When two or more substances are combined together but not in any particular or fixed proportion then it is called a mixture. A mixture can be classified into two categories - homogeneous mixture and heterogeneous mixture. A mixture that has a same composition throughout with uniform appearance is called homogeneous mixture.
A homogeneous mixture is also known as a solution. A heterogeneous mixture can be defined as a mixture which is composed of more than one phase and can be separated by mechanical means. Heterogeneous mixture do not have a uniform composition throughout the mixture.
Heterogeneous Mixture Explained
Suspensions and emulsions are two special types of heterogeneous mixture. A suspension is a mixture of solids and liquid in which the solids do not dissolve in the liquid solution. When a suspension is allowed to stand for some time and left undisturbed, the solid particles settle at bottom of the container. An example of suspension is sand in water and clay in water. An emulsion is a special type of suspension, where a mixture consisting of two liquids, do not mix together.
When two such liquids are kept together, they do not mix, no matter how much you try to do so by shaking or stirring. They will ultimately settle into two layers one above the other when left undisturbed for a considerable period of time. Examples of emulsions are kerosene and water and a mixture of oil and water. Both suspensions and emulsions show the Tyndall effect.
For example, if you mix sugar granules in a glass of water, the sugar granules completely dissolves in the water. This sugar solution is an example of homogeneous mixture. Now if you want to separate the sugar crystals from the sugar solution it is not possible to do so by mechanical means. For this you have to first boil the sugar solution so that all the water evaporates into the air and the sugar crystals will be left behind.
Now let us take the example of a heterogeneous mixture. If you have a mixture of two substances say, sand and iron fillings then you can easily separate them by mechanical means. This can be done by using a magnet to sift the iron fillings away. All you will be left behind is the sand. Thus a simple mixture can be separated by mechanical means. Some other mechanical means by which a mixture can be separated are sifting, filtration and by using gravity.
So this was all about heterogeneous mixture and how they can separated into their individual components. The most important thing to remember is that heterogeneous mixture consists of visibly different phases and substances and they are never chemically combined.