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Acids, bases and metals

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Indicators and the pH scale

When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline solutions. If a solution is neither acidic nor alkaline we call it neutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.

Litmus and universal indicator are two indicators that are commonly used in the laboratory.

Litmus

Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions - and it turns purple in neutral solutions.

Litmus paper is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.

  Red Litmus Blue Litmus
Acidic solution Stays red Turns red
Neutral solution Stays red Stays blue
Alkaline solution Turns blue Stays blue

Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the same', because it tells us the colour we actually see.

blue litmus paper turning red when dipped into acid

Acids turn blue litmus paper red

red litmus paper turning blue when dipped into alkali solution

Alkalis turn red litmus paper blue

Universal indicator and the pH scale

Universal indicator is a mixture of several different indicators. Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14.

Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases. In the middle, neutral pH 7 is indicated by green.

Photograph of different pH levels, randing from red to violet.

Universal indicator shows how acidic or alkaline a solution is

These are the important points about the pH scale:

  • neutral solutions are pH 7 exactly
  • acidic solutions have pH values less than 7
  • alkaline solutions have pH values more than 7
  • the closer to pH 0 you go, the more strongly acidic a solution is
  • the closer to pH 14 you go, the more strongly alkaline a solution is

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Acids, bases and metals activity

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