How acids and bases react
Acids react with metals to give hydrogen, with carbonates to give carbon dioxide, and with bases or metal oxides to give a salt and water.
Reactions of acids and bases
| Reaction | General rule | Example |
|---|---|---|
| Acid + metal | Salt + hydrogen | Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) |
| Base + metal (only some metals) | Salt + hydrogen | 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g); the salt is sodium zincate |
| Acid + metal carbonate | Salt + carbon dioxide + water | Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) |
| Acid + metal hydrogencarbonate | Salt + carbon dioxide + water | NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) |
| Acid + base (neutralisation) | Salt + water | NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) |
| Acid + metal oxide | Salt + water | CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l); the solution turns blue-green |
| Base + non-metal oxide | Salt + water | Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) |
Activity: HCl and NaOH with litmus, zinc and sodium carbonate
- 1Litmus: add dilute HCl to blue litmus solution, and dilute NaOH to red litmus solution.HCl turns blue litmus red; NaOH turns red litmus blue
- 2Zinc: drop zinc granules into dilute HCl in one test tube and into NaOH solution in another. Warm the NaOH tube gently.Bubbles of gas in both; the NaOH reaction is slower and needs warming
- 3★Test the gas: bring a burning matchstick near the mouth of each tube.The gas burns with a pop sound, so it is hydrogen
- 4Sodium carbonate: add dilute HCl to a little solid sodium carbonate, and pass the gas into lime water.Brisk fizzing (effervescence); the lime water turns milky, so the gas is CO₂
- 5Add NaOH solution to solid sodium carbonate.No reaction: bases do not react with carbonates
★ key step
The lime water test for carbon dioxide
- Pass the gas through lime water: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The white calcium carbonate makes it milky.
- Pass excess CO₂: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq). The milkiness disappears, because calcium hydrogencarbonate dissolves in water.
- Limestone, chalk and marble are all forms of calcium carbonate, so they too fizz with acids.
Metal oxides vs non-metal oxides
Neutralisation: acid + base → salt + water. The acid and the base cancel out each other's effect.
How to draw the gas-testing set-ups
- Hydrogen (zinc and dilute sulphuric acid): draw a test tube in a stand with zinc granules and dilute H₂SO₄. Fit a cork with a delivery tube that dips into soap solution in a trough.
- Show soap bubbles filled with hydrogen, and a burning candle held near one bubble. Label: zinc granules, dilute sulphuric acid, delivery tube, soap solution, bubble filled with hydrogen, candle.
- Carbon dioxide (sodium carbonate and dilute HCl): draw a test tube with sodium carbonate and HCl, closed by a cork with a bent delivery tube.
- The delivery tube dips below the surface of lime water in a second test tube. Label: dilute HCl, sodium carbonate, delivery tube, lime water turning milky.
- Draw in pencil, keep the labels outside the drawing, and draw the label lines with a ruler.
Curd, pickles and tamarind rasam are not stored in brass or copper vessels, because their acids react with the metal. A pinch of baking soda fizzes with sour curd or lemon, and the CO₂ makes dhokla soft and spongy.
1 marks: “Name the gas given off when zinc is warmed with sodium hydroxide solution.” Hydrogen (sodium zincate also forms).
2 marks: “Lemon pickle and tamarind are never stored in brass or copper vessels. Give the chemical reason.” They contain acids. The acids react with the metal and form harmful compounds that spoil the food.
3 marks: “Dilute hydrochloric acid is poured on marble chips. (i) What do you see? (ii) Name the gas and give a test for it. (iii) Write the balanced equation.” (i) Brisk effervescence (fizzing). (ii) Carbon dioxide; it turns lime water milky. (iii) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).
Metal gives Hydrogen, Carbonate gives CO₂, Base gives Water, and every acid reaction gives a salt.