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Pure substances and separating mixtures
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Sea water, ink and air are all mixtures. Chemists can separate mixtures and check whether a substance is pure.
Pure substances and mixtures
In science, a pure substance contains only one element or one compound, such as pure water.
A mixture contains two or more substances that are not chemically joined.
A pure substance melts and boils at a fixed temperature. A mixture melts and boils over a range of temperatures, so this is a way to identify pure substances.
A mixture contains two or more substances that are not chemically joined.
A pure substance melts and boils at a fixed temperature. A mixture melts and boils over a range of temperatures, so this is a way to identify pure substances.
Dissolving and diffusion
A solute dissolves in a solvent to make a solution. Salt is soluble in water; sand is insoluble.
When something dissolves, its particles spread out between the solvent particles. No particles are lost, so mass of solution = mass of solute + mass of solvent.
Particles spread out by diffusion because they move randomly. Diffusion is faster in gases than in liquids.
When something dissolves, its particles spread out between the solvent particles. No particles are lost, so mass of solution = mass of solute + mass of solvent.
Particles spread out by diffusion because they move randomly. Diffusion is faster in gases than in liquids.
Separating mixtures
Because the substances in a mixture are not joined, we can separate them using their different properties.
20 g of sugar dissolves in 100 g of water. What is the mass of the solution?
- No particles are lost when sugar dissolves.
- Mass of solution = 20 + 100
Answer: 120 g
Pure substance: one element or compound, fixed melting and boiling points. Solute + solvent = solution, and mass is conserved. Filtration, evaporation, distillation and chromatography separate mixtures.
The interactive lesson includes the diagrams for this topic.
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