Pure Substances

To start us off, let me ask you a question. Are rainwater and the water that comes out of your taps both safe to drink? The answer to this question is not really as straightforward as it seems, but let us briefly go over it. Firstly, in terms of tap water, we would say this is safe because it has gone through processing to become pure for us to drink. On the other hand, in terms of rainwater, you would automatically assume it is not safe to drink. Although this is true, did you know that any water waste we produce, is cleaned before being returned to lakes and rivers? Nevertheless, by the time the water cycle starts again other substances such as dirt or bacteria can make it impure, so you should avoid it. So, what does this have to do with what we are learning today? Well, as water is made ‘pure’ for us to drink, what does ‘pure' mean? This is what we will go through in this explanation.

• Firstly, we will explore what the definition of a pure substance is, and how it can differ for chemists.
• We will then explore how we can analyse a pure substance.
• Finally, we explore what a mixture is and how it is different to a pure substance.

Pure substance: definition

Generally, when you hear the word pure, automatically it is linked to something clean, safe to use or drink. If we refer back to our earlier example of tap water, we know that in the UK it has been processed to the point, that it is pure and therefore safe enough for us to consume. Another way we can explore this is orange juice. You may see in your supermarket that there are a variety of different types, and those that are advertised to be produced from freshly squeezed juice are deemed as pure.

This is a more general definition of pure, and there is a significant difference when we look at the definition of pure within chemistry.

Pure substance in chemistry

For chemists, when we look at pure substances, these are the ones that are made from one substance only. This can be either an element or a compound. Again, if we look at our tap water, in science we would not regard this as pure. This is because it has more than just molecules made of two hydrogens and an oxygen (H₂O). Also, if you carry on your studies in chemistry you will see when carrying out practicals you will not just use water from the tap, but rather specially prepared water from the technicians.

Another example of a pure substance in chemistry can be seen in the different chemicals you may use within your reaction. Let us say we are reacting dilute hydrochloric acid with some calcium carbonate; both of these will be pure when we use them. Substances need to be pure as if there are additional compounds within them, especially if it is unknown it can be a potential hazard as some toxic by-products can be produced.

Analysis of pure substances

To be able to investigate whether substances are pure or impure, we use two important factors:

• Melting point.
• Boiling point.

The melting and boiling points for different substances are different. Looking at the water, in particular, the melting point is 0℃ and the boiling point of water is 100℃. You can also investigate this yourself at home. You will notice that your freezer is set at below 0℃ and if you put some liquid water in the freezer, it will become solid ice. Then, if you take it out, as your general surroundings will most likely be above 0 ℃ the ice will melt. It is important to remember this is just the melting and boiling point of water.

To find this data for other substances there are many databases where it can be referenced and for mixtures, due to having different compositions, the melting and boiling point may vary slightly.

Now, when we analyse a substance, this is where we can use our database of melting and boiling points. This is because, if a mixture is not pure, the melting point tends to be lower, and it can also signify if the substance is solely pure or a mixture of different substances. When we carry out the experiment and if the substance is impure, looking at how far the melting or boiling point is compared to our data signifies to us how impure a substance can be. Such as with water, if a sample melted at a much higher or lower temperature than 0 ℃ we can assume it is quite impure or that another substance had been mixed with it.

The experiment

To test the purity of a substance we use a melting point apparatus which you can see above. You will notice it has a thermometer, which allows us to track the temperature throughout the reaction, it also has a capillary tube for our sample and finally a liquid such as oil or water that is heated, so the temperature is increased.

Analysis

After heating our mixture, to further determine whether our product is pure or impure, we can track how it cools. We have to record the temperature of the sample as it freezes. We then use this data to produce a cooling graph.

If you look at our first diagram, you will see that the point at which the sample freezes is around 44℃ and the melting point is quite sharp. Whereas, if you look at our second diagram, you will see the melting point has a more gradual decrease and the freezing point is slightly higher. This signifies that our second diagram is impure, as pure substances have a sharp melting point.

Difference between pure substance and mixture

If we look back at our scientific definition of a pure substance, we know it is made up of either one element or one substance. An element is a substance that is made up of atoms, which have the same number of electrons, neutrons and protons.

After exploring elements and substances by themselves, we can see how they are chemically pure substances.

This is vastly different to a mixture, which can be made up of multiple elements or substances. It is different to compounds because mixtures are not chemically combined and can be separated through physical methods.

There are many different types of the mixture, and the one we need to learn in particular are called formulations.

Formulations

Formulations are mixtures that are produced to be useful products, like medicines. Medicines are mixtures as they are made up of multiple substances, with the active drug making up around 5% - 10% of the whole medicine. The active drug is the part that when consumed facilitates the ease of symptoms and in some cases helps cure the illness. Aside from the active drug, whatever else is added is called excipients. This includes if a sweetener was added, any colours and if it was a tablet, to make it smooth, so it can be swallowed.

Examples of formulations

Paints are also another type of formulation: They contain pigments that give them the colour, a binder, so it can attach to the surface that is being painted and finally a solvent that helps the pigment and binder spread as it helps thin them out.

Chemicals that we use to clean are also a type of formulation. Looking at washing up liquid alone, we have: a surfactant that helps tackle the grease on dirty dishes, colour and fragrance to make it appealing to consumers and also water, so it can be disposed of from its packaging easily.

Make-up, some food products and even the fuel put into cars are also types of formulations.