Filtration and crystallisation teaching resources

Worksheets and lesson ideas to challenge students aged 11 to 16 to think hard about filtration and crystallisation (GCSE and Key Stage 3)
Filtration teacher brief

Overview: filtration allows us to separate an insoluble solid from a liquid or solution. Students will have studied filtration (using filter paper) and sieving at primary school, so it’s worth thinking about how to build on this knowledge and not just repeat information. Ask students to draw particle pictures to show how filter paper and filtration works to progress understanding. Make sure all students can fold filter paper correctly – they can do this before using any other apparatus and stick their folded paper in their books. Can students use terms such as filtrate and residue correctly? It’s important they can as this helps them to explain the process.

Key concept: filtration allows us to separate an insoluble solid from a liquid or solution because the substances have different sizes.

From big idea: all matter in the Universe is made from very small particles

Linked knowledge: mixtures, solutions, particle pictures

Misconception/error [scientific idea]: filtration can separate solutes from solvents [filtration using filter paper cannot separate solutions]; spelling of filteration [filtration]


Teaching resources

Where to start?

Ask students to label a set-up showing apparatus for filtration but purposely omit filter paper. Ask students to spot the error. Then add filter paper and ask students to make a prediction – what will happen when we filter seawater contaminated with bits of visible plastic? Why won’t we be able to separate the salt? Draw particle pictures to explain how the filter paper works. Avoid using single particles to represent substances in the residue – it’s important students get the idea that the residue has a solid arrangement and is not just one massive particle!

Carrying out a simple filtration

Making and separating a precipitate, made from adding sodium hydroxide to copper sulfate solution, can be a great practical to use. If you want a context to frame this practical around, copper hydroxide is used as a fungicide when growing fruits or vegetables, including wine.

Design a filter to purify dirty water for drinking (potable water)

This activity is taken from the secondary resource pack made for British Science Week where students design a filter to purify dirty water. There is an excellent paper that you can then discuss to develop interest and understanding outlining how scientists have designed filter paper made of cellulose (plant cell walls!) that removes viruses from water.

Click here for example 1 and example 2 of student work. This collection of resources was shared by George Rodrigues.

Filtration and crystallisation – separating soluble salts

KS3 practical activity to separate salt and sand. This practical is an open activity that asks students to come up with their own method to separate salt from sand. To increase the challenge, students should be allowed to select the apparatus they want to use. It’s worth pointing out the difference between evaporation and crystallisation – i.e. leave some water for crystallisation to take place in. At the end of the practical the percentage yield of each group is calculated and a winner declared (if you want to!). You could use this method for teaching about preparation of solution salts. (PDF)

Thinking deeper

  • Why can you not separate salt from seawater using filter paper?
  • What are the three main types of contaminant in drinking water? How could you ‘remove’ biological ones?
  • Where do you find filters in the human body?
  1. Filtration and crystallisation
  2. Distillation
  3. Chromatography

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