Topic outline

  • General

    If you have any questions about the work in science you can contact me using the forum below.

    At the end of this theme I will ask you to complete the two choice questions below so that I can get an idea of what you think about the work.

  • Topic 1

    What is the difference between an element, a mixture and a compound?

    Materials can be put into groups in many different ways. We could group materials as metals and non metals. Materials like copper and gold are metals because of their properties. They conduct heat, they conduct electricity, they are shiny and they are flexible. We can also group materials according to how their particles are arranged.

    solid liquid gas

    In the theme 'Classifying Materials' in year 7 the idea that all materials (solids, liquids and gases) are made of particles was explored. These particles are either atoms or molecules. There are many different atoms. In the natural world there are about 100 different types. There are iron atoms, gold atoms, oxygen atoms, chlorine atoms etc. There are many more types of molecule. Molecules are formed when atoms join together. Two oxygen atoms join together to form an oxygen molecule (O2). In most materials different atoms join together. Two simpler examples are carbon dioxide and water. A carbon atom and two oxygen atoms join together to form a carbon dioxide molecule (CO2). Two hydrogen atoms and one oxygen atom join together to form water (H2O).

    element and compound

    When a material is made of only one type of atom it is known as an element. Aluminium is an element. When a material is made of molecules containing two or more different atoms it is known as a compound. Salt (sodium chloride) is a compound. It is made of atoms of sodium and chlorine. Each sodium atom is joined to one atom of chlorine.

    mixtures

    Many materials are mixtures. They are either mixtures of elements (like brass), mixtures of compounds (like salt dissolved in water) or mixtures of elements and compounds (like air). 

  • Topic 2

    How can you get clean water from dirty water and clean salt from dirty rock salt?

    Whenever there is a mixture of materials there will always be a way of separating them. The methods to use however are not always obvious. It depends on the properties of the materials involved.

    filtering

    You can get clean water from dirty water by filtering. This works because the fine particles of dirt and sand are insoluble. This means they do not dissolve. When the dirty water is poured into the filter paper the water passes through the tiny holes in the paper but the insoluble particles of dirt and sand get caught.

    rock salt mine

    The picture above is of a machine cutting rock salt from a mine in Cheshire. This is where much of the salt that we use comes from. You can see that the salt is not clean and white. It is full of impurities. How is it possible to get clean salt from dirty rock salt?

    pure salt from rock salt

    The first thing to do is to mix it with water. When you do this the salt dissolves but the impurities that are insoluble stay undissolved. You then filter the dirty salty water. The insoluble impurities collect in the filter paper and you collect the clean salty water. Finally you can heat this clean salty water. The water evaporates and the pure salt is left behind. 

  • Topic 3

    Does a green pen only have green ink in it?

    Sometimes people need to see what is in a mixture of different liquids. They need to analyse the mixture. They can do this by using a process called chromatography.

    chromatogram

    In this simple chromatogram made using a green felt pen you can see how the different inks have separated. There is a light green ink and a blue ink.

    lolly chromatogram

    This is an example of a chromatogram made by some food safety inspectors. They have tested an orange flavoured ice lolly to see if it contains artificial colours that could cause allergic reactions in children. It is possible to do this by comparing the spread of colours for the ice lolly with the spread for three artificial food colourings.In this case we can tell that the ice lolly contains all three food colourings as the 'blobs' for each of the food colourings (tartrazine, sunset yellow and ameretto) match the 'blobs' for the ice lolly. 

  • Topic 4

    Do all metals go rusty?

    Metals are very useful materials because of their properties. Metals are dense. This means that they are heavy for their size. Metals are shiny. Metals are flexible. This means they can be bent without them breaking. Metals are also good conductors of heat and electricity.

    aluminium saucepan

    Aluminium is used to make saucepans because it is a good conductor of heat.

    copper cable

    Copper is used to make electrical cables because it is a good conductor of electricity.

    iron and steel

    Iron is the most common metal element. It is a very useful material but there are problems. The first problem is that it is not as flexible as many other metals. This problem can be overcome by mixing it with carbon. This makes the alloy that we call steel.

    rust

    The second problem that happens with both iron and steel is that they rust. This means that they react with water and oxygen in the air to make a reddy, brown crumbly material. This material is actually a chemical called iron hydroxide but in everyday life we call it rust. Other metals including copper, aluminium and lead do react or corrode more slowly when they are in contact with air and water but the problem is not as bad as that with iron or steel.

    rusty bridge

    Rust can make iron or steel very weak. For this reason people have found ways of stopping iron from going rusty.

    iron railings

    Painting these iron railings does not only make them look more attractive, it also stops them from going rusty. Paint stops iron going rusty because it blocks out the water and oxygen in the air. 

  • Topic 5

    What happens to a metal when you heat it up?

    All things are made from tiny particles that are so small that they can not be seen through an ordinary microscope. Some of these particles are atoms. Copper is made of copper atoms. Some of them are molecules. Water is made of molecules. Water molecules are made of two hydrogen atoms and a water molecule joined together. The particles in solids, liquids and gases are put together in different ways.

    solid liquid gas

    It is possible for a solid material to become a liquid if it is heated up. We call this melting. In some materials (like wax) there are only weak forces holding the particles together in the solid.

    melting

    In other materials (like most metals) there are strong forces holding the atoms together. This means that most metals have very high melting points.

    copper

    Sometimes before metals start to melt they react with oxygen in the air. This happens with copper. When copper is heated the outside layer reacts with oxygen in the air to form copper oxide. The copper oxide that is formed is a black crumbly layer. It is a compound made of copper and oxygen atoms joined together.

    magnesium

    When magnesium is heated in air it reacts quite violently with the oxygen and magnesium oxide is made. Magnesium oxide is a white powder. It is a compound made of magnesium and oxygen atoms joined together.

  • Topic 6

    What happens to metals when you put them in acid?

    calcium

    There are some metals that are very reactive. Calcium is a metal that reacts very quickly if you put it in water.

    calcium in water

    It splits up the water molecules. This releases hydrogen gas and the calcium atoms join with the remaining atoms to make calcium hydroxide. We can test the gas to show that it is hydrogen. When we hold a lighted splint at the end of a test tube with hydrogen in it we hear a popping sound.

    magnesium in acid

    Magnesium only reacts slowly if it is put in water but if we put it in hydrochloric acid it releases lots of bubbles of hydrogen very quickly. By seeing how metals react when we put them in water and acid we can work out which ones are more reactive and which ones are less reactive. We can work out the reactivity series for metals. The chart below gives an overview of the reactivity series.

    reasctivity series

  • Topic 7

    Why can’t you buy jewellery made out of magnesium?