1. Investigating the strength of the forces within selected solids

2. Sugar Salt Wax Tin Graphite Iodine Silicon Ice Sulfur Nine solids are place around the edge of a metal plate which is placed on a heating element.

3. The plate is at room temperature, and even before the heating element is turned on the ice begins to melt.

4. In less than a minute the ice has melted.

5. The wax starts to melt soon after the heating begins. It quickly turns to liquid and then vaporises. The heating element is turned on and slowly increases in temperature.

6. Next the iodine begins to sublime, forming faint clouds of purple vapour above the grey crystals. In the background the silicon is unchanged.

7. The liquid water begins to boil. At 100 °C the sulfur is still solid.

8. The iodine continues to sublime.

9. The sulfur starts to melt at 119 °C.

10. The iodine crystals have almost all vaporised now... …and the sulfur is completely melted.

11. The sugar starts to melt.

12. The strip of tin foil starts to soften and cling to the steel plate. All that remains of the iodine is a faint stain caused by reaction with the steel plate.

13. Meanwhile… The molten sulfur is getting darker… …while the sugar is beginning to caramelise as the molecules themselves start to break down.

14. The sugar continues to darken as it bubbles and breaks down. Eventually all that remains is black carbon.

15. The tin softens, then melts and shrivels into a few drops of molten metal.

16. All that remains of the sulfur is a blackened spot where the iron in the plate has reacted to form black FeS.

17. Meanwhile, …the salt, silicon and graphite remain unchanged.

18. Ice melts at 0 °C Wax melts at 66 °C Iodine sublimes as it warms. Under pressure, iodine melts at 114 °C. Water boils at 100 °C Sulfur melts at 119 °C Sugar melts at 186 °C and starts to decompose Tin melts at 232 °C Salt melts at 801 °C Silicon melts at 1410 °C Graphite melts at 3750 °C

19. Ice, wax, iodine, sulfur and sugar are all covalent molecular substances. As these substances melt or boil it is molecules which separate, not atoms. Water in either the liquid and gaseous phases is a molecular substance. These between-molecule inter-molecular forces are weak, which is why these substances melt easily.

20. Tin is a metal. Some metals have low melting points (mercury melts at –39 °C), while others have much higher melting points. The steel plate we used melts at around 1400 °C, while tungsten melts at 3410 °C. Metallic bonds between metal atoms can be weak or strong, depending on the metal. Sodium chloride is an ionic substance. All ionic substances have relatively high melting points because each ion is held in place by others above, below and beside it in 3 dimensions. Graphite and silicon are both covalent network substances. They have very high melting points because each atom is locked in place a number of strong covalent bonds.