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Acids and Alkalis 01

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The acid found in lemons is citric acid.

Acids and Alkalis 01

Explore KS3 Science with a quick look at acids, alkalis, and indicators. Read the tips, learn key terms, then try the quizzes below to check your understanding.

Explore the Topic →
(quiz starts below)

Fascinating Fact:

Lemons taste sour because they are full of citric acid, which triggers taste receptors on your tongue.

In KS3 Science, acids and alkalis sit at opposite ends of the pH scale. You’ll test substances with indicators, read colour changes, and explain why neutral solutions balance them out.

Key Terms

  • Acid: A substance with pH below 7 that forms H+ ions in solution.
  • Alkali: A soluble base with pH above 7 that forms OH? ions.
  • Indicator: A dye that changes colour to show whether a solution is acidic, neutral, or alkaline.

Frequently Asked Questions (Click to see answers)

What is the pH scale in KS3 Science?

The pH scale runs from 0 to 14. Acids are below 7, alkalis are above 7, and a pH of 7 is neutral.

How does universal indicator show acidity or alkalinity?

Universal indicator changes colour across the pH range: red/orange for strong acids, green for neutral, and blue/purple for alkalis.

Is neutral the same as pH 7?

Yes. Neutral solutions have equal amounts of H+ and OH? ions and are defined as pH 7.

Try These Related Quizzes

1 .
Which of the following pH numbers indicates a strong acid?
1
5
7
14
Be careful not to confuse a strong acid with a concentrated acid - a strong acid will always give a pH value of 1 or 2 until it is extremely dilute
2 .
Which of the following substances contain an alkali?
Baking powder
Coca cola
Distilled water
Vinegar
It is weak enough to be used in food
3 .
What colour is universal indicator in strong acid?
Green
Orange
Purple
Red
Red indicates a pH of 1 or 2
4 .
Which of the following pH numbers indicate a strong alkali?
1
5
9
14
Hydrangea flowers can be blue or pink depending on the pH of the soil
5 .
Strong acids and strong alkalis are.......
Able to turn blue litmus paper red
Corrosive and/or caustic
Sour
Sweet
We use the word corrosive for acids and caustic for alkalis, either way, if you get any on your skin, they are both harmful and will cause a chemical burn
6 .
Which of the following substances contain acid?
An orange
Bleach
Distilled water
Washing powder
Citrus fruit (oranges, lemons etc) contain citric acid
7 .
What colour is universal indicator in a strong alkali?
Green
Orange
Purple
Red
For weak alkalis, the colour is more of a blueish-green. It is often easier to see the colour using universal indicator paper
8 .
Which of the following acids do we have in our stomach?
Ethanoic acid
Hydrochloric acid
Nitric acid
Sulfuric acid
The acid kills bacteria and helps to begin the process of digesting proteins
9 .
Which of the following pH numbers indicates a neutral substance?
1
5
7
14
Pure water has this pH value
10 .
What colour is universal indicator in a neutral solution?
Green
Orange
Purple
Red
When using universal indicator liquid, you only ever need to add a few drops to see the colour
You can find more about this topic by visiting BBC Bitesize - Acids and alkalis

Author:  Sue Davison (Chemistry Specialist & KS3 Science Teacher, Quiz Writer)
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