Composition matters
A pure substance has a characteristic chemical composition. A mixture contains multiple substances physically combined in proportions that can vary.
Pure substances & mixtures
Use properties to classify and separate matter.
Class notes
Open PDF →Uniform does not mean pure
A solution is a homogeneous mixture. Salt water can be clear because dissolved salt is distributed throughout it. Muddy water is heterogeneous and suspended particles may settle.
Suspensions and colloids
Suspensions have particles that may settle or be caught by a suitable filter. Colloids have dispersed particles that remain spread through another substance; ordinary paper filtration usually does not separate them. Milk is an example of a colloid.
Choose a separation method
A paper filter can trap sand grains but dissolved salt passes through with water. Evaporation can leave non-volatile dissolved material behind; distillation can collect the evaporated water after condensation.
Interactive lab · change a variable
Choose a separation
Choose a mixture, then predict what ordinary filtration will remove.
A simplified model of an ordinary paper filter. It does not test whether water is safe to drink.
A clear sample leaves salt after evaporation. What does that tell you?
Reveal the reasoning
It contained water and dissolved salt. Its clear appearance did not make it a pure substance.
Try it yourself
Sand and salt are mixed with water. How could you separate them?
Give me a hint
Which material can an ordinary paper filter trap?
Compare with a worked response
Filter out the sand. The dissolved salt passes through with water. Evaporate the filtrate to recover the salt; distillation could also collect condensed water.
Investigate & explain
Design a separation sequence for a teacher-approved mixture of sand, salt and water. State what stays in the filter and what passes through, then explain how you might recover the salt.
Does removing visible particles prove the remaining water is safe to drink?
Compare your reasoning
No. Ordinary filtration can remove some suspended particles while dissolved substances and some microorganisms remain. Clear-looking water is not evidence of safe drinking water; this classroom separation is not a drinking-water treatment.
Common question: Clear water must be pure and safe.
Appearance cannot establish chemical composition or drinking-water safety.
Put the ideas to work
Choose an answer, then use the feedback to refine your reasoning.