Unit 8: Rates of reaction
Neutralisation — HIA International - Grade 8 Revision Notes & Topical Questions
5 min read 11 parts Lesson 2 of 10 2 quizzes
Lesson summary
Neutralisation — HIA INTERNATIONAL notes: Describe neutralisation as the reaction of an acid with an alkali or base, make a neutral solution and explain why neutralisation is important.Acid = a substance with a pH below 7 that turns blue litmus red, such as hydrochloric, sulfuric…
- Neutralisation is a chemical change
- Too much acid
- Too little acid
- Adding the acid a few drops at a time
- The salt name has two parts
- Hydrochloric acid makes chlorides
Section 1 of 11Acids, bases and alkalis
Acid
Acid a substance with a pH below 7 that turns blue litmus red, such as hydrochloric, sulfuric and nitric acid.
Base
Base a substance that reacts with an acid and neutralises it, such as a metal oxide, hydroxide or carbonate.
Alkali
Alkali a base that dissolves in water; its solution has a pH above 7 and turns red litmus blue.
Neutral
Neutral a solution with a pH of exactly 7, such as pure water.
Section 2 of 11What neutralisation is
1Neutralisation
- Neutralisation the reaction in which an acid and an alkali (or base) cancel each other out to make a neutral solution.
2Neutralisation is a chemical change
- Neutralisation is a chemical change and neutrality is a chemical property of a substance.
3
Word equation: acid + alkali (or base) → salt + water.
- Examplehydrochloric acid + sodium hydroxide → sodium chloride + water.
4Too much acid
- Too much acid leaves an acidic solution, with a pH below 7.
5Too little acid
- Too little acid leaves an alkaline solution, with a pH above 7.
6Adding the acid a few drops at a time
- Adding the acid a few drops at a time makes it easier to judge exactly when the solution becomes neutral.
Section 3 of 11Naming the salt
Naming the salt
The salt name has two parts
- The salt name has two parts the metal from the base or alkali, then the acid ending.
Hydrochloric acid makes chlorides
- Hydrochloric acid makes chlorides for example sodium chloride.
Sulfuric acid makes sulfates
- Sulfuric acid makes sulfates for example copper sulfate.
Nitric acid makes nitrates
- Nitric acid makes nitrates for example potassium nitrate.
Acid + metal carbonate → salt + water + carbon dioxide
- Acid + metal carbonate → salt + water + carbon dioxide the fizzing shows carbon dioxide and the acid is also neutralised.
Section 4 of 11Universal indicator colours
| Colour | pH | Type of solution |
|---|---|---|
| Red | 0–2 | Strong acid |
| Orange / yellow | 3–6 | Weak acid |
| Green | 7 | Neutral |
| Blue | 8–11 | Weak alkali |
| Purple / dark blue | 12–14 | Strong alkali |
Section 5 of 11Using a burette to neutralise accurately
Using a burette to neutralise accurately
A burette
- A burette adds measured volumes of acid slowly to an alkali in a conical flask that contains universal indicator.
The flask is shaken after each addition of acid.
At the start the pH is about 13
- At the start the pH is about 13 and the pH falls as acid is added.
At the end point the pH is 7 and the indicator is green
- At the end point the pH is 7 and the indicator is green the acid has neutralised the alkali.
Adding a little more acid
- Adding a little more acid makes the liquid weakly acidic, about pH 6.
Section 6 of 11Making a neutral salt solution
1
Measure a fixed volume of alkali into a flask and add a few drops of indicator.
2
Add the acid a little at a time, swirling after each addition.
3
Stop when the indicator shows the solution is neutral, green for universal indicator.
4
Record the volume of acid used and repeat without the indicator, using the same volumes.
5
Evaporate some of the water and leave the solution to crystallise to obtain the salt.
Section 7 of 11Rainbow neutralisation
Set-up
- Set-up a crystal of washing soda at the bottom of a test tube of water with universal indicator, with acid poured carefully on top.
Top of the tube
- Top of the tube the indicator is red, so it is strongly acidic.
Middle of the tube
- Middle of the tube the acid and the washing soda solution react and neutralise each other, so the indicator is yellow.
Bottom of the tube
- Bottom of the tube the washing soda is a strong alkali, so the indicator is purple or dark blue.
The tube must stay still
- The tube must stay still so that the acid and alkali particles spread by themselves and the colour bands form.
Section 8 of 11Neutralisation in everyday life
Indigestion
- Indigestion too much hydrochloric acid in the stomach is neutralised by antacid medicines, which are alkalis or bases.
Toothpaste
- Toothpaste contains an alkali that neutralises the acid made by bacteria feeding on food left on teeth, which prevents tooth decay.
Acid rain
- Acid rain some countries drop alkali into lakes to neutralise the acid, so plants and animals can live there.
Farming
- Farming farmers spread lime on acidic soil so crops grow better.
Bee stings
- Bee stings acidic stings can be treated with a weak alkali such as baking soda.
Section 9 of 11Testing the pH of soil
1
Shake a soil sample with water in a test tube.
2Filter the mixture
- Filter the mixture the filtrate is the liquid that comes through the filter paper.
3
Add a few drops of universal indicator to the filtrate and read the colour as a pH.
4Neutralisation releases heat
- Neutralisation releases heat so the temperature of the mixture rises, which makes it exothermic.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Neutralisation | The reaction of an acid with an alkali or base to make a neutral solution. |
| Salt | A compound formed when an acid is neutralised. |
| Base | A substance that reacts with and neutralises an acid. |
| Alkali | A base that dissolves in water. |
| Burette | A glass tube with a tap used to add measured volumes of liquid slowly. |
| Antacid | A medicine that neutralises excess stomach acid. |
| Lime | An alkali spread on acidic soil to neutralise it. |
| Filtrate | The liquid that passes through filter paper. |
Section 11 of 11Exam technique
1
Saying neutralisation always gives a pH of 7 — it does only when exact amounts react.
2
Saying acid + alkali makes only water — it makes a salt and water.
3
Confusing a base with an alkali — an alkali is a base that dissolves in water.
4
Adding the acid quickly near the end point — add it drop by drop so the neutral point is not missed.
Marked practiceQuizzes for this lesson
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