Acids and indicators
| English | Chinese | Pinyin |
|---|---|---|
| acid | 酸 | suān |
| indicator | 指示剂 | zhǐ shì jì |
| hydrogen ions | 氢离子 | qīng lí zi |
| litmus | 石蕊 | shí ruǐ |
| universal indicator | 通用指示剂 | tōng yòng zhǐ shì jì |
The chemistry of sourness
- Lemon juice, vinegar and stomach acid 酸 all share a sharp, sour character.
- They are acids, and we detect them with indicators 指示剂.
An acid produces which ion in water?
Acids release H⁺ ions in water.
What acids are
- An acid is a substance that forms hydrogen ions 氢离子 (H⁺) in water.
- Acids are sour, and common ones are hydrochloric, sulfuric and nitric acid.
Litmus 石蕊 and indicators show whether a solution is acidic or alkaline
The pH scale
Slide the pH or tap a substance — lemon, water, soap, bleach. The indicator goes red→green→purple, and each step down in pH is ten times more H⁺ ions.
Litmus turns which colour in an acid?
Litmus is red in acid, blue in alkali.
A substance that changes colour to show acidity is an ______.
Litmus and universal indicator are examples.
Indicators
- An indicator changes colour to show whether a substance is acidic or alkaline.
- Litmus turns red in acid; universal indicator 通用指示剂 shows a range of colours.
Litmus paper: blue turns red in acid, red turns blue in alkali
Acid + metal produces a salt and:
Acid + metal → salt + hydrogen.
Acid + carbonate produces a salt, water and carbon dioxide.
Carbonates release CO₂ with acids.
Reactions of acids
- Acid + metal → salt + hydrogen.
- Acid + base/carbonate → salt + water (+ carbon dioxide from carbonates).
Worked example. Adding hydrochloric acid to magnesium gives magnesium chloride + hydrogen gas (it fizzes). The fizzing is the hydrogen being released — a classic sign of acid + metal.
You've got it
- an acid produces H⁺ ions in water and tastes sour
- indicators (litmus, universal indicator) reveal acids and alkalis by colour
- acid + metal → salt + hydrogen; acid + carbonate → salt + water + CO₂