Sunday, 7 February 2016

Acids and Alkalis - Basic

What are acids and alkalis?

Acids are all: sour, corrosive and soluble in water
All alkalis are the chemical opposite of acids, have a soapy feel and can remove the sharp taste from acids.

Examples of acids:

  • Vinegar
  • Orange juice
  • Lemons and limes
Examples of alkalis:

  • Bleach
  • Toothpaste
  • Washing powder
  • Milk of magnesia


Indicators are used to test if a substance is acidic, they change colour when added to acids or alkalis. Many indicators are actually dyes that have been extracted from a natural source, e.g litmus. The table below helpfully shows some common indicators and the colours that they turn when you add them to acids or alkalis.

Universal Indicator

The universal indicator is used to find out how acidic or alkaline a substance is.
It's a mix of several different indicators and it can be used as a paper or as a liquid, it turns a different colour depending on what acid or alkali is added to it. You then match up the colour of the universal indicator with something called the pH scale. 

The pH scale.
Image courtesy of

Each colour on the pH scale corresponds to a number, and the number tells you the pH level of the substance you're testing. For example, the pH of pure water is 7.

What the pH level tells you about a substance

  • pH of less than 7 - acid
  • pH of 7 - neutral
  • pH of more than 7 - alkaline
pH meter
Image courtesy of

Another method of measuring the pH of a substance is to use a pH meter.
When you place the electrode into the solution, the electronic display shows the pH level.

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