Pure substances and formulations (AQA GCSE Chemistry): Revision Notes
Pure substances and formulations
What is a pure substance?
A pure substance contains only one type of material. This can be either:
- A single element (like oxygen or gold)
- A single compound (like water or carbon dioxide)
Pure substances have very specific properties. They melt and boil at exact temperatures that scientists have recorded in data books.
How to test if something is pure
Scientists use melting point to check purity. Here's how it works:
The melting point test
Worked Example: Testing Substance Purity
Step 1: Heat the substance gently in a special apparatus Step 2: Watch the temperature when it starts to melt Step 3: Record your observations:
- A pure substance will have a sharp melting point - it melts quickly at one specific temperature
- A mixture will melt over a range of temperatures - it takes longer and happens gradually
Comparing results
- Look up the expected melting point in a data book
- If your result matches exactly, the substance is pure
- If it's different or melts over a range, it contains impurities
Remember: Even small amounts of impurities will cause a substance to melt over a range of temperatures rather than at a single point. This is one of the most reliable tests for purity!
Different meanings of "pure"
Be careful - "pure" means different things:
In science: Pure means a single element or compound with nothing else mixed in
In everyday life: Pure often means "natural" or "nothing artificial added"
Don't confuse these two meanings! "Pure milk" in a shop means milk with no additives, but it's still a mixture of water, fats, and proteins - not scientifically pure.
What are formulations?
A formulation is a specially designed mixture where:
- Each ingredient has a specific job
- The amounts are carefully measured
- Everything is mixed for a particular purpose
Examples of formulations:
- Paints - contain pigments (colour), binders (stick to surface), and solvents
- Medicines - active ingredient plus other helpful substances
- Fuels - different chemicals mixed for best performance
- Cleaning products - various chemicals that work together
- Foods - ingredients combined for taste, texture, and preservation
Each part of a formulation does something important. Changing the amounts or removing ingredients would stop the product working properly.
Separating mixtures
When substances are mixed together, you can separate them using their different physical properties. Scientists use methods like filtration, distillation, and chromatography to split mixtures into their separate parts.
Different separation methods work because they take advantage of different physical properties - for example, filtration uses particle size differences, while distillation uses different boiling points.
Key Points to Remember:
- Pure substances are single elements or compounds that melt at specific temperatures
- Test purity by measuring melting point and comparing with data book values
- "Pure" in science means single substance; "pure" in everyday life means no additives
- Formulations are carefully designed mixtures where each part has a job
- Common formulations include paints, medicines, fuels, and cleaning products