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Module 5 - Acids & Alkalis

Module 5 - Acids & Alkalis - Revision

Here are the key ideas for module 5. When you have read through the notes you can test your knowledge in our quiz. Use the links page for more revision material.

The pH Scale

The pH scale is a measure of how acidic or alkaline a substance is. The scale goes from 1 to 14. The diagram shows the colour that Universal Indicator changes to for different pH values.

(R=Red, O=Orange, Y=Yellow, G=Green, B=Blue, P=Purple)

Here is a key to what the numbers mean:

  • pH 1 to pH 3 show that there is a STRONG ACID
  • pH 4 to pH 6 shows that there is a WEAK ACID
  • pH 7 shows that the substance is NEUTRAL (not acidic or alkaline)
  • pH 8 to pH 10 shows that there is a WEAK ALKALI
  • pH 11 to pH 14 shows that there is a STRONG ALKALI

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Where do I find acids and alkalis?

Lots of everyday substances contains acids or alkalis:

Acids are found in:

  • citrus fruits (lemon juice, orange juice)
  • vinegar
  • car batteries (sulphuric acid)
  • your stomach (hydrochloric acid)

Rainwater is a little acidic, but pollution (e.g. sulphur dioxide) from burning fossil fuels may make it even more acidic, forming acid rain.

When acids are present in food, they usually taste sour (think of the taste of lemon juice or vinegar). Strong acids are very dangerous.

Alkalis are found in:

  • oven cleaner (sodium hydroxide)
  • soap
  • cleaning fluid e.g. spray-and-wipe (ammonia)

Notice the connection between these substances? Alkalis are often found in substances for cleaning.

Strong alkali substances are just as dangerous as strong acidic substances, causing very serious burns if they come into contact with your skin.

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When an acid is added to an alkali or an alkali added to an acid, neutralisation takes place: the substance changes pH to become closer to being neutral.

  • If an alkali is added to acid then the pH will go up (e.g. from pH 3 to pH 7)
  • If an acid is added to an alkali then the pH will go down (e.g. from pH 12 to pH 7)

Neutralisation is sometimes very useful. For example, if your stomach makes too much acid, then you may suffer from stomach ache. Taking Alka-Seltzer (an antacid) will neutralise this extra acid and relieve the pain.

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