9.02.1 What is pure?

Syllabus

In chemistry, a pure substance is a single element or compound, not mixed with any other substance. Pure elements and compounds melt and boil at specific temperatures.
Melting point and boiling point data can be used to distinguish pure substances from mixtures.
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
You will not need to know the names of components in proprietary products.

4.8.1.2 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. These include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Students should be able to identify formulations given appropriate information.

Students do not need to know the names of components in proprietary products.

What does this mean?

What does Pure mean?

Pure generally means "the same throughout".

So we might say we have pure sand - so long as there is no gravel or mud mixed in with it. In pure sand every grain would have to be what we think of as sand even though they were all slightly different sizes.

But we can't really have pure soil because soil is a mixture of clay, sand, gravel, rocks, dead plant and animal matter etc. So it is impossible for every particle of soil to be the same as every other particle.

We think of drinking water being pure because we know it shouldn't contain any dirt or microbes that can make us sick. But to make this certain we add Chlorine to kill the microbes, and the water will have dissolved some substances from rocks. So it isn't really pure in a chemical sense.

When we boil a kettle repeatedly we often get limescale building up on the element - showing that the drinking water contained other chemicals - it was impure.

Everyday English is not the same as technical English.

If you had milk fresh from a cow it would be as pure as milk can be.

But if we leave it long enough (without letting it rot) it will begin to separate - the fat floats to the top, the watery part (whey) sinks to the bottom.

And the watery part contains minerals like calcium as well as sugars and other substances that dissolve in water.

So milk is also a mixture and can't be pure in the chemical sense.

When we say "pure milk" we mean "unadulterated milk" - milk with nothing added since it was produced by the cow.

Chocolate milk is neither pure nor unadulterated - it has had flavourings and sugar added.

And no one has yet invented a cow that produces chocolate milk naturally.

This Orange Juice may be unadulterated (only contain orange juice).

But it isn't chemically pure - it contains a mixture of many substances including:

sugars

acids

colours

and flavours

that all dissolved in water while the orange was growing.

And there's really no such thing as pure chocolate.

All chocolate is made by mixing cocoa, sugar and other ingredients.

But even the cocoa isn't pure because the beans that it comes from contain dozens or even hundreds of different natural chemicals

Manufacturers like to call things pure because some people believe that this means it is better for them.

What does Chemically pure mean?

The purest form of matter is an element - all the atoms are identical.

They might exist as a gas, a liquid or a solid but all the atoms are the same.

And you can't get purer than that!

Compounds are also pure.

A compounds contains two or more elements chemically bonded together.

Such as Water (H2O) or Carbon Dioxide (CO2)

Not every atom in water is exactly the same but every molecule is the same as every other molecule because they all contain one Oxygen and two Hydrogen atoms bonded together in exactly the same way.

So compounds are also pure.

Mixtures are not pure - some parts are different to others.

Air is a mixture of water vapour, Nitrogen, Oxygen, Argon and other gases.

So, if you took a small amount of air and counted the different types of molecules in it there may be a difference to another sample of air you took somewhere else (even though they'd be very similar)

So air can't be pure no matter where we are in the world - even if it is somewhere unpolluted.

How do we know when a substance is pure?

A chemically pure substance is the same in every part.

So every part will melt at the same temperature.

If we heat up a substance to well above its melting point we can watch it cool.

Its temperature will drop steadily and then stop for a few minutes before starting to cool again.

What is it doing in those few minutes?

It's freezing - so the freezing point (melting point) is very clear.

The same thing happens if we slowly heat up a pure substance until it melts.

The graph goes horizontal because the temperature doesn't change during melting

And it happens again when it gets hot enough to boil.

But if we heat up an impure substance different parts of it are not the same, so they melt at different temperatures.

So, we don't get a clear melting point.

And the graph will look like the one below.

It's easy to see when this substance is melting but it melts over a range of about 50 oC.

So it definitely isn't pure.

A test for the purity of a substance is to see if its solid completely melts over a narrow range of 2 or 3 oC.

And we can also use melting points to identify a pure substance.

Imagine you really need water to put out a fire but there are two identical bottles, one full of water and one full of flammable alcohol and you don't know which is which.

Without tasting it you could decide which was which by heatingsome of each and looking at the temperature at which they boil.

The alcohol should boil at 78oC and the Water at 100oC.

Although, by the time you find that out your house might have burned down anyway.

Formulations

All formulations are mixtures - so none of them are pure.

For example, the formulation for paint must contain colourings and thinners and binding agents that glue the colouring to the wall.

If you want a fast-drying paint you would have to change the formulation.

And if you wanted a different colour then you'd alter the colourings you would add.

Fuels, cleaning agents, medicines, alloys, fertilisers and foods are all formulations - mixtures made for a specific reason.

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