Explore the Use of Indicators

Acid or alkali? Strong acid or weak acid? Neutral solution? What pH?

There are many ways to identify whether a substance is an acid or an alkali......

One very simple method is called the litmus test. A special paper called litmus paper is dipped into the substance.

 Acids will turn litmus paper red and alkalis will turn litmus paper blue.

A neutral substance, one which is neither an acid nor an alkali, will turn litmus paper purple. 

There is another test, which can also tell us how acidic or alkaline a substance is - this is the pH test. You can see the pH scale below, which runs from pH0 (very acidic) to pH14 (very alkaline). There is a special solution called Universal Indicator solution. When a few drops of Universal Indicator are added to a substance, it turns a different colour. This colour matches one of the colours on the Universal Indicator scale below and that's how you determine how acidic or alkaline a substance is and so what its pH is.

Another way to determine if something is an acid or alkali is by using a pH probe. It is a digital hand-held device that, when dipped into the solution, will give you a precise pH value for that substance - sometimes even to fine detail eg pH 13.4!

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