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Indicator Art

Acid-base experiments can communicate an important message and look good too!

Australian Curriculum links

  • Science > Chemical Sciences > Year 9 > ACSSU179
  • Science > Chemical Sciences > Year 10 > ACSSU187
  • Science > Senior Secondary Curriculum > Chemistry > Unit 3: Equilibrium, acids and redox reactions

You'll need

  • A weak acidic solution e.g. 0.1M hydrochloric acid
  • A weak basic solution e.g. 0.1M sodium bicarbonate
  • Filter paper or paper towel
  • Universal indicator

Try this

  1. Place the filter paper on a tile or similar surface.
  2. Place a few drops of the weak basic solution onto the filter paper.
  3. Place a few drops of the weak acidic solution onto the filter paper. The basic and acidic solutions should just touch.
  4. Place some drops of universal indicator across the wet areas of the filter paper and observe the colour change.

Further investigation

Universal indicator is good for indicating both bases and acids, but there are many different indicators available. You can try this experiment with them to see how they vary.
A simple homemade indicator can be created by boiling chopped red cabbage in water, or blending a few large leaves in water and straining the lumps out. The remaining purple liquid can be used as an indicator.

What’s happening?

A pH indicator is something that is used to help determine a substance’s pH, using colour changes to indicate pH level.

The pH of a substance is its level of acidity or alkalinity. To be more precise, pH is a measure of the concentration of hydrogen ions (H+) relative to hydroxide ions (OH-) in a substance. The range of pH values is represented on a logarithmic scale from 1 to 14.If a solution contains more hydrogen ions than hydroxide ions it is considered acidic and will have a pH between 1.0 and 6.9. However, if there are more hydroxide ions than hydrogen ions, the solution will be alkaline and will have pH between 7.1 and 14. If the amounts of hydrogen ions and hydroxide ions are equal - such as in pure water - the solution is considered neutral and has a pH of 7.0.

Indicators, such as universal indicator, work by changing their colour with changes in pH. There are many different pH indicators and each indicator changes colour at a particular pH level. Often an individual indicator will only undergo one or two colour changes. This usually means that using a single pH indicator will tell you the general pH range of an unknown solution. Usually several different indicators must be used and the results compared in order to accurately determine the pH. The main components of universal indicator are thymol blue, methyl red, bromothymol blue and phenolphthalein. This mixture is important because, each component, changes colour depending upon the acidity or basicity of the solution being tested.