Stop 10 of 12 · The path
Acids & bases — sour and soapy
Acids give H⁺, bases take it. pH 0–14 reports how much: below 7 acidic, 7 neutral, above 7 basic. Meet in the middle: salt + water.
Bench tool
The pH slider
Drag across the scale:
Lemon ~2, rain ~5.6, pure water 7, soap ~10, bleach ~13.
Bench tool
Neutralise the acid
A flask of HCl waits. Slide to drip in NaOH and watch the pH cross 7:
HCl + NaOH → NaCl + H₂O
How it works
- Acids release H⁺ in water. HCl → H⁺ + Cl⁻. Sour, turn litmus red.
- Bases release OH⁻ (or grab H⁺). Soapy, turn litmus blue.
- Neutralisation: H⁺ + OH⁻ → H₂O. Plus a salt: HCl + NaOH → NaCl + H₂O.
Worked examples
Stomach acid + antacid. Excess HCl met by Mg(OH)₂ → MgCl₂ + water. Burn eases.
pH 3 vs pH 5. Each step is ×10: pH 3 is 100× more acidic than pH 5.
Vinegar + baking soda. Acid + carbonate → CO₂ fizz. Same gas as breath.
Where it goes wrong
Strong ≠ concentrated. Strong means fully split; concentrated means lots per litre. Dilute strong acid still burns less.
Try it at home: cabbage rainbow
Kitchen indicator. Soak red cabbage in adult-poured hot water and strain the purple liquid. Vinegar turns it pink; bicarbonate solution turns it green-blue — acids and bases push indicators opposite ways.
For teachers
Learners think all acids burn and all bases are safe — probe with vinegar vs oven cleaner. Quickest check: ask what acid + base always makes (salt + water), plus one example.