Separating Mixtures…

A mixture is any situation when two or more substances are mixed together. In a mixture, the different particles are close to each other but they are not joined. This means it should be possible to separate most mixtures out into their component parts. To do this, you need to look at the physical properties of the different parts of the mixture to work out a method for separating them. Here are some examples…

Sand from water

Sand is insoluble in water so, even if it is very well mixed in, forming a suspension where the particles are hanging throughout the water, a filter can be used to separate the sand from the water. The water will pass through the filter (called the filtrate) and the sand will be trapped by the filter (called the residue).

Salt from water

Salt dissolves in water to form a solution. The salt is known as the solute and the water as the solvent. Filtration will not work in this case because the salt has broken down into individual molecules and will pass through the filter paper. However, if you boil the mixture, the water will evaporate leaving the salt behind.

Salt from sand

A mixture of salt and sand can be separated by combining the two techniques above. First add water to dissolve the salt, filter to remove the sand and finally evaporate the filtrate (salt solution) to recover the salt. The diagram below shows this process.

Water from salt solution

If it is the water you want to collect from a solution, then evaporation alone is not enough. The water would just escape into the atmosphere. In this case, you need to catch the water vapour and condense it back into a liquid. This technique is called distillation. The solvent collected in this process is called the distillate. A Liebig condenser can be used to make the condensation more efficient. The two diagrams below show simple distillation and distillation using a condenser.

Ethanol from water

Ethanol (alcohol) mixes with water and is also a liquid but it has a lower boiling point than water. If it is distilled at a lower temperature then it can be separated from the water. This process is not perfectly efficient because water molecules are quite ‘sticky’ and will hitch a lift with the ethanol as it evaporates. Fractional distillation can ensure that the water and ethanol are completely separated by providing a large surface area for condensation in a fractionating column. Other mixtures of liquids such as crude oil and liquid air can be separated in a similar way.

Mixtures of dyes

Another physical difference that can be used to separate mixtures is colour. Chromatography is a technique that uses the different solubility of different molecules to separate them out. The dyes are carried up a piece of paper by a solvent and the more soluble molecules travel further than the less soluble one.

Iron filings and sulphur

What physical difference between iron and sulphur could be used to separate them? Neither is soluble in water; they are both solids at room temperature; heating the mixture would cause them to react and produce iron sulphide.

Iron is attracted to a magnet whereas sulphur is not so you could use this difference to separate the two.

Mixtures of soluble salts can also be separated by using differences in their solubility. Some salts dissolve more in hotter water (e.g. potassium nitrate) whilst others are not affected by the temperature of the water (e.g. sodium chloride).

Questions…

1. What word means “does not dissolve in”?
2. In sea-water, what is the solvent and what is the solute?
3. What separation technique would you use to find what colours were combined in a particular ink?
4. How would you separate…
   1. sulphur from water?
   2. copper sulphate from water?
   3. water from a cup of coffee?
   4. alcohol from wine?
   5. a mixture of solid copper sulphate and solid copper oxide?
5. Suggest a difference between mixtures and compounds?