Free Homework Activity – Catalysts, Investigating Apple Browning

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Investigating apple browning  – an investigation into catalysts that students can carry out at home.

Enzymes are biological catalysts: they help increase the rate of reactions taking place in living organisms. In this easy investigation, students use different liquids to see which are best at preventing apple slices from browning.

Each student will need:

  • An apple
  • Lemon juice, orange juice, lemonade, vinegar, water
  • Clock
  • Cups
  • Paper towels

Student instructions

You will have noticed that a cut apple goes brown quickly after cutting. This happens because apples contain an enzyme called phenolase. Enzymes are biological catalysts, they speed up reactions that happen in living organisms. In the presence of oxygen in the air this particular enzyme catalyses the formation of brown pigments called melanins.

In this investigation you will soak apple slices in a variety of different liquids to see which, if any, slow down this reaction.

  • Pour a small amount of each liquid you are going to test into a cup. Label them with the name of the liquid
  • Slice up the apple. Leave one apple slice on a paper towel. This is your control
  • Use the tongs to dip another apple slice into one of the test liquids for 30 seconds. Place it on the paper towel and label it with the name of the liquid
  • Repeat this for each liquid
  • Observe the slices (including the control) every 10 minutes for an hour. Write down your observations in a table stating the darkness of each apple slice

Now try to answer these questions:

Which liquid was best at preventing the apple from going brown? How do your results show this?

Can you come up with a scientific explanation as to why? You may need to do some research on the internet to find out.

Do you think the room temperature affects how quickly the apple slices go brown? Can you use what you know about chemical reactions to explain why? 

The students should find that the acidic solutions slow down the browning. This is because if the pH is reduced below 3, the enzyme will denature and no longer work. Placing the apple slices in water will temporarily inhibit the browning reaction since water prevents the oxygen reaching the apple tissue.

The warmer the room temperature, the quicker the browning. This is because an increase in temperature increases the rate of the reaction.

Your students can watch an exciting demonstration using a catalyst by watching the video: Elephant’s Toothpaste – A Reaction Using A Catalyst.

It might be a good idea to suggest to students that they read Smart facts  before starting the homework, as they give a little more information about enzymes which may help students to understand what is happening and think about why the reaction is slowing down in some cases.