Melting and solidification of solids
When a crystalline solid is heated its temperature first rises, but on reaching the melting point it stays constant until all the solid has melted. During this time the supplied energy does not increase the kinetic energy of the particles but is spent on breaking down the lattice, i.e. on breaking the bonds between particles. When a liquid is cooled, the temperature also stays constant at the solidification point: when the lattice forms, an amount of energy equal to that absorbed in melting is released. For a pure substance the melting and solidification temperatures are the same: ice melts at 0 °C and water freezes at 0 °C. On the graph this appears as a horizontal line (plateau). Different substances have different melting points (°C): mercury –39, lead 327, zinc 420, aluminium 660, copper 1083, iron 1539, tungsten 3410.
Safe observation: put crushed ice and water in a glass, insert a thermometer and write down the temperature every 2 minutes until the ice has melted. How did the temperature change during melting? Draw a graph. Work with hot objects only with an adult; do not go near a heater alone.