Meltdown

Meltdown: why ionic compounds and small molecules melt, boil and conduct so differently

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

Sodium chloride: a giant ionic lattice with a high melting point

For AQA GCSE Chemistry (spec point 4.2.2.3), ionic compounds have regular structures called giant ionic lattices. In sodium chloride every Na⁺ ion is surrounded by Cl⁻ ions and every Cl⁻ ion by Na⁺ ions, held by strong electrostatic forces of attraction in all directions between the oppositely charged ions. Ionic compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds. Sodium chloride melts at 801 °C and boils at over 1400 °C.

In Meltdown you see this happen. The lattice of violet Na⁺ and green Cl⁻ ions sits in a glass chamber with glowing links between neighbouring ions on every axis. As you turn the heat wheel up, the ions vibrate harder and harder, but the lattice holds until the furnace passes 801 °C. Only then do the ions break free from their fixed places and slide past each other as a liquid.

When ionic compounds conduct electricity

Ionic compounds conduct electricity when melted or dissolved in water, because the ions are free to move and so charge can flow. A solid ionic compound does not conduct: its ions are held in fixed positions and can only vibrate. It is the ions that carry the charge, not electrons, which is a common way to lose marks.

Every chamber in the game has a test head with two graphite electrodes and a bulb. Press TEST on solid salt and the bulb stays dark. Melt it, or add water to dissolve it, and the bulb lights while the Na⁺ ions drift towards the negative electrode and the Cl⁻ ions towards the positive one. Heat salt water past 100 °C and the water boils off, leaving solid salt that no longer conducts.

Small molecules: weak intermolecular forces, low melting and boiling points

Spec point 4.2.2.4 says substances made of small molecules are usually gases or liquids with relatively low melting and boiling points. Inside each molecule the atoms are held by strong covalent bonds, but between the molecules there are only weak intermolecular forces. When the substance melts or boils, it is these intermolecular forces that are overcome, not the covalent bonds. In Meltdown the covalent bonds are solid sticks and the intermolecular forces are faint threads between the molecules. When methane boils at −162 °C the threads snap and every molecule moves away still whole: it is still CH₄.

The intermolecular forces increase with the size of the molecules, so larger molecules have higher boiling points: methane −162 °C, ethane −89 °C, propane −42 °C, pentane 36 °C and hexane 69 °C. In the Boil-off Race you bet on the order three chambers will boil, then heat them and watch. Substances made of small molecules do not conduct, because the molecules have no overall electric charge, even when they can move as a liquid or a gas. Sugar solution doesn’t conduct either; salt solution does.

How to play Meltdown

Drag up anywhere to heat the furnace and down to cool it, or use the arrow keys, the mouse wheel or the HOTTER and COLDER buttons. One heat wheel sets the same temperature for every chamber, so you can compare substances side by side. Each chamber’s tag shows the state it is in now and the state the job needs; match every tag and hold steady. There are five levels: Salt, Small molecules, Boil-off race, Conduct or not (heat it, add water, test it, or stamp it WON’T CONDUCT) and a 75 second Meltdown Rush. Wrong moves cost points and open a short explanation with a link to the lesson. Build combos and beat your best score.

Exam tip: always name the particles and the force, then link the force to the energy. Write “strong electrostatic forces between oppositely charged ions” for ionic compounds and “weak intermolecular forces between molecules” for small molecules.