A world of mixtures
The ground that you stand on, the air that you breathe and the oceans of
the Earth are all mixtures. For example, air is a mixture of many gases,
including nitrogen, oxygen and carbon dioxide. Ocean water is a mixture of
pure water, salt and many other substances. Even the fresh water in lakes
and rivers is not pure and always contains small amounts of other substances.
These are all considered mixtures.
A mixture is two or more different materials that have been mixed together.
A mixture contains two or more substances that are not chemically
combined.
Activity 1: Mixing solids
Materials:
500g bag of barley (beans or lentils will also do)
500g bag of rice
small packet of sugar
clean sand
plastic spoons for scooping
paper cups for mixing
sieve (the type used for sieving flour)
Procedure and observations (Part 1):
1. Place 3 scoops of barley in the plastic cup.
2. Place 3 scoops of rice in the plastic cup.
3. Stir the barley and rice until they are mixed.
4. Answer the questions below.
1) Can you still see the individual rice and barley grains?
______________________
2) Make a labelled diagram of the mixture.
3) Separate the mixture into a pile of rice grains and a pile of barley
grains. Write a sentence to explain how you separated the mixture.
___________________________________________________________________________
___________________________________________________________________________
4) Did the barley and rice grains change in any way, or do they still look
the same as before they were mixed?
___________________________________________________________________________
Part 2
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Place 3 scoops of rice in the plastic cup.
Place 3 scoops of sugar in the plastic cup.
Stir the sugar and rice until they are mixed.
Answer the questions below.
1) Can you still see the individual rice and sugar grains?
_________
2) Can you think of a quick way to separate the mixture, using a sieve?
Describe what you would do to separate the mixture. Describe what
would happen to the mixture.
_____________________________________________________________________
_____________________________________________________________________
3) Did the sugar and rice grains change in any way, or do they still look
the same as before they were mixed?
_____________________________________________________________________
_____________________________________________________________________
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Part 3
Place 3 scoops of sand in the plastic cup.
Place 3 scoops of sugar in the plastic cup.
Stir the sugar and sand until they are mixed.
Answer the questions below.
1) Can you still see the individual sand and sugar grains?
_____________
2) Can you separate the mixture into a pile of sand grains and a pile of
sugar grains? How long would it take if you picked the sand grains out of
the sugar one by one?
_______________________________________________________________________
_______________________________________________________________________
3) Do you think that it would be possible to separate the mixture using a
sieve? Why do you think so?
_______________________________________________________________________
_______________________________________________________________________
In the previous activity we mixed solid materials with different sized grains and
learnt that:
• when the grains are large enough, we can separate them by hand
• when the two materials have grains of different sizes they can be
separated by sieving.
Separating mixtures
Some mixtures can easily be separated into their different parts. For example,
imagine that a few small iron nails have been dropped into a child’s sandpit
and have sunk into the sand so that they can’t be seen. One way of
separating the nails from the sand is to use a magnet. This works because the
nails and sand have different properties, or features. The nails are made from
a substance that is attracted to magnets, but the sand is not attracted to
magnets.
But what if plastic beads had been dropped into the sandpit instead of nails?
They can’t be separated from the sand with a magnet. The key to separating
mixtures is recognising the different properties of the plastic beads and the
sand. An obvious difference is size. The plastic beads are much bigger than
grains of sand so the sand grains pass through but the plastic beads don’t.
In some mixtures, the different materials are still clearly visible after mixing. In
other mixtures, the materials are mixed so thoroughly that it seems one
material has 'disappeared' into the other. Such mixtures are called solutions.