Cabbage water indicator

Description:

A reddish/purple solution turns red when acid (such as vinegar or lemon juice) but green when an alkali (base) is added (such as sodium carbonate, also known as washing soda or soda ash).

Requires:

  • Red cabbage
  • Water
  • Grater
  • Acids and alkalis (bases) to test

Method:

Take a red cabbage and shred it finely.  Boil until the cabbage is soft.  Strain the pieces from the water.  To concentrate the colour, boil the water to reduce the volume.

Principle:

This solution can be used as an indicator for acids and alkalis. Cabbage water turns red when acids are added to it and green when alkalis are added.  This can be used to test whether household materials are acid or alkali.

Vinegar and many fruit juices such as lemon and grapefruit are acidic whereas many cleaning materials such as washing soda are alkalis.  Milk of magnesia (magnesium hydroxide) will turn the indicator green adding vinegar will turn it pink.  The pink colour will slowly be replaced by green as more of the solid dissolves.  These colour changes will continue until the solid all dissolves completely.  At this point the colour will not change back to green.  (Note:  Products such as Maalox contains aluminium hydroxide and does not give the same result.)

Tip:

Many other highly coloured fruits and vegetables contain dyes which can be used in a similar way. Fruit teas such as blackcurrant also work.

Safety:

Take care with boiling water. Allow water to cool before pouring or straining. Heated water can be avoided by simply shredding the cabbage into a small volume of water and decanting or straining the water. Be aware that acids and alkalis can be caustic or harmful. If these are accidentally splashed onto skin wash the affected place with plenty of water.

Tags: Colour change, indicator, acid, base, alkali, turmeric, cabbage, equilibrium, fruit tea.