- Solids, Liquids and Gases
- Density
- Kinetic Theory of Matter
- Molecular Motion and Temperature
- Changing State
Solid, Liquids and Gases
The three physcial states of matter are soild, liquid and gas. Solids have well-defined rigid shapes and do not flow over surfaces. Liquids do not have well-defined shapes and they flow. A liquid has a well-defined volume and takes up the shape of the container containing the liquid up to the level defined by the volume of the liquid. Gases do not have a well-defined shape nor volume. Gases can flow and a gas will take up all available space of a container when placed in it.
Density
Whether an object of one material is heavier than an object of another material depends on the size of the objects. For a fair comparison, the volume of the objects must be the same. To compare which material is heavier, we compare the densities of the materials. Densityhas the unit of kg/m3 and is defined as:
density = mass/volume
The volume of an irregular solid can be determined by submerging the soild in some water and the rise in water level gives the volume of the solid. The volume of a liquid can be measured directly using a measuring cylinder while the volume of a flask use for measuring density of a gas can be measured by filling the flask with water then measure the volume of water using a measuring cylinder.
Kinetic Theory of Matter
The kinetic theory is a theory used to explain the behaviour of the physical states (solids, liquids and gases) in terms of moving particles. The models of the states are based on the concept that when particles are close together they attract each other.
In a solid, strong forces of attraction hold the atoms or molecules together in a regular grid or lattice. The particles vibrate but cannot break free so the solid has a fixed shape and volume.
In a liquid, the molecules are close together but they have enough energy to move about. As a result liquids can flow. At the surface the molecules attract one another forming a 'tension' surface above the liquid.
In a gas, the molecules are far apart and are moving so quickly that they do not really attract each other. The molecules move in straight lines but have many collisions with other molecules and with the walls of the container they are in. The collisioins with the wall give rise to the pressure at the walls.
The three assumptions behind the kinetic theory are that the particles are constantly in motion; that the particles collide with other particles; and that particles attract each other. The indirect evidences of these assumptions can be observed in diffusion, Brownian motion and surface tension of a liquid. Diffusion is when two gases or liquids merge together and their particles diffuse through each other to form a mixture (particles are constantly in motion). The Brownian motion of smoke particles is caused by air molecules bumping into the smoke particles (particles collide with other particles). Surface tension seen at the surface of liquids is caued by the particles in the surface layer are attracted to the particles in the liquid below them otherwise the surface particles would 'escape' (particles attract each other).
These kinatic theoty models can be used to explain the everyday observations. Solids expand when heated is due to the particles gaining more energy and so vibrate more vigorously causing the particles, on avaerage, to be further apart. On cooling the atoms vibrate less as they lose energy and so solids contract when cooled. Solids conduct heat because when heated at one end, the atoms near that end start to vibrate more vigorously, and this makes nearby atoms vibrate more and so gradually energy is passed along. This is called conduction. Liquids evaporate because sometimes a molecule gains enough energy to break free of the surface and escape. An interest point is that evaporation of a liquid causes cooling because after the faster molecules break free, the average speed of the molecules left is lower which means the liquid has a slightly lower temperature.
Molecular Motion and Temperature
Base on the kinetic theory, molecules move more quickly when a substance is heated. The energy from the heat source is transferred to the molecules as increased kinetic energy and the substance's temperature goes up. Thus temperature might be seen as a measure of the (average) kinetic energy of the molecules.
When a hot object is placed in contact with a cold object, the high-speed molecules of the hot object collide with the slower moving molecules in the cold object and they transfer some of their energy. The hot object gets slightly cooler and the cold object gets slightly warmer. Eventually the two objects are at the same temperature and they are in thermal equilibrium.
When an object cools down its molecules slow down and have less kinetic energy. If all the energy is removed the molecules would stop moving and the temperature could not be lower any further. The is the absolute zero of temperature and is about -273°C.
Changing State
When a solid is heated, the kinetic energy of the molecules in the solid increases and hence the temperature of the solid. At the melting temperature, further heating allow the molecules to gain enough energy to break free of the forces that hold them in place in the solid. The solid melts and turns into a liquid. Continue heating will increase the kinetic energy of the molecules in the liquid and hence the temperature of the liquid. At the boiling temperature, further heating allow the molecules to break free cpmpletely. The liquid boils and turns into a gas.
The energy needed to raise the temperature of 1kg of a solid, liquid or gas by 1 °C is called the material's specific heat. The energy needed to melt 1kg of solid without changing its temperature is called the material's specific latent heat of fusion. The energy needed to change 1kg of liquid into a gas at its boiling point is called the material's specific latent heat of vaporization.
For water , the specific heat is 4200 J/kg °C; the specific latent heat of fusion is 334kJ/kg; the specific latent heat of vaporization is 2260kJ/kg.
Note that energy is supplied to melt a solid or to boil a liquid. When a gas condenses or a liquid freezes energy is given out.