Kinetic Particle Theory
- 1. Introduction All substances are matter. This includes the air, sea, Earth, all living creatures and even the Sun and Stars. Matter is anything that has mass and takes up space. There are three forms of matter solid, liquid, gas. These three forms are called states of matter. Particle Theory Solid .Particles in solid are held tightly and packed fairly close together - they are strongly attracted to each other. .Particles are in fixed positions but they do vibrate. Liquid . Particles are fairly close together with some attraction between them. .Particles are able to move around in all directions but movement is limited by attractions between particles. Gas .Particles have little attraction between them. . Particles are free to move in all directions and collide with each other and with the walls of a container and are widely spaced out.
- 2. Properties of matter Solids Have a definite shape. Maintain that shape. Are difficult to compress as the particles are already packed closely together. Are often dense as there are many particles packed closely together. Liquids Do not have a definite shape. Flow and fill the bottom of a container. They maintain the same volume unless the temperature changes. Are difficult to compress because there are quite a lot of particles in a small volume. Are often dense because there are quite a lot of particles in a small volume. Gases Do not have a definite shape. Expand to fill any container. Are easily compressed because there are only a few particles in a large volume. Are often low density as there are not many particles in a large space.
- 3. Inter conversion of states of matter Melting 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. Boiling 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 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.
- 4. Evaporating 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.
- 5. Diffusion Diffusion is one of several transport phenomena that occur in nature. A distinguishing feature of diffusion is that it results in mixing or mass transport without requiring bulk motion. Thus, diffusion should not be confused with convection or advection, which is other transport mechanisms that use bulk motion to move particles from one place to another. From the atomistic point of view, diffusion is considered as a result of the random walk of the diffusing particles. In molecular diffusion, the moving molecules are self propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown. The theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein. Now, the concept of diffusion is widely used in science: in physics (particle diffusion), chemistry and biology, in sociology, economics and finance (diffusion of people, ideas and of price values). It appears every time, when the concept of random walk in ensembles of individuals is applicable.
- 6. Graham’s Law of Diffusion Inter mixing of two or more gases to form a homogeneous mixture without any chemical change is called "DIFFUSION OF GASES". Diffusion is purely a physical phenomenon. Gases diffuse very quickly due to large empty spaces among molecules. Different gases diffuse with different rates (velocities). The law stating that the rate of diffusion of a gas through a liquid (or the alveolar-capillary membrane) is directly proportional to its solubility coefficient and inversely proportional to the square root of its density. Graham's law is a quantitative relation between the density and rate of diffusion of gases. Mathematical Representation of the Law Since density a molecular mass, therefore, we can replace density d by Molecular mass M.
- 7. Effects of Pressure and Temperature on gases If the volume of the container is not fixed, increasing the temperature will cause a gas to expand (increase the volume), and contract when cooled (decreasing the volume). This would be the case for a gas inside a piston, or inside a rubber balloon. If the volume is fixed, then increasing the temperature will increase the pressure, and decreasing the temperature will decrease the pressure. This would be the case for a gas in a closed solid container, like a canister or sealed metal box. Increasing pressure will cause the gas to contract (reducing the volume), and decreasing the pressure will cause the gas to expand (increasing the volume). Again, this is if the volume is not fixed. If the volume is fixed, then increasing the pressure will increase the temperature, and decreasing the pressure will decrease the temperature. Gases can also be changed to a liquid or solid if the temperature is too low or the pressure is too high. As an example steam changes to a liquid when it touches a cold object, and nitrogen gas can be converted to liquid nitrogen by compressing it to very high pressures.