Melting
Have you seen icicles hanging from a roof? They are long pointed rods of ice. Some can be half a metre long or more. Icicles form in very cold weather. Water dripping from a roof freezes and makes their spiky shapes.

When the cold weather ends, the temperature of the air increases. The temperature of objects such as the tiles on the roof or the wood in a door increases but on the surface of the icicle the temperature stays the same for a short time.

The icicle is not warming up, unlike the rest of the objects, because the water is using the heat for something else. Ice is made of tiny particles which hold each other firmly together. In the icicle they actually make the shape of the spiky rod. When the air warms up around an icicle, the particles near the surface take in the heat. Heat is a form of energy and the particles use it to loosen the links between each other. When the links have loosened, the particles slide over each other and form liquid water. The ice has taken in the heat and used it to melt.

While the particles in the ice are losing their grip of each other the temperature of the ice does not increase. It stays constant. This temperature at which melting takes place is called the melting point. Different substances have different melting points. The melting point of ice, for example, is 0°C.

The ice in the icicle does not all melt at once. The ice nearest the warm air melts first. Once this layer of ice has turned to water, the temperature of the water can rise. This happens because heat is no longer needed to loosen links. It is used just to speed up the movement of the water particles.

The melted water on the icicle flows down its smooth surface, gathers at its tip and falls to the ground as drips. As the water moves away it exposes more ice which also becomes warm and melts. In time the icicle gets thinner and shorter and eventually it melts away.

The melting point can be described as the temperature at which a solid changes into a liquid. But something else happens at this temperature - the liquid turns into a solid. This process is called freezing and the temperature is also called the freezing point of the substance. For example the melting point of ice is 0°C and the freezing point of water is also 0°C. Usually you would not refer to both together but would use the one that helps you with your work. In this topic only melting is being studied so only the term melting point is used.

Water is unusual because its melting point is within the range of temperatures that we normally experience in the weather. For example, on a winter's day the temperature may drop to -10°C. At this temperature water is solid but when the temperature rises to 0°c the water melts and changes into a liquid. The melting point of most substances is much higher than normal temperatures. This means that they stay solid even on a hot summer's day.

When ice is very cold, say, -10°C does it warm up when the air around it warms up?
Yes, it does. The heat in the air passes into the ice. The particles in the ice use the energy from the heat to move about. As the particles are holding onto each other they cannot move about so they shake or vibrate. As the temperature rises they vibrate more strongly and the temperature of the ice rises. It is only at the melting point that the temperature of the ice suddenly stops rising. This happens because the energy is no longer used to make the particles vibrate but it is used to break the links between them.

Why does the temperature of the water rise once it has melted?
It rises because the energy in heat is used to make the particles move faster. The energy is no longer needed to break links between particles.

Is it correct to think that when the temperature of a substance rises, its particles move faster?
Yes, it is. In fact temperature is a measurement of how fast the particles are moving. When you dip the bulb of a thermometer into water, the particles in the water strike the sides of the bulb. When the bulb is struck by the particles it receives energy from them. The energy makes the liquid in the bulb of the thermometer expand and move up the scale of the thermometer to give you a reading.

Why does some chocolate melt in your hard when you pick it up?
The melting point of chocolate is about 33°C. When the chocolate is on a table it is at the temperature of the air which might be about 20°C. This temperature is below the melting point of chocolate so the chocolate remains a solid. When you pick up the chocolate your skin gives out heat to the chocolate because your body temperature is 37°C. As you are warmer than the melting point of chocolate, the chocolate melts.

Is it safer to put the chocolate in your pocket?
No, this could make things worse. The chocolate could become even runnier. The reason for this is that it is hotter in your pocket than on your skin. The material of your clothes acts as an insulator and heat passing from your body into the cavity of your pocket becomes trapped. This makes the temperature in your pocket higher than the temperature on your skin so the chocolate makes an even stickier mess.