Why are melting and boiling points helpful

Identify substances with their properties

Material properties enable us to say exactly which material it is. One possibility is to want to differentiate between two substances. This is often not that difficult at all. But if you don't know which substance it is, you first have to determine its properties and then compare them with a collection, for example in a table. An example that you may already know from your chemistry class is the distinction between iron and sulfur. With these two elements we now want to look at how a combination of properties enables identification.

colour

Sulfur is yellow. Iron is gray. But many other fabrics can also have these colors ...

Physical state

Both substances are solid at room temperature. So that doesn't help at first.

shape

Sulfur is powdery or in lumps that crumble easily. Iron, on the other hand, can exist in many forms: as shavings, as powder, as a nail, as a plate ...

magnetism

Sulfur is not magnetic, but iron is.

density

It is commonly said that iron is "heavier" than sulfur, but what is meant is density: How heavy is a certain volume of a substance? If you divide the mass of a substance by its volume, you get the density (density ρ = mass m / volume V). This is given in the unit g / cm³. The density of iron is ρ (Fe) = 7.874 g / cm³. The density of sulfur, on the other hand, is ρ (S) = 2.07 g / cm³. These values ​​are much more specific than the properties we have considered so far.

Melting temperature and boiling temperature

One of the most important properties when you want to know what substance it is are melting and boiling points. These depend on the conditions under which they are determined, for example the air pressure. It is therefore important to always state the conditions under which the experiment was carried out. If you carry out an experiment in the chemistry room, "standard conditions" prevail almost everywhere. Schools that are very high up, for example in the Alps, are an exception. There the air pressure may be lower. The melting temperature of iron is 1538 ° C, its boiling temperature 3000 ° C. For sulfur, the melting temperature is 115 ° C, the boiling temperature is 445 ° C. These exact values ​​are also helpful.

Responsiveness

If you have a suspicion and want to prove that it is a specific substance, you can carry out so-called detection reactions. Detection reactions describe chemical reactions that only certain substances enter into. However, these fall under a different area of ​​the chemistry book.