Neutralisation and Everyday Applications
AdvancedAcid plus alkali gives a salt and water — neutralisation. Stomach medicine, sting care and fixing acidic soil all rely on this reaction.
- Neutralisation: acid + alkali → salt + water.
- At exact neutralisation the pH is 7; excess alkali makes it higher.
- Everyday uses: antacids for stomach acid, baking soda for stings, toothpaste for teeth.
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
1Swallowing a stomach tablet, dabbing baking soda on a bee sting, a farmer spreading lime on a field — three unrelated acts sharing one chemical reaction: neutralisation.
2The principle is simple: when an acid meets an alkali, their properties cancel out and a salt plus water are formed. Take hydrochloric acid and sodium hydroxide: acid + alkali → salt + water. When they exactly neutralise each other, the solution sits at pH 7 — neither acidic nor alkaline.
3The applications run through daily life: antacids neutralise excess stomach acid; toothpaste neutralises the acids mouth bacteria make, protecting enamel; slaked lime restores acidic soil to a crop-friendly pH; power stations even wash acidic flue gases with alkali to reduce acid rain.
4From chemistry we now turn to physics, and to a big question: where does electricity come from?
1Summary 重點摘要
Acid plus alkali gives a salt and water — neutralisation. Stomach medicine, sting care and fixing acidic soil all rely on this reaction.
2Vocabulary cards Vocabulary
The chemical reaction in which an acid and an alkali cancel each other out, forming a salt and water.
Neither acidic nor alkaline; pH 7, like pure water.
The general name for compounds formed when an acid neutralises an alkali, e.g. sodium chloride — not just table salt.
The dilute hydrochloric acid made by the stomach to digest food; too much causes discomfort.
A medicine containing an alkaline substance, such as milk of magnesia, that neutralises excess stomach acid.
Sodium hydrogencarbonate, a mildly alkaline powder used to neutralise acids and in baking.
Calcium hydroxide, the alkaline powder farmers spread on fields to neutralise acidic soil.
A tooth-cleaning paste containing mild alkalis that neutralise acids made by mouth bacteria, protecting enamel.
3Step by step Steps
- The basic equationWhen an acid reacts with an alkali the products are a salt and water — this is neutralisation. Hydrochloric acid reacts with sodium hydroxide to give sodium chloride and water: the acidic and alkaline properties cancel out and the solution moves towards neutral. Inside the reaction, hydrogen ions from the acid join hydroxide ions from the alkali to form water, and the remaining ions make up the salt, which dissolves in it — sodium chloride, for instance, is table salt. Neutralisation is widely applied in daily life, farming and industry.
- Use 1: excess stomach acidStomach acid is dilute hydrochloric acid that helps break down proteins, but too much of it causes discomfort and a burning feeling. Antacids in stomach medicine, such as magnesium hydroxide, neutralise the excess acid, turning it into a salt and water to bring relief. This is the most personal application of neutralisation and shows how chemistry can directly improve health. Medicines must still be taken as instructed, because too much alkaline remedy is harmful too.
- Use 2: bee and wasp stingsInsect stings are treated with the same principle: bee stings are acidic, so a paste of baking soda, a weak alkali, neutralises them and eases the pain; wasp stings are alkaline, so a weak acid such as dilute vinegar is applied. First decide whether the secretion is acidic or alkaline, then treat accordingly — a wrong choice makes things worse and deepens the pain. That is why baking soda and vinegar belong in a home first-aid kit.
- Use 3: fixing acidic soilOverly acidic soil stops crops taking up nutrients, and long-term fertiliser use or acid rain can make soil acidic. Farmers spread slaked lime (calcium hydroxide) on fields, using its alkalinity to neutralise the excess acid and lift the pH from about 4 back to between 6.5 and 7, so crops grow healthily. The technique is long established, cheap and effective — a classic case of chemistry improving farming.
- Use 4: protecting teethAfter sweets, bacteria in the mouth break down sugar and produce acids that slowly erode the enamel, causing tooth decay. Brushing with mildly alkaline toothpaste neutralises these acids and protects the enamel. After brushing, avoid rinsing with large amounts of water, so a little toothpaste stays on the teeth and the protection lasts longer. This is why brushing before bed matters most, so acid cannot keep eroding teeth overnight.
- Use 5: fighting acid rainBurning coal and oil in power stations releases sulphur dioxide, which turns into acid in the air and falls as acid rain. Modern power stations install scrubbers that wash the flue gas with alkaline solution such as lime slurry, neutralising the acidic gases before release — a technology known as desulphurisation that cuts sulphur dioxide sharply. Neutralisation thus reaches from the laboratory into environmental protection, benefiting air quality as well.
4Diagrams Diagrams
5Worked example Example
Example: sodium hydroxide solution is added drop by drop to dilute hydrochloric acid. The pH rises from 2 to 7 and then to 11. Explain the whole process.
- At first the beaker holds only acid, so the pH is 2 — strongly acidic.
- As alkali is added drop by drop it neutralises the acid, and the pH keeps rising.
- At pH 7 the acid and alkali have exactly neutralised each other, giving a salt and water.
- Beyond that the alkali is in excess, so the pH climbs to 11 and the solution is alkaline.
- Answer: At first the beaker holds only dilute hydrochloric acid, so the pH is 2 and the acid is strong. As sodium hydroxide solution is added drop by drop, the alkali steadily neutralises the acid and the pH rises; at pH 7 the acid and alkali have exactly neutralised each other, forming a salt and water. Beyond that, extra alkali is in excess, the solution turns alkaline and the pH climbs to 11. The whole process shows acid and alkali cancelling each other out.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the two products of neutralisation.
Q2Why do antacids relieve excess stomach acid?
Q3Should you treat a bee sting with an acidic or an alkaline substance? Give an example.
Q4What substance do farmers use to treat acidic soil?
Q5What is the pH of a solution when acid and alkali have exactly neutralised each other?
8Recap Recap
- Neutralisation: acid + alkali → salt + water.
- At exact neutralisation the pH is 7; excess alkali makes it higher.
- Everyday uses: antacids for stomach acid, baking soda for stings, toothpaste for teeth.
- Slaked lime fixes acidic soil; industry washes flue gases with alkali to cut acid rain.
- Method: identify acid or alkali first, then choose the right substance to neutralise.