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The graph shows how the temperature (in this case of some ice) changes over time when left out on a laboratory bench at 20 °C. The ice changes from being in a solid state to a liquid state. This change of state is a phase transition.
There are five distinct regions of the graph which can be explained by the behaviour of the water molecules. The temperature of an object is a measure of the average kinetic energy of the particles from which the object is made. Individual molecules may have more or less energy than their neighbours.
a – the block of ice starts at -8 °C. Its molecules are closely packed, in a fixed arrangement and vibrating. Weak bonds between the water molecules hold the solid structure together. Since the air in the room is at a higher temperature than the ice, energy flows from the room to the ice gently warming it up. This makes the molecules vibrate more vigorously. Eventually, at 0 °C, some of the water molecules have enough energy to break their bonds and the block of ice begins to melt.
b – the ice is melting so the temperature remains constant until all the ice has changed to water. The temperature of the ice and water are in equilibrium. Imagine a point where it is 90% ice and 10% water at 0 °C. The air is still at 20 °C so it heats the water up a little. Now the water is hotter than the ice, so heat will be transferred from the water to the ice. This energy breaks a few more bonds between the molecules so a little more ice melts and the water cools back to 0 °C. This carries on with a higher proportion of liquid water and less ice for every moment that passes.
c – all the ice has melted. The particles in liquid water are still very close together but they are moving around and able to slide over one another. The air is at a higher temperature than the water so energy continues to move from the air to the water. This makes the molecules move a little faster (their average kinetic energy increases) and the temperature of the water rises.
d – the air and water are very nearly at the same temperature so the rate of energy transfer from the air to the water slows down.
e – the average energy of the water molecules is equal to the average energy of the air molecules. In other words, they are the same temperature and equilibrium has been reached. No further temperature changes occur.
Over time, even at only 20 °C, the water will eventually completely evaporate. Every now and again a random water molecule will have enough energy that it will be able to break the bonds holding it in the liquid phase and escape into the air as a free molecule of water vapour. This means that the average energy of the remaining molecules has gone down slightly; this is known as evaporative cooling. Since the air is now very slightly warmer than the water, energy will flow from the air to the water. As a result another molecule will be able to escape the water into the air. This process will continue until all the water molecules have turned into water vapour and the remains of the original ice cube have completely dried up.
This evaporation assumes that the humidity of the air in the laboratory is less than 100%. Dry, moving air will speed the evaporation up as the air above the puddle of water is replaced by new dry air. If the air is very still then water vapour will increase in concentration above the puddle making it harder for more water molecules to escape. If the air becomes saturated then evaporation will stop; a slight breeze will keep moving the water molecules away ensuring that the air above the puddle never becomes saturated with water.
Boiling water remains at 100 °C no matter how much more heat is put into it because of the effects of evaporative cooling. If you heat water in an enclosed space, where it cannot turn from liquid into gas because of the pressure, then it can be superheated above 100 °C. This is the principle behind a pressure cooker.
The state of matter for a particular substance depends upon both temperature and pressure. The lower the pressure the more likely the substance is to be a gas. At higher pressures, it is more likely to be a liquid or a solid. Diagrams called phase change diagrams show how the state of matter changes for different substances.
The triple point is a region where the material can exist in all three phases. For water this is just slightly over zero Celsius at a low pressure well beneath normal atmospheric pressure (often called 1 atmosphere and about 10 N/cm2).
Questions…
- What is the boiling point of pure water at 1 atm?
- What is the melting point of ice at 1 atm?
- What word describes the direct change from a solid to a gas?
- Suggest a substance that can change directly from a solid to a gas.
- Energy (heat) is released when chemical bonds form. Melting involves breaking the bonds between molecules. Is melting likely to be an exothermic or an endothermic process?
- Is condensation exothermic or endothermic? Explain your answer.
- Why does a pressure cooker cook food more quickly than either boiling or steaming?
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