What are materials made of?
       4. Elements and compounds
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What are elements and compounds?

Water is a liquid. But what is it made of? If we pass an electric current through water it splits into two gases: hydrogen and oxygen.

So what are hydrogen and oxygen made of? No matter how hard we try, we can’t split them up into anything simpler. They are elements.

What is an element?
Elements are substances (like hydrogen and oxygen) that can’t be split into simpler substances.

A substance like water, that is made up of two or more elements, is called a compound.

What is a compound?
Compounds are usually very different from the elements that have combined together to make them. Water, for example, is a liquid that we can use to put out fires. Both oxygen and hydrogen are gases, and hydrogen easily catches fire.
Sodium and chlorine make salt
Picture 4.1 Would you put sodium and chlorine on your chips?
Photos of sodium and chlorine
Picture 4.2 A lump of sodium and green chlorine gas in a gas jar.

Are compounds like their elements?
The scientific name of common salt is sodium chloride. It is another compound. It is made up of the elements sodium and chlorine. We often add sodium chloride to our food during cooking.

Sodium, by contrast, is a silvery solid that reacts violently with water. And chlorine is a green, poisonous gas. Clearly, when they join to form the compound sodium chloride, there is a fundamental change in the properties.

This is true of all compounds. They no longer have the properties of the elements that make them up. They have their own properties.

Compounds and mixtures
Mixtures are different from compounds. They are still a bit like the materials that make them up.

For example, sodium chloride (salt) and water are both compounds. They mix together when salt dissolves in water to form salt solution. A solution of salt in water is sometimes called brine. Salt solution is a mixture not a compound. It still has the liquid properties of water and the taste of salt. We can fairly easily separate the salt and water from each other.

It is much, much more difficult to un-combine the elements that make up compounds.

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Question 16
Look at the table below. There are some statements and some materials.

For each statement, decide whether it describes a mixture or a compound and check the box.

For each material, decide whether it is a mixture or a compound of the materials shown in brackets. Then tick the box.

  compound mixture
It keeps the properties of the materials that make it up.
It is difficult to separate into the materials that make it up.
There was a chemical reaction when it was formed.
Salt (sodium and chlorine)
The air (oxygen, nitrogen and carbon dioxide)
Water (hydrogen and oxygen)
Rock salt (sand and salt)