Pure Substances & Mixtures

Activities to try

1. 1

   Purity tester: reading heating curves of pure vs impure samples

   Why it works: A thermometer can act as a purity detector: at a given pressure a pure substance changes state at a single sharp fixed temperature, while a mixture usually changes state over a range and at shifted values — so melting/boiling behaviour is a test for purity, not just a property to memorise.

   1. Heat a sample of ice made from distilled water, stirring it, and watch a temperature-vs-time graph: observe that it stays roughly flat at about 0°C (at normal air pressure) while it melts, then climbs once it has fully melted.
   2. Beside it, heat frozen salt water and observe that it starts melting below 0°C and that its temperature creeps up while it melts, instead of holding at one clean plateau. The exact temperatures depend on how salty the water is.
   3. State in your own words what the SHAPE of each curve shows: one sharp flat step suggests a pure substance; a sloped spread across a range suggests a mixture (a useful rule at this level, although a few special mixtures can also change state sharply).
   4. Take a 'mystery sample' round: given unknown heating curves, judge whether each sample is pure or a mixture and give your reason from the curve shape.
   5. As a bonus, heat salty water next to pure water under the same conditions and compare. The salty water starts boiling at a slightly higher temperature (usually only about a degree, even with a lot of salt), and its temperature keeps creeping up as water boils away and the solution gets saltier. This shows that impurities both shift AND spread the transition.
2. 2

   Chemistry 'pure' vs supermarket 'pure'

   Why it works: The scientific meaning of 'pure' differs sharply from everyday use: orange juice and spring water are mixtures even though adverts call them 'pure', because scientifically 'pure' means a single substance only.

   1. Examine an everyday label such as '100% pure orange juice — all natural' and decide whether a chemist would call it pure.
   2. Compare a tap-to-reveal contrast of pure-substance and mixture particle pictures alongside annotated sharp-vs-range curves.
   3. Write a sentence distinguishing the two meanings: everyday 'pure' = natural/nothing added; chemical 'pure' = a single element or compound with nothing else in it.
   4. Classify examples: spring water (a mixture), distilled water (pure), salty water (a mixture that still looks clear).
3. 3

   Formulations: why we deliberately make mixtures

   Why it works: Not all useful materials are pure: many are formulations — mixtures designed to a precise recipe so each ingredient contributes a needed property. Purity is desirable for identification, but mixtures are often what we actually want.

   1. List everyday designed mixtures — medicines, alloys, paints, fuels — and identify what useful job each component does.
   2. Explain in a sentence why a 'formulation' (a mixture made to a precise composition) is deliberately not pure.
   3. Discuss that the substances in a mixture are not chemically joined, so each keeps its own chemical identity and the mixture can in principle be separated by physical means, even though the mixture as a whole can have useful properties that its separate components lack (for example, an alloy is often harder than the pure metal).