Mixtures

Rarely in nature do we find pure substances. Often chemists have to spend a lot of effort into making pure substances. Instead most items we see around us are made of mixtures.

Mixtures vs. Compounds

A mixture is simply where two or more different compounds or elements are brought together, but are not bonded together. As an example let's go back to the table salt. If we want to break apart the salt compound, then this is something we cannot do easily. This is because the sodium and chlorine are bonded together strongly. We have to put a lot of energy in to break them apart.

Now let’s take the salt and place it in to water. If we stir the water until the salt dissolves then we have a glass of salty water. Is it possible to easily separate them again?

Well yes, it is actually fairly easy to separate them. One way we could do this is to simply leave the glass of salty water near a window. The water would slowly evaporate and then after a few days the salt would be all that is left. You can try this for yourself - sugar would also work instead of salt. This is a slow process, so we can speed it up. We could do this by putting the salty water into a pot and boiling it.<\p>

What is happening? In both cases we are using the fact that the water and the salt have different boiling points. They are only physically mixed together, not chemically bonded. By boiling the salty water, the water is turning into steam and leaving the pot. However, the salt has a much higher boiling point. The salt does not leave the pot and instead remains behind. We once again have pure sodium chloride and all we need to do was change the physical state of the water. Most of the time the mixtures can be separated by some kind of physical method as described above.

There are two types of mixtures. Mixtures where the substances are evenly mixed throughout (called homogeneous mixtures) and mixtures where it is unevenly mixed (which we call heterogeneous mixtures).

Homogeneous Mixtures

Homogeneous mixtures (sometimes also called homogenous mixtures) are mixtures where the two or more substances are mixed together so that if you take any part of that mixture then it will have roughly the same percentages of each substance. Our example above, using salty water, is an example of an homogeneous mixture. Other examples are things like milk, stainless steel (a mixture of iron, cabon, nickel, chromium and other metals), blood, and air.

Heterogeneous Mixtures

Heterogeneous (or heterogenous) mixtures are where the mixture is not even throughout. In this case if we took 3 different samples of the mixture from different places we would expect to see different percentages of the substances. An example of this is if we mix sand with water. The sand does not dissolve but instead sinks to the bottom of the container. There will be less sand at the top of the water than at the bottom. If we mix it vigorously, then we get the mixture to be homogeneous, but only for a very short time, before the sand settles back down to the bottom again.

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