound.
2. Why does curd or pickle spoil the taste in a brass or copper vessel?
Curd and pickle contain acids. Acids react with metals like copper and zinc (in brass) to form salts that are harmful and taste bad. So they are kept in glass, steel or ceramic vessels.
3. Metal compound A reacts with dilute HCl to give a gas that turns lime water milky. The solution left contains calcium chloride. Identify A and write the equation.
The gas is CO₂, so A is a carbonate; the salt is CaCl₂, so the metal is calcium. A = CaCO₃. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
4. A copper oxide sample is black. Dilute HCl is added. What do you see and what does it prove?
The black powder dissolves and the solution turns blue-green (CuCl₂). CuO + 2HCl → CuCl₂ + H₂O. Since a metal oxide reacts with an acid to give salt and water, metal oxides are basic.
5. Why does dry HCl gas not turn dry blue litmus red?
Acid behaviour comes from H⁺ ions, and HCl gives H⁺ only in water. With no water, there are no H⁺ ions, so no colour change. Moist litmus turns red.
Common mistakes
- Saying every compound with hydrogen is an acid. Glucose (C₆H₁₂O₆) and alcohol have hydrogen but give no H⁺ ions in water.
- Adding water to concentrated acid. Always add acid slowly to water, with stirring.
- Thinking all bases are alkalis. Only bases that dissolve in water are alkalis.
- Writing that phenolphthalein turns pink in acid. It is colourless in acid and neutral solutions, pink only in bases.