What are the properties of pure substances?

Pure substances have a fixed composition, consistent properties, and cannot be separated into simpler substances by physical methods.

Pure substances are a key concept in chemistry. They are materials that are made up of only one type of particle, whether it be an atom, molecule or ion. This means that they have a fixed composition. For example, pure water is always made up of two hydrogen atoms and one oxygen atom. This is different from mixtures, which can have varying compositions.

One of the most important properties of pure substances is that they have consistent properties. This means that no matter where you find a pure substance, it will always have the same physical and chemical properties. For instance, pure gold will always have the same density, colour, melting point and reactivity, whether it's found in a jewellery shop in London or a mine in South Africa.

Another key property of pure substances is that they cannot be separated into simpler substances by physical methods. This is because all the particles in a pure substance are the same. For example, you cannot separate pure water into hydrogen and oxygen by filtering or distilling it. Instead, you would need to use a chemical reaction, such as electrolysis, to break the bonds between the hydrogen and oxygen atoms.

In summary, pure substances are unique in their fixed composition, consistent properties and the fact that they cannot be separated by physical methods. Understanding these properties is crucial for many areas of chemistry, from identifying substances to predicting how they will behave in different conditions.