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Particles in a solid vibrate around fixed positions, in a liquid move around one another, and in a gas are widely separated and move randomly. Heating can increase kinetic energy or supply energy for a change of state. Melting and boiling do not create a new element or change every molecule into individual atoms. A pure substance has a characteristic melting or boiling point at a stated pressure; mixtures often change state over a range.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain changes of state using particles
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which method separates an insoluble solid from water?
Filtration. The solid is retained by the filter.
After watching 2: What happens to particle identity when pure water boils?
Water molecules remain water molecules. This is a physical change.
What you'll be able to do
Particles in a solid vibrate around fixed positions, in a liquid move around one another, and in a gas are widely separated and move randomly. Heating can increase kinetic energy or supply energy for a change of state. Melting and boiling do not create a new element or change every molecule into individual atoms. A pure substance has a characteristic melting or boiling point at a stated pressure; mixtures often change state over a range.
Filtration removes an insoluble solid from a liquid. Crystallisation recovers a dissolved solid by concentrating its solution and cooling. Simple distillation collects a solvent; fractional distillation separates miscible liquids with different boiling points using repeated vaporisation and condensation. These methods rely on physical properties rather than chemical reactivity. Select a method by stating what needs to be collected, not merely by naming equipment.
A mobile solvent carries components over a stationary phase. Different attractions make components travel different distances. Draw a pencil baseline above the solvent and mark the solvent front. Rf is spot distance divided by solvent-front distance, both from the baseline. It has no unit. Reference and unknown samples must run under the same conditions, and equal Rf values support but do not uniquely prove an identity.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A spot moves 2.4 cm and the solvent 6.0 cm. Find Rf.
0.40.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.