7. METHODS FOR SEPARATING MIXTURES
If you had a mixture of iron and aluminium filings, how would you separate them? We often find that we have homogeneous and heterogeneous mixtures in which we have to separate the components without altering the nature of the pure substances in the process.
The most common methods for separating the components of a mixture are:
7.1. Magnetic separation
You may have thought of using a magnet as the easiest and quickest way to separate the iron from the aluminium and you'd be right. If you don't have a magnet, you could use magnetic fasteners on handbags or the covers for mobile phones as they both possess magnetic properties. The iron is attracted to the magnet and the aluminium isn't.
This method for separating the two components of a heterogeneous mixture is called magnetic separation. It can only be used when one of the components has magnetic properties (like iron) and the others don't.
We use decantation for liquids with different densities that don't mix together (immiscible), such as oil and water. For this, you use a decantation funnel:
- Pour the mixture into the funnel, making sure beforehand that you have closed the tap at the bottom (turned it to a horizontal position) so that the mixture doesn't come out.
- Leave the mixture to settle until the two liquids have separated.
- Put a beaker below the funnel and open the tap. The denser liquid (the one at the bottom of the mixture) will begin to flow out.
- Close the tap when all the denser liquid is out.
- The less dense liquid will stay in the funnel. To get it out, simply pour it out the top of the funnel so it doesn't mix with the residue of the other substance that has stayed in the tap.
We use this method to separate a solid from a liquid that hasn't dissolved (insoluble), such as sand and water. To do this, we pass the heterogeneous mixture through a filter with the correct pore size (smaller than the particles that we want to separate). The filter usually goes through a funnel.
We use distillation to separate soluble liquids with very different boiling points from each other, such as water and alcohol.
Put the mixture into a round-bottom distillation flask and heat it up. When it reaches the lower boiling point of one of the two substances, this substance will turn to vapour and pass through the condenser, where it will cool down and condense. The resulting liquid, called the distillate, is collected in a container (a beaker, for example).
7.5. Evaporation and crystallisation
We use this method to separate a solid dissolved in a liquid, such as salt in water. The process starts with evaporating (naturally or forced via heat) the solvent and ends with the depositing of the solid in the form of crystals at the bottom of the container (usually a crystalliser). The slower the evaporation of the solvent, the larger the crystals will be.
We use this to separate two components in a mixture according to how soluble they are in a particular solvent.
One of the simplest techniques is chromatography in paper, which uses a strip of filter paper.
- The components of mixtures can be separated using different methods.
- The most common methods are magnetic separation, decantation, filtration, distillation, evaporation and crystallisation and chromatography.
- The separation methods make use of the different properties of the substances in a mixture.
Put a tiny drop of the mixture onto the strip of filter paper and put the bottom part of the paper into a solvent, such as alcohol. This will move slowly up the filter paper by capillary action, pulling the different components of the mixture with it. Because each component dissolves to a greater or lesser extent in a particular solvent, those that are higher up the strip at the end of the process are more soluble than those at the bottom. This method can be used, for example, to separate photosynthetic pigments (chlorophyll and carotenes) present in spinach and other vegetables.