Accessible Filtration

Colour Method

Background

There are multiple ways of separating mixtures. Filtration is a method by which an insoluble solid is removed from a liquid. An insoluble solid is a solid that does not dissolve in the liquid solution it is in. Filtration is especially important in purifying water- where we need to remove chemicals, germs and bacteria- in the pharmaceutical industry- for example nanofiltration- and in everyday life such as making a cup of tea! 

Experiment

This experiment uses rock salt and sand to demonstrate how two substances can be separated from one another. It introduces the ideas of density, dissolution and separation.

Aim

To separate out a mixture of sand and salt using filtration.

You will need:

  • Small zip lock/ seal bag (13cm by 9cm works well)
  • 1 x heaped teaspoon rock salt
  • 1 x level teaspoon sand
  • Colour dye
  • 25mL water, dyed
  • 2 x 100mL plastic beakers, one with gems stuck at the 25mL level (see image below in step 3)
  • 1 x funnel
  • 1 x filter paper
  • 1 x colour detector

Method

  1. Measure out one heaped teaspoon of rock salt and one level teaspoon of salt. Add to ziplock bag, and shake for thirty seconds. Feel through the ziplock bag to determine if there are two distinct layers. (It may help to turn the plastic bag at a 45 degree angle to do this, as shown in the picture below.)
  2. Pour in the coloured water and shake for two minutes or until you cannot feel the rock salt anymore. What do you observe about the temperature of the bag? Why do you think this is?
  3. Pour the contents of the bag to the funnel with the filter paper on (use a spatula if needed). Filter the solution. The end of the filtration can be indicated when the colour detected detects the volume of solution at the level where the gems are attached on the beaker.

Pictorial guide to the experiment

This image shows a small plastic bag with a mixture of sand and rock salt in. The mixture has formed two distinct layers, with the sand forming a layer at the bottom of the bag, and a layer of rock salt on top of the sand.
Step 1 as an image- the students should be encouraged to feel that there are two different layers formed
The image shows the small plastic bag after dyed water has been added to the rock salt/ sand mixture solution. The sand remains undissolved in a layer at the bottom of the bag, with a layer of salt water above it.
Step 2 as an image – again students should be encouraged to feel the difference between the solid sand layer and the liquid layer.
The image shows a 100mL plastic beaker, filled to 25mL with the dyed salt water solution. There are gems stuck onto the beaker at the 25mL level.
Step 3 as an image- because a known volume was added (and the density of salt water is practically the same as pure water), the end of the filtration can be indicated by the point where the volume reaches the gems. This can be done experimentally by the student using the colour detector (yellow button) at points during the filtration, and encouraged to feel when the colour detector senses the water level at the same point where the gems are attached.

Weighing Method

This is an alternative method to the above if you do not have access to a colour detector but instead requires a talking balance.

The aim of this experiment is the same as the above, but the idea is that students will weigh the sand and salt mixture before any water has been added, and then at the end of the reaction weigh what is left in the filter paper, demonstrating that the salt solution has been removed.

You will need:

  • Small zip lock/seal bag (13cm by 9cm works well- larger bags require larger masses of rock salt and sand and larger volumes of water)
  • 10g of rock salt
  • 10g of sand
  • 25mL water
  • 2 x 100mL plastic beakers
  • 1 x funnel
  • 1 x filter paper
  • 1 x talking balance

Method

  1. Measure out 10g of rock salt into a weighing boat, and then add sand until the scales read 20g.
  2. Add the mixture to the zip lock bag and shake for thirty seconds. Feel through the zip lock bag to determine if there are two distinct layers.
  3. Pour in the water and shake for two minutes or until you cannot feel the rock salt anymore. What do you observe about the temperature of the bag?
  4. Pour the contents of the bag to the funnel with the filter paper on (use the spatula if needed). Filter the solution for 5 minutes. Whilst you are waiting, have a look at some of the questions below.
  5. After the solution has been filtering for 5 minutes, take the filter paper off the funnel and use a spatula to transfer the solid to a weighing boat and weigh it. What do you notice about the weight compared to the weight of the sand and the salt?
  6. If you have any extra time, transfer the solid you have weighed out back to the funnel with a new piece of filter paper in. Slowly pour over some water. Let the solution filter for another 5 minutes, before weighing it out again. Why do you think we wash the solid with water? What has happened to the weight?

Questions for these experiments (applies to both the colour method and the weighing method)

  • Why do the rock salt and the sand form two different layers? 
  • Could you just pick handpick out all the salt from the mixture? What problems might you have if you try this with other mixtures? 
  • What happens to the salt when you add water? 
  • Why does the bag go cold when you add water? 
  • Why does the water pass through the filter paper but sand doesn’t? 
  • How might you get rock salt back from the solution? 

Experiment documents for download here.

Page author: Charley Schofield