Science 8th grade
UNIT 8: Mixtures 8.2 More about mixtures
Science 8th grade
UNIT 8: Mixtures 8.2 More about mixtures
In science the word "pure describes something only containing a single substance. Pure water contains just water, with no other substances mixed with it.
A mixture is not pure. It is made up of different kinds of particles that are mixed together. The mixture may be of clements, compounds or both. There are solids, liquids and gases that are mixtures.
Question
1) The diagram on the right shows some particles in air. The orange circles represent oxygen atoms. The blue circles represent nitrogen atoms. The white circles represent hydrogen atoms. The black circles represent carbon atoms.
a: Which is the most common element in air?
b: How many different kinds of molecules are shown in the diagram? Can you name them all?
Metal mixtures
Metal mixtures are called alloys. Alloys are made by mixing different metals together and then melting them. The atoms of the different metals mix together, but they do not bond together to form a new compound.
Bronze is an alloy made by mixing copper and tin. Bronze is harder than either copper or tin.
Steel is an alloy, but a rather odd one because one of the elements in the mixture is not a metal. Steel is a mixture of iron and carbon. Sometimes chromium and nickel are also added to steel. This type of steel does not rust and is used for cutlery.
Questions
2) Explain what an alloy is.
3) Give two examples of alloys.
4) If you had some copper and tin, how would you make bronze?
$A+I$: 5) Suggest why a helmet made of bronze is more useful than a helmet made of copper or tin.
Mineral water is a mixture
If you look at the label on a bottle of mineral water, you will sce that many minerals are listed. There is more than just water in the bottle. The bottle contains a mixture of water and other substances.
These minerals are dissolved in the water. The mineral water is a solution. A litre of water may have about $0/5g$ of minerals dissolved in it. You will find out more about solutions on pages 100-103.
Question
6) Look at the picture of the mineral water label.
List the three most abundant minerals in this bottle of mineral water.
Activity 8.2 (Is it a mixture?)
SE: Your teacher will give you a container of water. Your task is to discover if there is any other substance mixed with the water.
1) Place the water in an evaporating basin and heat until it boils.
Safety: Wear safety glasses.
2) Continue to heat gently.
Safety: The solution may start to spil.
3) When you have evaporated off some of the water (or the solution starts to spit) remove from the heat and leave the evaporating basin to cool.
Safety: Do not touch the evaporating basin with your hands - usc tongs.
The water may take a day or two to evaporate completely. It will depend on the temperature.
Questions
${A_1}$: Use ideas about particles to explain why the water evaporated.
${A_2}$: What was left in the evaporating basin?
${A_3}$: Where has this substance come from?
${A_4}$: Was the water you were given pure water, or was it a mixture of water and other substances? Explain your answer.
${A_5}$: Why did you need to wear safety glasses?