Solids, Liquids and Gases Lesson PlanIGCSE Chemistry 0620 · for teachers, tutors and self-study

Read out or write the four questions on the board. Students answer in their exercise books (optional: mini whiteboards) and hold them up on your signal. For the first two, they can show one finger for solid, two for liquid, three for gas.

Spend most of the time on question 4. Many students say the space between gas particles is "air" — ask "What is air made of?" to show that air is itself a gas made of particles, so the space between the particles is empty. Keep the answers on the board for the next stage.

Answer each question in your exercise book, then hold it up or show fingers (1 solid, 2 liquid, 3 gas) when asked.

Compression demonstration (optional: two sealed syringes, one of air and one of water). Push each plunger: the air compresses, the water barely moves. No syringes? Describe the experiment and take a vote with hands up: "Which one can I push in?" Then ask why — this leads to particle spacing.

Draw a table on the board with three rows — separation, arrangement, motion — and a column for each state:

• Solid: particles touching / very close; regular pattern; vibrate about fixed positions.
• Liquid: particles close together, mostly touching; irregular (random); move around, sliding past each other.
• Gas: particles far apart; random; move quickly in all directions.

Stress that each row answers a different question: separation is how far apart, arrangement is the pattern, motion is how they move. Then draw a second, separate table of bulk properties (fixed shape? fixed volume? flows? compresses easily?) and say clearly that these describe the substance, not the particles. Model Worked example 1: Particle diagrams and the properties of the three states, drawing three boxes of circles on the board.

Finally draw the changes-of-state triangle (solid, liquid, gas) with the five names: melting, freezing, boiling or evaporating, condensing. Mark energy taken in for melting, boiling and evaporating, and given out for freezing and condensing. Contrast boiling (throughout the liquid, only at the boiling point) with evaporation (at the surface, below the boiling point), as in Worked example 2: Naming the changes of state.

Copy both tables into your exercise book, keeping particle statements and bulk properties apart. Copy the changes-of-state triangle with energy in or out on each arrow. Then answer the two check questions with a partner.

Heating curve (Extended). Sketch a heating curve on the board for a solid heated at a constant rate: rising line, flat section, rising line, a longer flat section, rising line. Label each section with the state or states present: solid; solid and liquid (melting); liquid; liquid and gas (boiling); gas. Then explain the flat sections in particle terms:

• Energy is still being supplied, but it is used to overcome the forces of attraction between the particles, not to make them move faster, so the temperature stays constant.
• The boiling section is longer than the melting section: boiling separates the particles completely, so more energy is needed (Worked example 9: Why boiling takes longer than melting).

Model Worked example 4: Interpreting a cooling curve in particle terms. On a cooling curve the flat sections line up with the boiling point and the melting point: a pure substance condenses at the temperature it boils and freezes at the temperature it melts. While a liquid freezes, energy is released as the particles are attracted into fixed positions, which replaces the energy lost to the surroundings.

Gases. Optional demonstration: put a sealed syringe of air (free-moving plunger) into hot water — the plunger moves out. No equipment? Describe it, or have students imagine a balloon left in a hot car, and ask them to predict with thumbs up (bigger) or thumbs down (smaller).

• Core (describe): at constant pressure, a higher temperature gives a larger volume; at constant temperature, a higher pressure gives a smaller volume.
• Extended (explain): at a higher temperature the particles have more kinetic energy and move faster, so they hit the walls more often and with more force; the gas expands until its pressure balances the pressure outside, and the particles end up further apart. A higher pressure pushes the particles closer together. The particles themselves never get bigger. Point to Worked example 7: Gas syringe in a water bath: temperature and gas volume.

Copy the labelled heating curve and write one sentence beside each flat section explaining it. Answer the two questions in your book, then compare with a partner.

With equipment (optional practical). Before the lesson, melt the stearic acid in the boiling tubes in a beaker of hot water at about $80\,^\circ\text{C}$, with a thermometer in each tube. Pairs lift a tube out with a test-tube holder, stand it in a rack or an empty beaker, start the stop-watch and read the temperature every $30$ seconds for $10$ minutes. Answer the questions below between readings. Safety: eye protection; hot water and molten stearic acid can scald, so handle tubes with holders; never heat stearic acid directly in a flame; do not pull the thermometer out once the solid has set — re-melt it in hot water afterwards.

No equipment (default). Copy the class data table below onto the board as "results from a pair who did the experiment". Students read the table, describe the pattern and answer the questions in their books; use fingers or hands up to check answers question by question.

Note: pure stearic acid melts at about $69\,^\circ\text{C}$. Lab-grade stearic acid is often a mixture, so a class may see a less sharp flat section a few degrees lower — a good chance to mention that impurities change melting points.

Either take readings every 30 seconds in a table in your book, or read the class data table on the board. Answer the questions in your book between readings, and hold up your book when the teacher checks each answer.

Students plot the cooling curve (their own results or the class data) on a grid ruled in their exercise books (optional: graph paper), then work through the three tiers. Check that lines of best fit are drawn as smooth curves and a horizontal straight line, not joined dot to dot — examiner reports regularly note that a best-fit line can be curved and does not have to pass through every point. Circulate and check the particle explanations: each must say what the particles do, not what the substance does. Unfinished graphs go to homework.

Plot the cooling curve, then answer the Core, Extended and Challenge questions in your book with full sentences for each explanation.

Write the three exit-ticket questions on the board. Students answer on scrap paper or the back page of their exercise book and hand it in (or hold it up if you prefer a quick check). Sort the answers into three piles (all correct / one wrong / two or more wrong) to plan the next lesson. Finish by returning to the starter: "What is between the particles in a gas?" — the class should now answer empty space.

Answer the three questions on your own, then check them against the success criteria at the top of the lesson.