Measuring Acidity

What's this Science Bite about?

Try our science experiment – measuring acidity – where you will prepare your own acid indicator by extracting a pigment from a red cabbage and use it to look at how acidic some of the things in our kitchen are!

Strong acids are corrosive solutions that break things down e.g. hydrochloric acid breaking down zinc metal powder. This is the same acid that you have in your stomach! Not all acids behave like this though, some are less harmful and these are called weak acids.

You might have heard the term acid related to things we eat and drink. We can measure the acidity of substances using a chemical indicator. Acid indicators tell us how acidic something is by the colour it turns. Red cabbage contains a purple pigment called flavin which changes colour depending on the acidity of the environment it is in. This makes it a good acid indicator.

Always remember to ask your parent or guardian to help you.

What you'll need

  • Gloves – red cabbage juice stains your hands!
  • Red cabbage
  • Large bowl/container
  • 2 jugs – one empty, one with water
  • Sieve
  • See-through containers – e.g. plastic cups
  • Items to test – e.g. vinegar, toilet bleach, fizzy juice, ketchup…

How to do the experiment

Follow the instructions in the video below.

Can't see the video above? Watch it on Youtube.

  1. Rip off the several of the darkest leaves of your red cabbage into a large bowl.

  2. Add enough water to just cover the leaves.

  3. Wearing gloves, mash the leaves for several minutes to extract the colour. Alternatively you can extract the colour using a blender or boiling the leaves in a pot of water. This is the easiest – and perhaps most fun – way to do it.

  4. 3.	Wearing gloves, mash the leaves for several minutes to extract the colour.
  5. Once you have a really intense purple colour, sieve the solution into another jug. The more intense the colour is, the easier it will be to see any colour changes. You now have you indicator solution and are ready to start testing things!

  6. Pour some indicator solution into a few see-through containers.

  7. 5.	Pour some indicator solution into a few see-through containers.
  8. Leave one container as your control. This is what you will use to compare the colour of the indicator once you’ve added something. Having a control is good scientific practise.

  9. To each of the other containers, add different substances to the indicator and see what happens. Only add one foodstuff per container.

  10. 7.	To each of the other containers, add different substances to the indicator and see what happens. Only add one foodstuff per container.

Find out more...

Vinegar is a weak acid, also called acetic acid. It should turn the indicator from purple to pink. The opposite of an acid is a base, or an alkali. A lot of cleaning products are alkaline so adding something like toilet bleach to the indicator will change it from purple to green. Make sure you are wearing gloves when handling cleaning products.

Neutral substances will not change the colour of the indicator because it is already in a neutral environment, water.

Water is made up of tiny particles called molecules. There are millions of these in just one drop! Water molecules have the chemical formula H2O meaning each molecule contains two hydrogen atoms and one oxygen atom. Hydrogen and oxygen are elements, the ingredients that make up water.

Sometimes water molecules split up into charged fragments, called ions. The two ions water usually form are H+ and OH-. H+ is pretty unstable by itself so it usually attaches itself to another water molecule forming H3O+, but H+ is usually written for simplicity.

measuring acidity4

When there are equal numbers of H+ and OH ions in the solution, it is neutral, which is why water is neutral. A solution is acidic if it has more H+ ions than OH- ions in it and alkaline/basic if there are more OH- ions than H+ ions. The presence of metals or non-metals in the water change the number of ions free in the solution.

Scientists measure acidity using the pH scale, which measures the concentration of H+ ions. Neutral solutions have a pH of 7, acidic solutions have a pH less than 7 and basic solutions have a pHmore than 7.

pH scale

Weak acids have pH values near 7 while strong acids have low pH values. You have hydrochloric acid in your stomach, which is so strong it can dissolve zinc metal! The acid needs to be strong so that all the food you eat can be broken down. You stomach has a special outside layer that protects it from digesting itself.

Visit BodyWorks at Glasgow Science Centre - an interactive exhibition all about human health and wellbeing in the 21st Century.

Curriculum Links

Chemical Changes

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