Common Alkalis and Indicators
FoundationDrain cleaner contains sodium hydroxide and window cleaner contains ammonia — alkalis are everywhere too. Litmus turns red in acid and blue in alkali, phenolphthalein turns pink in alkali, turmeric turns red-brown in alkali.
- Common alkalis: sodium hydroxide (drain cleaner), ammonia solution (window cleaner), calcium hydroxide (limewater).
- Alkali solutions are bitter, feel slippery and have a pH above 7.
- Indicator rules: litmus — red acid, blue alkali; phenolphthalein — pink in alkali; turmeric — red-brown in alkali.
文體小知識：科普知識文章用生活實例解釋科學概念，先看現象再講原理，讀完就能解釋日常見到的東西。
§Text Passage
1The drain cleaner in your kitchen is labelled caustic soda, and the window cleaner lists ammonia solution among its ingredients — alkalis, like acids, have long been part of daily life. Alkali solutions taste bitter, feel like soapy water, and have a pH above 7.
2Alkalis are not gentle by any means: sodium hydroxide dissolves grease and hair in drains, and it burns skin just the same. In the laboratory, handle alkalis with goggles and avoid skin contact.
3To tell acids from alkalis, scientists use indicators — substances whose colours change with acidity. Purple litmus turns red in acids and blue in alkalis; phenolphthalein is colourless in acids and pink in alkalis; turmeric powder from the kitchen turns from yellow to red-brown in alkalis. Meanwhile, clear limewater turns milky with carbon dioxide — the classic test for the gas.
4What happens when an acid meets an alkali? Neutralisation — the topic of the next lesson — holds the answer, and it runs everything from stomach medicine to soil improvement.
1Summary 重點摘要
Drain cleaner contains sodium hydroxide and window cleaner contains ammonia — alkalis are everywhere too. Litmus turns red in acid and blue in alkali, phenolphthalein turns pink in alkali, turmeric turns red-brown in alkali.
2Vocabulary cards Vocabulary
A compound that dissolves in water to give a pH above 7; its solution tastes bitter and feels soapy.
A strong alkali, also called caustic soda, used in drain cleaners and soap making.
A weak alkali used in window cleaners and as a raw material for fertilisers.
Also called slaked lime; its clear solution, limewater, is used to test for carbon dioxide.
A common indicator: it turns red in acids and blue in alkalis.
An indicator: colourless in acidic or neutral solutions, turning pink in alkalis.
A natural indicator: yellow in acid or neutral solutions, turning red-brown in alkalis.
The clear solution of calcium hydroxide, which turns milky when carbon dioxide passes through it.
A highly alkaline and corrosive alkali, such as sodium hydroxide, requiring great care.
3Step by step Steps
- Alkalis at homeAlkalis are as common at home as acids: drain cleaner contains sodium hydroxide, window cleaner contains ammonia solution, and soap itself is made from sodium hydroxide and fats. Because alkalis dissolve grease, they clear greasy drains and hair, and kitchen sprays rely on alkalinity to cut stubborn oil. Knowing which household products are alkaline makes us handle them more carefully, keeping them away from skin and eyes.
- General properties of alkalisAlkali solutions share common properties: they feel slippery, taste bitter and have a pH above 7, with larger numbers meaning stronger alkalinity. Soapy water feels slippery, and sodium hydroxide solution can reach pH 14, among the most alkaline of common liquids. Alkalinity is measured with pH paper or universal indicator. Because alkalis react with fats and oils, skin that touches them turns rough, so rinse thoroughly with water after use.
- Strong alkalis are dangerous tooStrong alkalis are no less dangerous than concentrated acids: caustic soda (sodium hydroxide) dissolves grease and hair in drains and burns skin just as badly, and in the eyes it can damage sight. So never lean over the container when using it, in case the solution splashes; point the opening away from you when opening, and wear goggles and gloves. Both strong acids and strong alkalis must be stored carefully and never kept in drink bottles.
- Litmus testLitmus is the most familiar indicator: purple litmus turns red in acidic solution and blue in alkaline solution — red acid, blue alkali. Added to lemon juice it turns red, and to soapy water blue, telling acid from alkali at a glance. If the colour change is faint, dip litmus paper straight into the solution and compare the colour. Litmus can only tell acid from alkali, not show strength — that is its limit.
- Phenolphthalein testPhenolphthalein is the specialist detector for alkalis: it stays colourless in water and in acids, but turns pink the moment it meets sodium hydroxide solution, and it is very sensitive. A colour change with phenolphthalein therefore confirms the solution is alkaline. Experiments also use phenolphthalein to judge when neutralisation is complete: as alkali is added drop by drop to an acid, the appearance of pink shows the acid has been fully neutralised.
- Turmeric testIndicators do not have to come from a laboratory, and turmeric is a natural example: turmeric water is yellow at first, turns red-brown when ammonia or baking soda solution is added, and keeps its yellow in acids. The curry powder in the kitchen contains turmeric and works as an indicator too. Its clear colour contrast, easy availability and safety make it a good home demonstration, showing that science lives in everyday life.
- Testing carbon dioxide with limewaterClear limewater is the standard test for carbon dioxide: blow through a straw into limewater and the carbon dioxide in your breath reacts with calcium hydroxide, forming insoluble calcium carbonate, so the liquid turns milky. Keep blowing and the calcium carbonate converts further into soluble calcium hydrogencarbonate, and the liquid clears again — the reaction happens in steps. This matches the marble reaction from the last lesson, which also forms calcium carbonate.
4Diagrams Diagrams
5Worked example Example
Example: two colourless solutions, one dilute hydrochloric acid and one sodium hydroxide solution. Using only litmus and phenolphthalein, how do you tell them apart?
- Take small samples of both and add purple litmus to each.
- The one turning red is dilute hydrochloric acid (red for acid).
- The one turning blue is sodium hydroxide solution (blue for alkali).
- Add phenolphthalein to the blue one: it turns pink, confirming it is alkaline.
- Answer: First take samples of both solutions and add purple litmus to each: the one turning red is dilute hydrochloric acid, because litmus turns red in acids; the one turning blue is sodium hydroxide solution, because litmus turns blue in alkalis. Then add phenolphthalein to the blue one: it turns pink, confirming the solution is alkaline. Using the two indicators together lets the results confirm each other, making the identification reliable.
6Common mistakes Mistakes
7Quick check Quick check
Q1What colour does purple litmus turn in soapy water?
Q2What colour is phenolphthalein in an acidic solution?
Q3Which strong alkali is commonly used in drain cleaners?
Q4What happens when carbon dioxide passes through clear limewater?
Q5What colour does turmeric indicator turn in alkalis?
8Recap Recap
- Common alkalis: sodium hydroxide (drain cleaner), ammonia solution (window cleaner), calcium hydroxide (limewater).
- Alkali solutions are bitter, feel slippery and have a pH above 7.
- Indicator rules: litmus — red acid, blue alkali; phenolphthalein — pink in alkali; turmeric — red-brown in alkali.
- Strong alkalis are corrosive and as dangerous as concentrated acids — wear goggles.
- Limewater turning milky is the test for carbon dioxide.