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The kinetic theory of matter can be used to explain how solids, liquids and gases are interchangeable as a result of increase or decrease in heat energy. When an object is heated the motion of the particles increases as the particles become more energetic. If it is cooled the motion of the particles decreases as they lose energy.
In a solid the strong attractions between the particles hold them tightly
packed together. Even though they are vibrating this is not enough to disrupt
the structure. When a solid is heated the particles gain energy and start to
vibrate faster and faster. Initially the structure is gradually weakened which
has the effect of expanding the solid. Further heating provides more energy
until the particles start to break free of the structure. Although the particles
are still loosely connected they are able to move around. At this point the
solid is melting to form a liquid. The particles in the liquid are the same
as in the solid but they have more energy. To melt a solid energy is required
to overcome the attractions between the particles and allow them to pull them
apart. The energy is provided when the solid is heated up. The temperature at
which something melts is called its "melting point" or melting temperature.
At room temperature a material is a solid, liquid or gas depending on its melting
temperature. Anything with a melting temperature higher than about 20oC is likely
to be a solid under normal conditions. Materials have widely differing melting
temperatures e.g. mercury -39oC, ice 0oC, salt 1081oC, aluminium 660oC and steel
1535oC. Everyday materials such as ice, butter and wax have different melting
temperatures and can be used as examples with children.
Ice is probably the melting substance most children are familiar with. It can be problematic when trying to develop an understanding of melting as it does often appear to them to melt without any source of heat. Ice melts at room temperature because the surrounding air is warmer than the ice and at a temperature above the melting temperature. The heat energy required to melt the ice comes from the surrounding air which will consequently become a little cooler.
Not all solids melt when they are heated. Some may undergo chemical changes as a result of heating. For example paper burns rather than melts.
Within a liquid some particles have more energy than other. These "more
energetic particles" may have sufficient energy to escape from the surface
of the liquid as gas or vapour. This process is called evaporation and the result
of evaporation is commonly observed when puddles or clothes dry. Evaporation
takes place at room temperature which is often well below the boiling point
of the liquid. Evaporation happens from the surface of the liquid. As the temperature
increases the rate of evaporation increases. Evaporation is also assisted by
windy conditions which help to remove the vapour particles from the liquid so
that more escape.
Evaporation is a complex idea for children for a number of reasons. The process involves the apparent disappearance of a liquid which makes the process difficult for them to understand. It is not easy to see the water particles in the air. Also, evaporation occurs in a number of quite differing situations - such as from a puddle or bowl of water where the amount of liquid obviously changes, to situations where the liquid is less obvious - such as clothes drying or even those where there is no obvious liquid at all to start with - such as bread drying out. A further complication is that evaporation may be of a solvent from a solution e.g. water evaporating from salt water to leave salt. These situations are quite different yet all involve evaporation.
Evaporation may also involve liquids other than water e.g. perfume, petrol, air fresheners. The particle model can be used to explain how it is possible to detect smells some distance away from the source.
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If a liquid is heated the particles are given more energy and move faster
and faster expanding the liquid. The most energetic particles at the surface
escape from the surface of the liquid as a vapour as it gets warmer. Liquids
evaporate faster as they heat up and more particles have enough energy to break
away. The particles need energy to overcome the attractions between them. As
the liquid gets warmer more particles have sufficient energy to escape from
the liquid. Eventually even particles in the middle of the liquid form bubbles
of gas in the liquid. At this point the liquid is boiling and turning to gas.
The particles in the gas are the same as they were in the liquid they just have
more energy. At normal atmospheric pressure all materials have a specific temperature
at which boiling occurs. This is called the "boiling point" or boiling
temperature. As with the melting point the boiling point of materials vary widely
e.g. nitrogen -210oC, alcohol 78oC, aluminium 459oC.
Any material with a boiling temperature below 20oC is likely to be a gas at room temperature. When liquids boil the particles must have sufficient energy to break away from the liquid and to diffuse through the surrounding air particles. As these particles cool down and lose energy they will condense and turn back to liquid. When steam is formed by water boiling at 100oC the particles quickly condense as the surrounding air temperature is likely to be much less that 100oC so the particles cool rapidly. In fact the "steam" coming out of a boiling kettle can only be seen because some of the gas particles have condensed to form small droplets of water.
When a gas turns to a liquid (condenses) or a liquid turns to a solid (solidifies) the particles lose energy to the surroundings.
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Website maintainer: R. Jones Updated: November 13, 2000