How do melting and boiling points indicate purity?

Melting and boiling points indicate purity as pure substances have specific, consistent melting and boiling points.

In more detail, every pure substance has a unique and specific melting and boiling point. These are physical properties that can be used to identify a substance or to check its purity. When a substance is pure, it will melt or boil at a specific temperature. For example, pure water boils at 100 degrees Celsius at standard atmospheric pressure. If the water is impure, it will boil at a slightly different temperature.

Impurities in a substance disrupt the regular arrangement of particles, which affects the forces between them. This means that more or less energy is needed to change the state of the substance, which changes the melting or boiling point. For instance, if there are impurities in a sample of water, these impurities can disrupt the hydrogen bonding between water molecules, causing the water to boil at a temperature lower or higher than 100 degrees Celsius.

In a practical sense, if you were to heat a pure substance, it would melt or boil at a single, sharp temperature. However, a mixture would melt or boil over a range of temperatures, as different components in the mixture would have different melting and boiling points. This is why melting and boiling points are often used in laboratory settings to test the purity of a sample.

In conclusion, the melting and boiling points of a substance are a clear indication of its purity. Any deviation from the known melting or boiling point of a pure substance indicates the presence of impurities. Therefore, by comparing the observed melting or boiling point with the known values for a pure substance, one can determine whether a substance is pure or not.