Pure substances and formulations (AQA GCSE Chemistry Combined Science): Revision Notes
Pure substances and formulations
What are pure substances?
A pure substance contains only one type of element or one type of compound. Nothing else is mixed in with it.
In science, we use the word "pure" to mean exactly one element or one compound. In everyday life, "pure" often means something has nothing artificial added to it. For example, "pure milk" usually means milk with no additives.
Key point: Pure substances are made of only one thing - either a single element (like oxygen gas) or a single compound (like water).
Testing purity using melting points
Pure substances have a sharp melting point. This means they melt at one exact temperature.
When you test a pure substance:
- It melts completely at a specific temperature
- The melting point matches the value in data books
- The temperature stays constant while melting happens
Worked Example: Testing Substance Purity
Step 1: Heat the substance gently in a melting point apparatus
Step 2: Watch the thermometer as the solid melts
Step 3: Record the exact temperature when melting occurs
Step 4: Compare your result with the data book value
If your substance is pure, your melting point will match the data book exactly.
If your measured melting point doesn't match the data book value, or if melting occurs over a range of temperatures, your substance contains impurities.
What happens with impure substances?
Mixtures and impure substances behave differently:
- They have a variable melting point (melting happens over a range of temperatures)
- They don't have one sharp melting point
- The melting point won't match data book values exactly
Worked Example: Identifying Impurities
If aspirin should melt at 136°C but your sample melts between 132-136°C, it contains impurities. The wider the melting range, the more impurities are present.
Understanding formulations
A formulation is a mixture that has been specially designed for a specific job. Each ingredient in the mixture has its own important purpose.
Key features of formulations:
- They are complex mixtures
- Each component serves a specific function
- The amounts of each ingredient are carefully measured
- They are designed to do a particular job
Examples of formulations:
- Paints: contain pigments (for colour), binders (to stick), solvents (to spread easily)
- Medicines: contain active ingredients (to treat illness) plus other substances (to help absorption)
- Cleaning products: contain different chemicals that work together to clean effectively
- Fuels, fertilisers, and foods are also formulations
Each part of a formulation is there for a reason. Remove one component and the formulation won't work properly.
Separating mixtures
Mixtures can be separated using physical methods. The components keep their own properties, so we can use these differences to separate them.
Common separation methods include:
- Filtration (for solids and liquids)
- Distillation (for liquids with different boiling points)
- Chromatography (for separating dissolved substances)
Key Points to Remember:
- Pure substances contain only one element or one compound
- Pure substances melt at a sharp, specific temperature that matches data books
- Mixtures have variable melting points over a range of temperatures
- Formulations are useful mixtures where each component has a specific purpose
- Examples of formulations include paints, medicines, fuels and cleaning products