The pH scale

• The pH scale shows how acidic a substance is. It can be measured using a pH meter which gives a numerical value.
• The pH scale ranges from 0 (very acidic) through 7 (neutral) to 14 (very alkaline).
• pH can be also be measured using an indicator and comparing the colour with a comparison chart.

Play this game to see where different common substances appear on the pH scale.

Watch this video to find out more about the pH scale.

The pH scale is a number scale from 0 to 14. It tells us how acidic or alkaline an aqueous solution is. The pH scale is used to classify solutions as acidic, alkaline or neutral.

• Neutral solutions are exactly pH 7.
• Acidic solutions have pH values less than 7. The closer to pH 0, the more acidic a solution is.
• Alkaline solutions have pH values more than 7. The closer to pH 14, the more alkaline a solution is.

Did you know?

Everyday substances are not pure. This makes it difficult to give them a pH value. One type of soap might have a pH of 8, but another have a pH of 9.

Stomach acid has a range of pH values, depending on how long ago you last ate!

Solutions can be acidic, alkaline or neutral. For example, pure water is neutral with a pH of 7. A solution of common salt (sodium chloride) is also neutral, so it has a pH of 7 too.

In science we often use the term aqueous solution. It describes a solution that’s made of water. pH can only be measured in water-based solutions.

Therefore, we can measure a substance’s pH by dissolving it in water.

For example, copper sulfate is a solid in the form of blue crystals. When copper sulfate is dissolved in pure water, we can measure the pH of the solution. The pH is approximately 6 which shows that copper sulfate solution is slightly acidic.

A solution is a liquid made when a solute is dissolved in a solvent.

In the example above, copper sulfate would be the solute and water the solvent.

Did you know?

Classify is a word often used in science. We use it when talking about how we sort things into groups according to their properties, such as acidic, alkaline or neutral.

The pH of a substance can be tested if it is an aqueous solution. The substance must be dissolved in water.

There are 2 methods for testing for pH:

• Using a pH meter
• Using an indicator

A pH meter gives a numerical value of the pH. As long as the pH meter has been looked after properly, the measurement should be accurate. This means that the measurement is close to the true value.

An indicator is a substance which will change colour depending on the pH of the solution it is mixed with. Some indicators are liquids, meaning we can add drops of the indicator to the solution being tested. Other indicators are strips of paper, and we can dip these into the solution.

Litmus is an example of an indicator. It turns red in solutions that are acidic and it is blue in alkaline solutions.

Universal indicator is a special indicator because it turns into a different colour at many different pH values. The colour of universal indicator at different pH values is shown further below.

Did you know?

Even red cabbage can be used as an indicator!

Universal indicator is supplied as a solution or as universal indicator paper. It is a mixture of several different indicators. Unlike litmus, universal indicator can show us how strongly acidic or alkaline a solution is, not just that the solution is acidic or alkaline. This is measured using the pH scale, which runs from pH 0 to pH 14.

Universal indicator has many different colour changes, from red for strongly acidic solutions to dark purple for strongly alkaline solutions. In the middle, neutral pH 7 is indicated by green.

When using universal indicator paper, we get more accurate results by putting only a small spot of the test solution on the paper, and then leaving the colour to develop for about 30 seconds before comparing it with the colour chart.

Lots of highly coloured vegetables and flowers can be used to make indicators just by mashing them in hot water. Red cabbage is one of the best, and can be compared to the colour charts above.

Other good natural indicators are beetroot, tea, turmeric and blackberries.

To make your own indicator, you will need:

• Red cabbage
• A kettle and water
• Household substances to test, such as lemonade, vinegar, baking powder or toothpaste. These are your substance samples.

1. Chop up the red cabbage into small pieces and place it in a mug.
2. Add freshly boiled water from the kettle.
3. Stir the cabbage and boiling water until a deep purple solution is made.
4. Put drops of your substance sample onto a white plate and then add your red cabbage solution.
5. Use the colour comparison chart to find the pH of your substances.

Write a short conclusion for your experiment - make sure you refer back to your results. Is red cabbage a good indicator?

Find out more about how to conclude and evaluate your work in this guide.