Learn about colours and how to use a smartphone, some fruit juice, a filter and a torch to measure changes in concentration across different juice drinks

In this video, education coordinator John demonstrates a simple experiment for measuring concentration that touches on many core chemistry concepts such as particle theory, colour, spectroscopy, colorimetry, dilutions and concentrations. This simple activity can be set for learners to try at home with a responsible adult or used as a hands-on classroom experiment or demonstration.

Equipment list

  • A small torch
  • A smartphone
  • Some identical drinking glasses
  • Fruit juice (blackcurrant works best)
  • Water
  • A measuring jug
  • A spoon
  • A green filter (sweet wrapper or drinks bottle)

Health and safety

  • If your green light filter is from a green drinks bottle, please make sure to be extra careful when cutting it out with scissors. 
  • Always ask for the permission of the smartphone owner before downloading any apps.

Activity instructions

  1. Download a light meter app to the smartphone, there are many free versions available across all platforms. They are sometimes called a ’lux meter’ because they provide us with ’lux numbers’. 
  2. Prepare your green light filter. You can get this from a sweet wrapper or by cutting it out of a drinks bottle. 
  3. Using the measuring jug, measure an equal amount of water into each glass. It doesn’t matter how much you use as long as it’s the same amount in each glass. 100 ml should be sufficient. 
  4. Add a different amount of blackcurrant juice to each glass. You can use the spoon to prepare specific concentrations of drink to observe the relationship, eg sample one contains one spoon of juice and sample two contains two spoons etc. 
  5. Ensure each drink is thoroughly mixed using the spoon. 
  6. Set up the experiment with the torch and green filter on one side of a glass and the smartphone on the other side. Ensure the camera is directly across from the torch. 
  7. Repeat the experiment for each drink, recording the lux number each time. 
  8. Observe that the lux number is inverse to the concentration, so the higher the concentration the lower the lux number. 

Explanation

The reason we see colour is because everything around us absorbs different wavelengths of light. The colour wheel can help us keep track of what colours are being absorbed. In this experiment our blackcurrant drink is purple so it will absorb green light – green is opposite purple on the colour wheel.

Using identical drinking glasses and an equal amount of water in each drink is important because it reduces the number of variables in the experiment. In this instance the glasses and volume of water become our constants. The only variable should be the amount of blackcurrant juice that is added to each drink sample. It is always important to discuss which are the constants and variables with learners, particularly when setting up experiments.

In diluted drinks there is only a small amount of blackcurrant juice available to absorb the light. Most of our green light from the torch will pass straight through the drink and into the camera of the smart phone giving us a large lux number. However, in concentrated drinks there is more blackcurrant juice available to absorb the light. In this case only a small amount of light will pass through the drink and into our smartphone camera which will give us a smaller lux number. 

Also check out

  • You can take this activity further using the Beer’s law simulation
  • More simple experiments using everyday equipment which your learners can try at home or you can bring to the classroom on our YouTube playlist.
  • Read the CLEAPSS guidance on practical activities for pupils at home during extended periods of school closure, GL339.
  • Read the SSERC guidance for primary home learning.

Downloads