Elements, Mixtures and Compounds

  • An element is a pure substance made up of only one type of atoms.
  • A compound is a substance which contains two or more elements, chemically bonded together in a fixed proportion.
  • A mixture is a group of substances that are physically bonded together.

Differences between a mixture and a compound

CompoundsMixtures
Proportions of elements are fixed.Proportions may vary.
Properties different from those of the mixed elements.Properties are simply those of mixed the elements.
Cannot be separated into elements by physical means.Can be separated by physical means.
There is usually an energy change when a compound is made.Usually no energy change when the elements are mixed to form a mixture.

Separation techniques for separating mixtures

A mixture can be seperated by the following physical means.


Solid and Liquid

  • If the solid has not dissolved in the liquid (i.e. is suspended in the liquid), the two substances can be separated by filtration.nThe liquid filtrate passes through the filter paper and the undissolved solid residue stays on the paper.
  • If the solid has dissolved in the liquid, forming a solution and if only the solid is required, it is obtained by evaporation else if the liquid is required, it is obtained by distillation.

During distillation, the solution to be separted is placed in a flask and heated. The liquid evasporates, and its vapour passes into a condenser, where it cools and turns back to liquid. The solid remains behind in the flask as a residue.

Distillation

  • X is the solution to be separated.
  • Y is the level where the thermometer bulb should be, if you decide to add a thermometer.
  • Z is the condenser.

Liquid and Liquid

  • If the liquids are immiscible seperate by using a seperating funnel. When the two liquids do not mix (e.g. paraffin and water), they can be separated simply by running the denser liquid from a separating funnel by opening the tap.
  • If the liquids are miscible they can be separated by fractional distillation. Both liquids are evaporated, and their vapours pass into a fractionating column, where they are condensed and re-boiled many times. The vapour of the liquid which has the lower boiling point. emerges from the top of the column first, and passes into the condenser. When all of this liquid has distilled, it is followed by the liquid having the higher boiling point.

Fractional Distillation

Solid and Solid

  • By dissolving. Use a solvent that will dissolve one solid but not the other. Then filter off the residue on the filter-paper and dry it. To obtain the second solid (now in solution, in the filtrate), evaporate the filtrate and crystallise the solid produced.
  • By chromatography. This is a method for separating two solids that are both soluble in the same solvent. Make a solution of the mixture and place a drop of the solution on a piece of filter-paper. Allow a suitable liquid to spread gradually across the paper. The solid that is more soluble in the liquid will move through the greatest distance, and the solid that is least soluble will move least. Thus the dissolved solids are separated on the paper.