C2.1 - Purity And Separating Mixtures

• Their relative masses. - Their masses compared to the mass of a proton.

Relative atomic mass, Ar, is the mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.

A chemical formula tells you how many atoms of each element there are in a unit of a substance.

Relative formula mass, Mr, is the mean mass of a unit of a substance compared to 1/12 the mass of a Carbon-12 atom.

An empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound.

A balanced chemical equation shows the formulae (how the atoms are rearranged) and the relative amounts of each substance involved.

• Pure in everyday life describes natural substances that have not been processed or changed. • In science, a pure substance consists of just one element or compound.

A mixture contains two or more different substances that are not chemically joined together, (two or more different compounds), so are therefore impure.

• Just one atom or molecule of something else makes the substance impure. • Substances can easily become contaminated such as with the carbon dioxide from the air which itself is a mixture of nitrogen, oxygen, and other substances.

An alloy is a mixture of a metal with at least one other element.

The melting point of a substance is a single temperature.

• Its melting point is less that that of the pure substance. • It often melts over a range of temperatures. • Greater the difference between the measured melting point and the accepted melting point, the lower its purity is likely to be.

Heating the substance and: - Measure the temperature at which it melts. OR - Measure its temperature at regular time intervals and plot a temperature against time graph.

• Heating the substance slowly - Allows the temperature of the whole sample to increase. • Stirring the substance - Ensures the entire sample is at the same temperature. • These two actions improve the accuracy of a measurement of the melting point of a sample.

A solution is a mixture formed when one substance dissolves in another.

• A solute is a substance that dissolves in a solvent. • A solvent is a substance that can dissolve a solute to form a solution.

When a substance dissolves, its particles separate and become completely mixed with the particles of the solvent.

• A soluble substance is one that can dissolve in the given solvent. • An insoluble substance is one that can’t dissolve in the given solvent.

• Filtration separates an insoluble substance in the solid state from substances in the liquid state. • It works because filter paper has tiny, microscopic holes where small molecules can pass through and larger insoluble substances can’t.

• Filter paper, funnel, residue, conical flask, filtrate. • Scientists flute filtrate paper to produce a larger surface area for the filtrate to pass through.

Crystallisation is the process by which crystals are formed during evaporation of a solvent from a solution.

• Use a water bath to heat up the solution, (filtrate - depending on context), until the solution becomes saturated (when no more solute can be dissolved at that temperature). • Crystals will start forming at this point. • Let the solution cool slowly - because the solution then decreases the solubility of the solute which results in more crystals forming. • Separate the crystals from the remaining solution by filtration, and dry them in a warm oven or by patting them with filter paper.

• If you heat the solution too strongly, you get a powder. • If you allow the solvent to evaporate slowly, you get regularly shaped crystals.

• Simple distillation separates a solvent from a solution. • It relies on the solvent having a much lower boiling point than the solute.