Almost everything is made of different substances mixed
together. Things are only easy to recognize as mixtures if the PARTICLE SIZE of each substance is big enough to see. The flakes, nuts, and
raisins in a bowl of cereal are a mixture that is easy to see. A fruit drink,
though, doesn’t look like a mixture because the particles of fruit and
water are so small. It is a type of mixture called a SOLUTION,
made of different, very tiny particles dissolved (evenly spread out) in
water.
Rock, sand, and seawater are all mixtures of the same substances
– such as the minerals feldspar, mica, and quartz – but in
different particle sizes. Rock contains these substances in chunks or veins;
sand has them as small grains; and seawater contains them as tiny dissolved
particles that are invisible to the eye. Rain and rivers dissolve the minerals
as they wash over the rock on their way to the sea.
All rocks are mixtures of naturally occurring substances called
minerals. Granite is a common rock made of three differently coloured minerals
called feldspar, mica, and quartz. The pink grains in granite are feldspar, the
black grains are mica, and the light grey, glass-like grains are quartz.
Granite is usually about 75% feldspar, 5% mica, and 20% quartz. These
proportions can vary and the rock often contains small amounts of other
minerals as well.
There are many different types of mixtures, which are
divided into groups based on how small their particles are. A mixture such as
sand has a large particle size. Mud stirred in water is a type of mixture
called a suspension; the particles are too small to see when mixed, but they
eventually settle out. A mixture such as fog (water and air) is called a
colloid; its particles are too small ever to settle out.
The particles of some mixtures are large enough to see without a
microscope. When you look closely at a handful of sand, for example, you can
make out the different coloured grains mixed together. Some sands have smaller
grains than others. The smaller the grain size, the softer and more powdery the
sand feels.
A colloid is a mixture containing tiny particles of one substance
scattered throughout another substance, such as dye particles mixed with glass
in a marble. The particles are smaller than those in a suspension, but larger
than those in a solution. The particles are so small and light, they do not
ever settle out.
Milk is made up of tiny globules of fat scattered throughout
water. It is an example of an emulsion, a special type of colloid in which oils
or fats are mixed with water to create a creamy liquid or paste. Other examples
of emulsions are mayonnaise, emulsion paints, lipsticks, and face creams.
A solution is a mixture in which the different particles
are tiny and are mixed completely evenly. Solutions are often made by
dissolving a solid, such as sugar, into a liquid, such as water. The sugar is
called the solute and the water is called the solvent. Water is the most common
solute. Solutions can also be a liquid dissolved in another liquid, for example
antiseptic liquid. This is water and alcohol. Or they can be a gas dissolved in
another gas, such as oxygen dissolved in nitrogen in the air.
Wood’s Metal is found in automatic fire sprinklers. It is
an alloy (mixture of metals) containing bismuth, lead, tin, and cadmium. This
mix of metals has a low melting point of 71ºC (158ºF). It is used
as a sensor in automatic fire sprinklers; if the temperature gets too high, the
metal alloy melts and releases the water.
