Acids and indicators
Introduced| English | Meaning |
|---|---|
| acid/ˈæsɪd/ | A proton donor; in water, it increases the hydrogen-ion concentration. |
| indicator/ˈɪndɪkeɪtə/ | A substance that shows a change in chemical conditions through an observable change, often in colour. |
| hydrogen ions/ˈhaɪdrədʒn ˈaɪɒnz/ | hydrogen ions |
| litmus/ˈlɪtməs/ | An indicator that is red in acidic conditions and blue in alkaline conditions. |
| universal indicator/ˌjuːnɪˈvɜːsl ˈɪndɪkeɪtə/ | A mixture of indicators that shows different colours over a range of pH values. |
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?
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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?
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Litmus is red in acid, blue in alkali.
A substance that changes colour to show acidity is an ______.
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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:
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Acid + metal → salt + hydrogen.
Acid + carbonate produces a salt and hydrogen.
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False. Acid + carbonate gives a salt, water and carbon dioxide. Hydrogen comes from acid + a reactive metal.
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
Write from an acidic solution of pH 2
- A solution turns universal indicator red.
- Its pH is 2. Neutral in the next lesson is pH 7.
Is the solution acidic, neutral or alkaline?
pH below 7 is acidic.
What is a solution of pH 2?
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Red universal indicator and a low pH mean acid.