Pure Substances and Mixtures; Separation of Mixtures
ICSE Class 6 · Chemistry · Chapter 5
What this chapter covers
- Pure substances and mixtures
- Mixtures: variable composition, properties retained
- Differences between mixtures and compounds
- Separation techniques: sieving, sedimentation, decantation and filtration
- Separation techniques: evaporation, sublimation and magnetic separation
- Choosing a separation technique
Key definitions
A pure substance is a substance that is made of only one kind of matter and has the same properties all through.
A mixture is a material made of two or more substances that are simply mixed together and not joined chemically.
A component is one of the substances that make up a mixture.
Composition is the amount of each component present in a mixture.
Separation is the process of taking the components of a mixture apart from each other.
Sieving is a method of separating a mixture of solid particles of different sizes by passing it through a sieve.
A sieve is a utensil with a mesh of small holes in its base.
Sedimentation is the settling of heavy, insoluble particles at the bottom of a liquid when it is left undisturbed.
Sediment is the solid layer that settles at the bottom of the liquid.
Decantation is the method of gently pouring out the clear liquid from above a sediment, leaving the solid behind.
Filtration is the method of separating insoluble solid particles from a liquid by passing the mixture through a filter.
A filter is a material with very fine holes that lets a liquid pass through but stops solid particles.
Key points
- A pure substance is made of only one kind of matter; elements and compounds are pure substances.
- A mixture is made of two or more substances mixed together but not joined chemically.
- The components of a mixture can be present in any proportion, so its composition can vary.
- Each component of a mixture keeps its own properties.
- In a compound, elements join in a fixed proportion and the compound has new properties.
- The components of a mixture can be separated by simple methods; a compound needs chemical methods.
- Sieving separates solid particles of different sizes.
- Sedimentation lets heavy particles settle; decantation pours off the clear liquid above them.
- Filtration separates an insoluble solid from a liquid; the liquid is the filtrate and the solid is the residue.
- Evaporation gets back a dissolved solid, such as salt from sea water.
- Sublimation separates a solid that turns straight into a gas, such as ammonium chloride or iodine.
- Magnetic separation removes iron from a mixture with a magnet.
- The method chosen depends on how the components differ in size, weight, solubility, magnetism or behaviour on heating.