Substances & properties

Is the powder in the kitchen sugar or salt? Is the necklace real gold? Chemistry answers questions like these not by guessing, but by looking closely and measuring. That is exactly where all chemistry starts: with the properties of substances.

Properties can be measured

Every substance has a whole set of properties: colour, smell, density, solubility in water, electrical conductivity, or whether it is magnetic. Many of them can be measured. Density, for example, tells you how much mass fits into a certain volume: water (H₂O) has about 1.0 g/cm³, iron 7.9 g/cm³, and gold as much as 19.3 g/cm³. These values do not depend on how big your piece is. A magnet tells you whether a substance is magnetic: iron is attracted to it, aluminium and copper stay put.

Same properties, same substance

This is the key idea: if two samples match in all their substance properties, they are the same substance. Only properties that do not depend on the size of the sample count here, so density, melting point or solubility, but not mass and volume. And the other way round: if they differ in even one of those properties, they are different substances. Sugar and salt look almost identical, but sugar melts in a pan at around 170 °C and turns brown, while salt just lies there unchanged and only melts at about 800 °C. One property is enough to tell the two apart.

A substance's profile

In chemistry, the most important properties of a substance are collected in a profile: colour, density, melting and boiling point, solubility, conductivity, magnetic or not. Density needs two measurements: you read the mass off the scales, and you get the volume of an irregularly shaped object by dipping it into a glass filled to the brim with water. The water that spills over has exactly the object's volume. With such a profile you can identify unknown samples like a detective: measure, compare, recognise.