In a mixture there are two or more substances combined which have not reacted chemically with each other. In other words, nothing further than mixing has taken place. Each constituent remain intact within it, and are able to demonstrate all the chemical properties that characterize it as a distinct substance. Therefore, a mixture of salt and sugar is both salty and sweet. A mixture of sand and gunpowder will certainly explode if lighted (of course, not as well as pure gunpowder would). Some people like to drink shandy, which gives them a taste of beer and lemonade at the same time. This is because both those beverages are in the mixture, not having changed to anything else. The air that you breathe is also a mixture of quite a few gases. A candle burns, demonstrating that there is oxygen in it. Blow into limewater and it turns milky, demonstrating that there is carbon dioxide in it too.
Mixing also takes place in a compound, with the difference that a fundamental chemical change has also taken place. Therefore, the original substances mixed together to make a compound are no longer present in it. Oxygen reacts violently with hydrogen to produce water. In water there is neither hydrogen nor oxygen, at least none in so far as the original gases are concerned. To start off with, water is a liquid at room temperature while the other two are gases. Water does not help to burn a candle, so there is no oxygen there. It does not explode in the presence of oxygen, showing that there is no hydrogen there either. Water is something entirely new from the constituents that produced it, having distinct chemical properties of its own.
A few other distinctions can be made between a mixture and a compound. A mixture can come in any ratio whatsoever – a pinch of salt in a cup of sugar, a lot of lemonade to make a very light shandy etc. But in a compound the ratio of its constituents is always the same. Therefore, it is always the case that two volumes of hydrogen react with one volume of oxygen to produce water.
It is also emphasized that, with a mixture, the original substances can no longer be retrieved. Chemists have an armory of physical techniques to do this – sedimentation, distillation, filtration, chromatography etc. But after chemical change has taken place – i.e. the compound has been formed – this is no longer possible, at least not by any physical means.
This distinction between a mixture and a compound was not even clear a century and a half ago. In fact, it was the effort to clear this confusion that gave rise to modern chemistry. Before Dalton’s atomic theory, early in the 19th century, the elements were thought to be four – earth, water, air and fire (and sometimes a fifth, ether; the study of these was the preserve of alchemy). So, these were the basic building blocks of nature, and everything was a “mixture” of these, in a certain ratio. If one thing changed into another, the ratio of the elements had changed. Therefore, to burn a piece of wood is to add fire to it, and the new mixture is charcoal.
When Dalton proposed the atom to be the fundamental constituent of matter, he not only predicted the existence of elements (atoms of a certain kind), but also compounds (two or more ‘elemental’ atoms coming together). Dalton had the theory, but chemists in reality were faced with a confusing picture. They were not unaware of chemical change, therefore knew how two substances chemically reacting with each other was different from a mere physical mixture. But the alchemical notion that elements combine in all ratios was hard to wear off.
The principal source of confusion was the alloys. The mixing of metals had been practiced from early history, and a certain alloy (a mixture of two metals in a certain ratio) produced desirable improvements from the constituent metals that made it up, yet remained metallic in nature. Was an alloy a mixture or a compound? It certainly appeared to be more than an ordinary mixture. And it was also noted that alloys can be obtained in any ratio. Dalton’s theory predicted chemical combinations in only certain ratios.
Also, iron was known to combine with oxygen to produce iron oxide. However, there was not only one, but three iron oxides, each with a different set of properties. The French chemist Louis Berthollet demonstrated that the difference between them was due to the proportion of oxygen that each contained. Other metals displayed similar properties. Therefore, the evidence was that substances combined in different ratios.
However, Dalton’s theory was vindicated in the end. The observation of Berthollet was explained as due to certain metals having more than one valancy (i.e. had and atomic structure that allowed for more than one combination). Alloys were found to be mixtures (or something more than a mixture, and that allowed for the improvement of certain properties, but not yet a chemical compound). Therefore, looking back at its history, we can see that the difference between a mixture and a compound is highly instructive. It was the original question that gave rise to modern chemistry.