Purity and Separating Mixtures

Elements, Compounds and Mixtures

Pure Substances

A pure substance is either a single element, or compound, that is not mixed with any other substance.

A mixture consists of two or more elements/compounds, that are not chemically combined together and can be separated by physical processes.

Pure substances will melt and boil at specific temperatures, so we can use the temperature that a substance melts/boils at to tell how pure it is. Impure substances will melt over a range of temperatures. If a substance is impure it will generally also have a lower melting point and a higher boiling point.

Pure Substance
Impure Substance

Elements, Compounds and Mixtures

RAM and RFM

Relative Atomic Mass  (RAM or Ar )

This is the average relative mass of the atoms of different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom.

An atom’s mass number is the sum of its protons and neutrons. Carbon has 6 neutrons and 6 protons so we say it has a mass of 12. You can find this information on a Periodic Table.

However, relative atomic mass takes into account that some elements have isotopes. These are atoms of the same element, but with different numbers of neutrons. The Periodic Table contains a list of every element, with their relative atomic masses.

We can take an average of all an element's isotope mass numbers to work out the relative atomic mass of that element for ourselves (see diagram).

Relative Formula Mass  (RFM or Mr )

This is like relative atomic mass, but we are working out the mass of a compound. You have to add all of the relative atomic masses of each atom together to calculate an RFM, for example water (H₂O):

  • there are two hydrogen atoms, each with an atomic mass of 1... so 1 x 2 = 2
  • there is one oxygen atom, which has an atomic mass of 16
  • in total the relative formula mass of water is 16 + 2 = 18

Empirical Formula

An empirical formula is the simplest whole number ratio of atoms in a substance. 

  • H2O has the empirical formula of H2O
  • C4H8 has the empirical formula of CH2
  • Al4O6 has the empirical formula of Al2O3

Formulations

A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties. Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

It is important that formulations keep the same ingredients, so that the formulation is always produced the same way. People buy these products for continuity, especially paints - it would be pointless buying the same shade of paint that was actually different colours.

Alloys

Most metals in everyday use are alloys. An alloy is a mixture of two or more elements, where at least one of the elements is a metal. Bronze is an alloy of copper and tin. Brass is an alloy of copper and zinc.

Pure metals are generally too soft to use as they have a regular lattice structure. When a force is applied to the metal, the regularly aligned layers are able to easily slide over each other (this makes them malleable).

Alloys are made of different sized atoms, distorting the regular layers so that they are not able to easily slide over each other any more - increasing the strength of the metal (they are less malleable, and more brittle).

Pure Metals vs Alloys

Separation Techniques

Filtration is a method for separating an insoluble solid from a liquid. A mixture of liquid and solid is passed through filter paper into a flask below. The solid stays on the filter paper (residue), and the liquid passes through to the container below (filtrate).

Crystallisation/evaporation is a method that separates a soluble solid from a liquid. A solution of liquid and dissolved solid is heated, causing the solvent to evaporate and leaves solid crystals behind.

Simple distillation is a method for separating the solvent from a solution, leaving behind the solute. This method works because the solvent has a much lower boiling point than the dissolved solute. Heating the solution allows the solvent to evaporate, and then it passes through a condenser, where it is cooled and condensed into a separate container. The solute does not evaporate and so it stays behind.

Fractional distillation is a method of separating multiple liquids from each other. The technique works in the same way as distillation, but on a much larger scale.

Separation Techniques

Paper Chromatography (TLC)

Chromatography is a method for separating dissolved substances from one another. It works because some of the coloured substances dissolve in the solvent better than others, so they travel further up the paper. This technique is often used to separate out inks, or food colourings.

Paper chromatography consists of two stages:

  • the stationary phase (a.k.a. the phase that doesn't move - this is the paper)
  • the mobile phase (a.k.a. the phase that moves - this is the solvent)

Separation by chromatography produces a chromatogram, and can be used to distinguish between a pure substance (that produces one spot), and a mixture (impure) which produces multiple spots. Different substances will move at different rates through the paper.

When setting up the experiment you must make sure to draw the baseline in pencil, otherwise the ink from a pen would also dissolve in the solvent. 

The Rf value of a spot can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value will always be the same for each substance (when the same solvent is used). It can be calculated by using:

Rf value = distance moved by substance ÷ distance moved by solvent

Chromatography

Gas Chromatography

Gas chromatography consists of two stages:

  • the stationary phase (a.k.a. the phase that doesn't move - thin layer of unreactive liquid)
  • the mobile phase (a.k.a. the phase that moves - this is an unreactive gas like nitrogen or helium)

A detector measures the amount of each substance in a mixture as it leaves the column:

  • different substances travel at different speeds through the column
  • they leave at different times - we call these the retention times

Two main pieces of information can be gathered from a gas chromatogram:

  • the number of compounds in the mixture - the number of peaks
  • the amount of each compound present - the height of each peak (higher = more)

The Gas Chromatogram here shows that

  • four different substances were in the mixture (there are four peaks)
  • A was present in the smallest amount (it has the smallest peak)
  • D was present in the greatest amount (it has the largest peak)

Gas Chromatogram