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🧪 Acids, Alkalis, and Chemical Reactions

Discover why lemons are sour and soap feels slippery, and how the pH scale, indicators, and neutralisation reveal the hidden chemistry all around you.

3
lessons
~30 min
to learn
🔬 Science
subject
Teens
level
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What you’ll learn

  1. The pH ScaleUnderstand that pH measures how acidic or alkaline a substance is, with below 7 acid, 7 neutral, and above 7 alkali.The pH scale runs from 0 to 14 and measures how acidic or alkaline something is. Below 7 is an acid, exactly 7 is neutral like pure water, and above 7 is an alkali or base. Lemon juice and vinegar are acids, while soap and baking soda are alkalis. The further a substance is from 7, the stronger it is.
  2. IndicatorsLearn how indicators such as litmus, universal indicator, and red cabbage juice change colour to reveal pH.An indicator is a substance that changes colour depending on the pH. Litmus turns red in acids and blue in alkalis. Universal indicator shows a full range: red or orange in acids, green when neutral, and blue or purple in alkalis. Even red cabbage juice works as a natural indicator, so pH can be tested safely without tasting anything.
  3. NeutralisationUnderstand that acid plus alkali makes a salt and water, and see how neutralisation is used in real life.When an acid reacts with an alkali they neutralise each other, always making a salt and water and ending near neutral. Neutralisation is used to calm an acidic stomach with antacids, to fix acidic soil with lime, and to soothe acidic bee stings. Acids also react with carbonates to make salt, water, and carbon dioxide, and with metals to make a salt and hydrogen.

Questions this course answers

What does a pH of exactly 7 tell you about a substance?

On the pH scale, 7 is the middle point that means neutral, neither acidic nor alkaline.

Lemon juice has a pH of about 2. What kind of substance is it?

Any substance with a pH below 7 is an acid, and 2 is well below 7.

As a substance's pH number gets higher above 7, it becomes more...

Numbers above 7 mean alkali, and the higher the number, the more strongly alkaline the substance is.

What colour does litmus paper turn when you dip it in an acid?

Litmus turns red in an acid and blue in an alkali, so an acid gives red.

Universal indicator turns green in a liquid. What is that liquid?

Universal indicator shows green at neutral, red or orange in acids, and blue or purple in alkalis.

What is an indicator?

An indicator changes colour depending on the pH, which lets you tell acids and alkalis apart safely.

Grounded in trusted sources

  • Royal Society of Chemistry
  • BBC Bitesize
  • Britannica

Every Wunder lesson is built from real, reputable sources — never invented.

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