Mixtures What do they look like? This activity accompanies the Education in Chemistry article ‘How to teach mixtures and solutions’ by David Paterson: rsc.li/2FxsuyJ Images © David Paterson

Preparation Prepare a set of containers (eg jam jars, plastic pots) that contain different arrangements of simple objects (eg marbles, beads). An example set: Just red, just blue, just yellow and just green beads in separate pots A mixture of red and yellow beads A mixture of green and blue beads A mixture of yellow, blue and green beads A mixture of yellow, blue, green and red beads A mixture of smaller and larger red beads Images © David Paterson

Preparation Prepare a set of sealed boiling tubes containing: salt sand salt and sand copper turnings zinc granules copper turnings and zinc granules

Investigation You have 2–3 minutes in your groups to look at and discuss the contents of the pots. You should think about: Which contain mixtures? How can you tell? What properties of the ‘particles’ are to make this decision? Be ready to feedback to the class.

Group feedback Which pots contain mixtures? How can you tell? What properties of the ‘particles’ are you using to make this decision? Do you agree/disagree with what other groups are saying? Explain why. For those producing more sophisticated arguments, you can ask more probing questions. For example: Is it sufficient to say the beads are different because they have different sizes? Would you make the same argument for apples of different sizes?

Pure substances Here we have red ‘particles’ blue ‘particles’ yellow ‘particles’ green ‘particles’ Each container only contains only one type of particle – so these are pure substances.

Mixtures Here we have red and yellow ‘particles’ together green and blue ‘particles’ together. The containers have two different types of particles in – so these are mixtures.

Mixtures Here we have Blue, green and yellow ‘particles’ together Blue, green, yellow and red ‘particles’ together The containers have more than one type of particles in – so these are mixtures.

Mixtures Here we have red and yellow ‘particles’ together We also have two pots containing only red ‘particles’. small and large ‘particles’ medium and large ‘particles’ All the pots contain mixtures – the ‘particles’ differ by colour or size. Different properties can make ‘particles’ different. Those providing more sophisticated reasoning may challenge the classification of the different sized red beads as different substances – see the copper & zinc ‘Pure substances’ slide (12).

Investigation You have 2–3 minutes in your groups to look at and discuss the contents of the boiling tubes You should think about: Which contain mixtures? How can you tell? What properties of the contents are you using to make this decision? Be ready to feedback to the class.

Group feedback Which contain mixtures? How can you tell? What properties of the contents are you using to make this decision? Do you agree/disagree with what other groups are saying? Explain why.

Pure substances Here we have salt sand zinc pieces copper pieces Each boiling tube only contains one chemical substance – these are pure substances. Those providing more sophisticated reasoning may challenge the classification of the different sized red beads as different substances. Here, the pieces of zinc and copper will likely show some variation, however we are saying these are not mixtures. This can be used as a lead-in to discussions of chemical elements, and the different types of atoms as the defining characteristic of the chemical elements.

Mixtures Here we have sand and salt zinc and copper The containers have more than one chemical substance in – so these are mixtures.

Mixtures The ‘particles’ in the pot are the beads. Large red beads are different to small red beads so this counts as a mixture. In the tube, the copper pieces are different sizes. However, they are all made of copper atoms, and all copper atoms are the same. Therefore, this doesn’t count as a mixture. Watch out for the very keen student who may know of isotopes, and challenges the idea that all copper atoms are the same. You could explain the ‘sameness’ of copper atoms is based on the number of protons in the nucleus, although this is taking the student(s) into 14-16-level chemistry.