Compounds and mixtures
Compounds are chemicals made from atoms of different elements joined by chemical bonds. They can only be separated by a chemical reaction.
A mixture is made from molecules of elements and compounds that are simply mixed together, without chemical bonds. Mixtures can be separated using techniques such as filtration, chromatography, evaporation and distillation.
This Revision Bite covers:
Atoms of different elements can join together in chemical reactions to form compounds. For example hydrogen and oxygen are elements. They react together to form water, a compound.
There are countless different ways for the elements to join together, and millions of compounds are known.
The properties of compounds are usually very different from the properties of the elements they contain. For example hydrogen and oxygen are both gases at room temperature, but water is a liquid.
The reaction between iron and sulphur to make iron sulphide is often used in school to study elements and compounds. Look at the animation to remind you what happens in this reaction.
The table compares the properties of iron and sulphur (the two elements), and iron sulphide (the compound).
|is it attracted to a magnet?||yes||no||no|
|reaction with hydrochloric acid||hydrogen formed||no reaction||smelly hydrogen sulphide formed|
The atoms in a compound are chemically joined together by strong forces called bonds. You can only separate the elements in a compound using another chemical reaction. Separation methods like filtration and distillation will not do this.
Remember that an element is one type of atom, like carbon, gold or chlorine. We will look at three ways that atoms can exist.
The atoms of some elements do not join up with other atoms. They stay as single atoms.
The element helium is like this. Helium is an unreactive gas. Helium atoms do not join up with each other or any other element.
When atoms of the same element join together we get a molecule of that element.
Oxygen is like this. Two oxygen atoms join together to make an oxyen molecule. Most of the oxygen in the air is in this form. Hydrogen and chlorine also have molecules with two atoms.
Some elements have molecules with more than two atoms. Sulfur atoms can make molecules of eight atoms joined together.
A compound is made when atoms of different elements join together by chemical bonds.
This means that compounds will always exist as molecules, not separate atoms. The diagrams show some molecules of common compounds.
|Carbon dioxide molecule|
Remember that we use chemical symbols to stand for the elements. For example, C stands for carbon, O stands for oxygen, S stands for sulphur and Na stands for sodium. For a molecule we use the chemical symbols of the atoms it contains to write down its formula.
For example the formula for carbon monoxide is CO. It tells you that each molecule of carbon monoxide consists of one carbon atom joined to one oxygen atom.
Take care when writing your symbols and formulae. Be careful about when to use capital letters. For example CO means a molecule of carbon monoxide but Co is the symbol for cobalt.
The word 'formulae' ("form-u-lee") is the plural of 'formula'. If we have more than one formula, we don't say formulas, we say formulae.
If the molecule contains more than one atom of an element we use numbers to show this. The numbers are written below the element symbol. For example, the formula for carbon dioxide is CO2 . It tells you that each molecule has one carbon atom and two oxygen atoms.
Take care when writing these formulae. The small number go at the bottom. For example CO2 is correct but CO2 is wrong.
Some formulae are more complicated. For example, the formula for sodium sulphate is Na2SO4 . It tells you that sodium sulphate contains two sodium atoms (Na2 ), one sulphur atom (S) and four oxygen atoms (O4 ).
All compounds have a definite composition. Let's look at water as an example. A water molecule always has two hydrogen atoms and one oxygen atom - it cannot be a water molecule if it has different numbers of these atoms. Its formula is always H2O.
Some metals are very unreactive. That means they do not easily take part in chemical reactions. For example platinum does not react with oxygen in the air, even if it is heated in a Bunsen burner flame.
Some metals are very reactive. They easily take part in chemical reactions to make new substances.
Magnesium is like this. If it is heated in a Bunsen burner, it ignites and burns with a brilliant white flame.
Other metals may be more reactive than magnesium, or in between magnesium and platinum. If we put the metals in order of their reactivity, from most reactive down to least reactive, we get a list called the reactivity series.
If you want to learn the reactivity series, you could try making up a mnemonic or silly sentence to help. Here's one - can you do better?
"Pond slime can make a zoo interesting - the long crinkly sort goes purple."
Remember that metals react with oxygen in the air to produce metal oxides, like magnesium oxide.
Non-metals react with oxygen in the air to produce non-metal oxides. Here are two examples for the non-metals carbon and sulphur.
Carbon burns in air to form carbon dioxide:
carbon + oxygen → carbon dioxide
Sulphur burns in air to form sulphur dioxide:
sulphur + oxygen → sulphur dioxide
Non-metal oxides such as sulphur dioxide and nitrogen oxide are responsible for acid rain. They dissolve in the water in the clouds to form acidic solutions. Acid rain damages rocks and buildings, and harms wildlife.
There are some important differences between the physical and chemical properties of metal oxides and non-metal oxides. The table shows some of these differences.
|Metal oxides||Non-metal oxides|
|solids at room temperature||usually gases at room temperature|
|bases – if they dissolve they form alkaline solutions||dissolve in water to form acidic solutions|
A mixture is made from different substances that are not chemically joined.
For example powdered iron and powdered sulphur mixed together makes a mixture of iron and sulphur. They can be separated from each other without a chemical reaction, in the way that different coloured sweets can be picked out from a mixed packet and put into separate piles.
Mixtures have different properties from compounds. The table summarises these differences.
|Variable composition – you can vary the amount of each substance in a mixture.||Definite composition – you cannot vary the amount of each element in a compound.|
Joined or not
|The different substances are not chemically joined together.||The different elements are chemically joined together.|
|Each substance in the mixture keeps its own properties.||The compound has properties different from the elements it contains.|
|Each substance is easily separated from the mixture.||It can only be separated into its elements using chemical reactions.|
|Air, sea water, most rocks.||Water, carbon dioxide, magnesium oxide, sodium chloride.|
Remember that iron and sulphur react together when they are heated to make a compound called iron sulphide. What are the differences between a mixture of iron and sulphur, and iron sulphide? Here are some of them:
The mixture can contain more or less iron, but iron sulphide always contains equal amounts of iron and sulphur.
The iron and sulphur atoms are not joined together in the mixture, but they are joined together in iron sulphide.
The iron and sulphur still behave like iron and sulphur in the mixture, but iron sulphide has different properties from both iron and sulphur.
You can separate the iron from the mixture using a magnet but this does not work for iron sulphide.
Can you recognise elements, compounds and mixtures?
An element contains just one type of atom.
A compound contains two or more types of atom joined together.
A mixture contains two or more different substances that are not joined together.
The different substances in a mixture can be elements or compounds.
The table shows some examples.
|Pure compound||carbon dioxide|
|Mixture of elements||oxygen and helium|
|Mixture of compounds||alcohol and water|
|Mixture of elements and compounds||air|
Notice that the different substances in a mixture can be single atoms, molecules of elements or molecules of compounds.
The different substances in mixtures are usually easily separated from one another. The method you use depends upon the type of mixture you have.
This is good for separating dissolved substances that have different colours, such as inks and plant dyes. It works because some of the coloured substances dissolve in the liquid better than others, so they travel further up the paper.
Filtration is good for separating an insoluble solid from a liquid. (An insoluble substance is one that does not dissolve).
Sand, for example, can be separated from a mixture of sand and water using filtration. That's because sand does not dissolve in water.
This is good for separating a soluble solid from a liquid (a soluble substance does dissolve, to form a solution).
For example copper sulphate crystals can be separated from copper sulphate solution using evaporation. Remember that it is the water that evaporates away, not the solution.
This is good for separating a liquid from a solution. For example, water can be separated from salty water by simple distillation. This method works because the water evaporates from the solution, but is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.
This is good for separating two or more liquids from each other. For example, ethanol (alcohol) can be separated from a mixture of ethanol and water by fractional distillation. This method works because the two liquids have different boiling points.
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