An absolutely pure substance that does not contain impurities of other molecules is extremely difficult to obtain. Therefore, in practice, a pure substance is a substance consisting of molecules of the same type, in which other molecules are so small that they do not affect its properties.
For example, we consider spring water to be quite clean and drinkable because the tiniest impurities in it are simply imperceptible to us. But for some scientific and technical purposes, such water is completely unsuitable. For example, in closed circuits of nuclear power plants, even cleaner water is needed, otherwise, the pipes through which it circulates will quickly fail. But sometimes even cleaner water is needed! To obtain and store such ultrapure water, even glassware is no longer suitable, because other molecules get into the water from the glass. Only quartz dishes are suitable, and there should be no air above the water surface because many gaseous substances (carbon dioxide, nitrogen, oxygen) are soluble in water and "pollute" it.
Real substances always contain some impurities. Even pharmaceuticals with special purity requirements always contain minor impurities of other substances. But these impurities are so few that our bodies simply do not notice them. At the same time, a medicine with expired shelf life can become inactive or even have a side effect, because the amount of impurities in it has accumulated above a certain limit.
There is no need to think that pure substances are better than mixtures in all cases. For example, you cannot breathe pure oxygen. If nitrogen, which "dilutes" oxygen to a safe level, were to disappear from the atmosphere for some reason, all living organisms on land would die within hours, and vegetation on all continents would be completely destroyed by a monstrous fire.
Many medicines are mixtures. Pharmacists in pharmacies prepare mixtures from pure drugs, which means mixtures in Latin.
Solutions are special cases of mixtures. If a mixture of clay and sand can be distinguished by the eye from sand and clay, then dissolving the salt in water, we get a transparent liquid that looks indistinguishable from water. However, in this mixture, the particles that used to make up salt crystals are evenly distributed in the solvent (water) molecules. If water is allowed to evaporate, the salt will fall to the bottom of the vessel in the form of the same crystals. There is no chemical transformation - salt and water remain by themselves.
Even if the drugs are not absolutely pure, then what can be called (in the broadest sense) a pure substance, and what - a mixture? Fortunately, there are physical properties of the substance that stop changing after the amount of impurities decreases to a certain limit. For many purposes, a substance with such invariable properties can be considered pure. These physical properties are the melting point and boiling point. If the melting of a sample of a substance is not stretched several degrees but occurs within one degree - then such a substance is usually called pure by the chemist. If the substance boils at the same temperature, which does not change with the evaporation of the sample - it is also likely to be a pure substance. Of course, there are exceptions to these rules, so the final conclusion about the purity of the substance can only be drawn from the analysis. What chemical analysis is - we will talk about it later.
How do pure substances are released from mixtures? There are several ways to do this. Let's describe some of them.
To make the task look useful from a practical point of view, we will consider water purification. Suppose we have about half a liter of contaminated water at our disposal. It is not a problem to find such water in any settlement (you can take water from a puddle or a pond). Let's start cleaning it first by simple, and then by more and more complicated methods.
- DEVELOPMENT
- FILTRATION
- REGION or DISTILLATION
Thus, not only heterogeneous mixtures (water with soil particles) but also homogeneous mixtures (solutions) can be separated by distillation. By defrosting and filtering, homogeneous mixtures (solutions) cannot be separated.