The following are the elaboration of the three state of matter and how they can be transformed from one state to another
So, each of the many millions of substances around us can be classified as a solid, a liquid or gas. Look around you and name substances that are solids, liquids and gases. The state in which any matter exists depends on temperature and sometimes pressure conditions. One substance may exist as a solid in one condition and as a liquid or gas under a different condition. Water is an example of such substances. This change is called a change in the state of matter.
The three physical states of matter differ in the way they respond to temperature and pressure. All three states can increase in volume (expansion) when the temperature is increased. They decrease in volume (contraction) when the temperature is decreased. Gases are easily compressed. Liquids are only slightly compressible. Solids are incompressible. They are not affected by change in pressure.
Investigation of the compressibility of solids, liquids and gases
Procedure
- Take three new syringes and fill them with sand, water and air respectively (figure 5.1).
- Try to push in the end of each syringe.
- Observe what happens.
Figure
Compressibility of solids, liquids and gases
Observation
Which of the substances under investigation can compress into a smaller volume?
Findings
You should have found that a solid (sand) and a liquid (water) cannot be compressed but a gas (air) is easily compressed.
The three states of matter differ in their physical properties. These differences in properties are summarized in table bellow
Differences in properties of the three states of matter
| Property | Physical state | ||
| Solid | Liquid | Gas | |
| Shape | has a definite shape | no definite shape, takes shape of the container | no definite shape, occupy whole container |
| Volume | has a fixed volume | has a fixed volume | variable (depending on temperature and pressure) |
| Fluidity | does not flow | generally flows easily | flows easily |
| Expansion on heating | low | medium | high |
| Compressibility | incompressible | almost incompressible | highly compressible |
| Motion of particles | slow | high | very high |
| Density | high | moderate to high | low |
| Tangibility | tangible | tangible | intangible |
| Visibility | visible | visible | invisible |
One State of Matter to Another
Change one state of matter to another
We have seen that matter exists in three different states - solids, liquids and gases. We can use the kinetic theory of matter to explain how a substance changes from one state to another. Basically, changes from one state to another are caused by alterations in temperature and pressure. Normally molecules, ions or atoms of a substance move faster when the temperature is increased.
Melting and freezing
Melting is a change from solid to liquid state. When solids are heated, their constituent particles (atoms, molecules or ions) get energy and vibrate more violently. Vibrations of these particles overcome (exceed) their binding forces. The particles become mobile. The crystalline structure of solid is destroyed. A liquid state is reached and the particles are free to move. The temperature at which this happens is called melting point of the solid.
The melting point of a solid tells us something about the strength of forces holding its constituent particles together. Substances with high melting points have strong forces between their particles. Those with low melting points have weak forces between their particles.
Change in state from solid to liquid
Freezing is a change from liquid to solid state. Freezing is the opposite of melting. The process is reversed at the same temperature if a liquid is cooled. The temperature at which a substance turns to a solid is called freezing point. The melting point and freezing point of any given substance are both the same. For example, the melting and freezing of pure water takes place at 0°C. Melting is not affected by any changes in atmospheric pressure.
Evaporation and boiling
Boiling is a change from liquid to vapour state at a particular temperature. Evaporation is the change from liquid to vapour state at any given temperature. If a liquid is exposed to open air, it evaporates. Splashes of water evaporate at room temperature. After rain, small pools of water dry up. When a liquid changes into a gas at any temperature, the process is called evaporation. Evaporation takes places from the surface of the liquid. The larger the surface area, the faster the liquid evaporates. The warmer the liquid is, the faster it evaporates. Thus, surface area and temperature affects the rate of evaporation of a liquid.
When a liquid is heated, its molecules get more energy and move faster. They knock into each other violently and bounce further apart. As the heating goes on, its molecules vibrate even faster. Bubbles of gas (due to air dissolved in water) appear inside the liquid. The whole process is called boiling. The temperature at which a liquid boils is called boiling point.
The molecules at the surface of the liquid gain enough energy to overcome the forces holding them together. They break away from the liquid and from a gas (vapour). As more of the liquid molecules escape to form a gas, a liquid is said to evaporate. This occurs at the boiling point of a liquid.
Change in state from liquid to gas
The temperature at which a liquid boils explains how strong the forces holding its particles (molecules) together are. Liquids with high boiling points have strong forces of attraction between their molecules than those liquids with low boiling points.
The boiling point of a liquid can change if the surrounding pressure changes. If the surrounding pressure falls, the boiling point also falls. The boiling point of water at standard pressure (760 mmHg) is 100°C. On a high mountain, where pressure is low, it is lower than 100°C. If the surrounding pressure is increased, the boiling point rises. The same behaviour is experienced by a gas when the pressure is either increased or decreased.
The melting and boiling points of some common chemical substances at standard temperature and pressure (s.t.p)
| Substance | Physical state at room temperature (20°C) | Melting point (°C) | Boiling point(°C) |
| Oxygen | gas | -219 | -183 |
| Nitrogen | gas | -210 | -196 |
| Ethanol (alcohol) | liquid | -117 | 78 |
| Water | liquid | 0 | 100 |
| Sulphur | solid | 115 | 444 |
| Common salt (sodium chloride) | solid | 801 | 1465 |
| Copper | solid | 1083 | 2600 |
| Carbon dioxide | gas | sublimation point (°C): -78 | |
From the above explanation, obvious differences between evaporation and boiling can be detected. See table bellow
| Evaporation | Boiling |
| 1.Occurs at all temperatures | Occurs at one particular temperature (boiling point) |
| 2.Occurs on the surface of the liquid | Occurs both inside and on the surface of the liquid |
| 3.Takes place slowly | Takes place faster |
| 4.Bubbles are not necessarily formed |
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