Kinetic theory 1.- States of matter 2.- Changes of state 3.- Kinetic theory 4.- Pressure of a gas 5.- Gases laws 1.- Matter Matter is anything which has mass and volume. In other words, it occupies a space and has an inertia. Some examples of matter could be sand or water. On the other hand, light is an example of non-material phenomenon. Activity 1: Try to classify the following physical systems in material or non-material: air, colours, oil, sound, granite and wind Different kinds of matter can be identified by their characteristic properties, or physical quantities which have the same value for every sample of the same type of matter. An example of characteristic property is density, the ratio of mass to volume, because every sample of a certain substance has the same density, despite of its different size. We can find matter in different forms which are called states of matter. Generally speaking, in the Universe there are four different states of matter, solid, liquid, gas and plasma 1. Solids Solids hold a permanent shape which doesn't change. They are incompressible, that means they don't reduce its size when they are forced by pressure. However, they can dilate (become bigger) as temperature increases. Most of metals, rocks and minerals are solids at usual temperature 2. Liquids Liquids can take the shape of its container, so they can flow through pipes or tubes. They are also incompressible and dilate as temperature increases. That means liquids have a proper volume at certain temperature or, in other words, its density just depends on temperature. Water, oil or alcohol are examples of liquids at usual temperature 3. Gases Gases try to fill the whole volume of its container and can flow as liquids. That s why we call fluids both of them. But gases are compressible fluids, so they can reduce its volume as we force them under a pressure. Besides they also have thermal dilation so you need to fix both pressure and temperature to know the volume of a gas or its density. Air, carbon dioxide or methane are examples of gases 4. Plasmas Plasma is a portion of matter which has enough energy to break its particles, forming electric subatomic particles such as protons and electrons. Therefore the main difference between gases and plasmas is that gases are formed by neutral particles, such as atoms or molecules, but plasmas are formed by charged particles, called ions, so they have electric properties. Plasma is the most common state of matter in the Universe, but not on Earth, because all the stars are made of plasma. Besides flames, northern lights and fluorescent lights are plasmas as well.
Activity 2: You know a bulb is made of metal and glass. What could happen if these two materials had different thermal dilation properties? Experiment 1: Fill a balloon with water and another one with air. What are the differences when you try to compress both of them? Activity 4: Think about a hydraulic mechanism, for instance, the brakes of your car. Try to imagine what could happen if liquids would be compressible Activity 5: Try to explain why some trainers have an air filling inside its soles. 2.- Changes of state A change of state is a process in which a portion of matter is transformed from an original state of matter to a different one. We have shown you the different changes of state at the image below. Notice that you need heating matter to reach to the states placed at the top of the image, and, on the other side, you need to freeze matter to go to the bottom. Melting is the process in which a solid becomes liquid and freezing is the opposite process in which a liquid becomes solid. Every chemical substance has its own melting point, the temperature in which these two processes are carried out Vaporization is the process in which a liquid becomes gas and and condensation is the opposite process in which a gas becomes liquid. There is also a boiling point, the temperature in which both processes can be carried out, but on a general basis, vaporization can be done at any temperature. As a matter of fact everybody know that sea water is transforming all the time into gas and creating clouds even if water don't reach its boiling point. That s why we have to distinguish two different kinds of vaporization: evaporation and boiling. Evaporation is a process in which a portion of liquid just in its upper surface become gas at any temperature. On the other hand, boiling is the process in which the whole liquid becomes gas at certain temperature, its boiling point Experiment 3: Pour an amount of water in a pressure cooker. Close it and wait overnight. When you open the lid you'll see little drops of water on the top. How can they reach to the top of the pressure cooker? Experiment 4: Open the door of your freezer. Why can you find water vapour inside? Sublimation is the process in which a solid becomes gas and deposition is the opposite process, in which a gas becomes solid The process in which a gas becomes plasma is called ionization and the opposite process is deionization Changes of state can be represented using a heating curve, a graph of temperature versus heat added. When we are heating a solid, its temperature increases until a point where the solid starts to melt. As the melting process is carried out, the temperature is fixed because the whole energy we are giving to the solid is just used for the melting process.
When the melting process is finished and all the matter has become liquid, temperature increases again until it reaches to a point where the liquid starts to boil. Then the liquid becomes gas and temperature remains constant as the vaporization process is carried out. We can realise again that the whole amount of energy supllied to the system is used for performing the change of state instead of increasing its temperature. As vaporization process is finished and all the system is gas, temparature rises again. Gases can be transformed into plasmas by different ways, because we can supply thermal energy by means of increasing temperature or electric energy, by means of an increase of voltage. 3.- Kinetic theory Kinetic theory is a physical theory about the microscopic constitution of matter. Despite this theory is very simple it can explain quite a lot of experimental evidence, such as the different states of matter and its main physical properties, diffusion of gases or Brownian motion. It could be summed up according to the following statements: 1. Matter is composed by tiny particles, called atoms or molecules 2. Particles which form matter has a continuous and random motion which depends on its thermal energy. Furthermore, its average kinetic energy is proportional to its temperature. In other words, the greater the temperature, the faster its motion 3. Interactions between particles are inversely proportional to a power of the distance between them. This statement means that as the particles are closer, they will be more attracted, and as the distance increases, the interaction will decrease. Now we are going to explain the main features and physical properties of the states of matter, according to its microscopic structure Solids are formed by particles which are arranged in a periodic and ordered structure known as crystal. Each particle is as close to the others as possible and is placed at the same distance of everyone. As the distance between particles is the shortest, the attractive interaction is great, so these particles cannot leave its place in the crystal nor become closer to their neighbours. That s why solids have a permanent and characteristic shape and are incompressible. However this structure allows the particles to vibrate in different directions. As temperature increases, this motion becomes faster and the average distance between them increases as well. This is the reason of its thermal dilation. As a solid is heated, it reaches to a point where the average of kinetic energy of its particles equals to the amount of attractive interactions among them so the particles can leave their own places in the crystal and move to different locations. This is the moment when the change of state to liquid is produced. Liquids are formed by particles which are relatively close, but they are free to move from a position to another one. As the particles are continuously moving, the shape of a liquid can change, which explain that liquids conform to the shape of its container. Although its motion, particles in a liquid are very close to the others and they are still attracted very tightly, so they are incompressible and as one of them go out, the others follow the first and the liquid can flow as a whole set. When a liquid is heated, it reaches to a point where the kinetic energy of its particles is greater than the attractive interactions, so particles start to scape from the liquid, and the liquids become gases. Gases are formed by particles which are far away from the
others, so there is almost no attractive interaction amongst them. They are moving very fast in straight lines, colliding with other particles and with the walls of the container. Thats why gases can fill up the whole volume of their container or even they can scape if there is a hole in the container. As the average distance between the particles is great compared to their own size, they can approach and remain closer when they are forced by a pressure, so gases are compressible. We can explain the thermal dilation if we remember that kinetic energy of the particles depends on temperature. As we heat a gas, its particles will move faster and the average distance between them will be increased Plasmas are similar to gases with just one difference: they are formed by ions instead of neutral atoms Activity 6: Try to explain why the smoke of a cigarette is spread in a whole room using the kinetic theory. This physical phenomenon is called diffusion of a gas Activity 7: In 1827 an Scottish biologist called Robert Brown discovered that pollen grains move random and continuously on the surface of water. This motion is called Brownian motion and was explained in 1905 by A. Einstein as a result of collisions between these grains of pollen and the molecules of water. Try to find an animation Experiment 5: Pour water in a glass and drop a little amount of milk. Try to explain the mixing process according to the kinetic theory 4.- Pressure of a gas Experiment 6: Place a closed and empty plastic bottle in your freezer (no water inside!). Wait for a couple of hours. What does it happen?. Can you explain what you have observed? Experiment 7: Open a yogourth. Why doesn t it fall down when I turn it downwards? What does it happen when a hole is opened at the bottom of the container? Although gases have a low density due to the great space between their particles, they have a valuable mass and we can measure the weight of a gas: There are tons of gas above our heads!. Consequently we must feel the effect of such force and so we do. In 1644 Torricelli performed an experiment to measure the effect of this huge weight of the atmosphere. He filled a long glass tube with mercury. Then he tipped it down and observe that mercury falled down until a height of 760 mm over the surface of mercury dish. Therefore the weight of the atmosphere equals to the weight of this amount of mercury. We usually measure this weight as a pressure, which is the force applied to an area. It is measured in atmospheres (at) or mm Hg 1 at = 760 mm Hg We can explain the existence of pressure in a gas by means of kinetic theory. As the molecules have a random motion, they hit against the walls of the container and exert a force on them. So, according to kinetic theory, the pressure of a gas is the result of collisions of molecules against the outer surface of the gas.
5.- Gases laws The gas laws were stated during the XVII th and XVIII th century when the scientist started to develop instruments to measure accurately pressure and temperature at the same time. Then they realized that all gases behave almost the same because there is no specific interaction between their particles. We are going to state the most important gas laws and justify them according to the kinetic theory. Boyle's Law It was discovered by an English physicist, Robert Boyle, in 1662. It states that if temperature remains constant, the pressure of a gas is inversely proportional to its volume. In other words, the greater the pressure, the lower the volume. Mathematically, it could be state that pressure times volume equals constant p 1.V 1 = p 2.V 2 = constant We can explain this law according to the kinetic theory if we assume that pressure of a gas is caused by collisions of its molecules against the walls of the container. As the volume of a gas is reduced, the molecules will be closer to the walls of the container. As a result, the number of collisions will be increased and the pressure of the gas as well Charles' Law It was discovered by an English physicist, Charles, in 1678. It states that if pressure remains constant, the volume of a gas is directly proportional to its absolute temperature. In other words, the greater the temperature, the greater the volume. Mathematically, it could be stated like this: V = a. t + b where temperature is measured in Kelvin, volume in liters and a and b are constants.
There is no problem when you want to heat a gas, because it expands continously and its volume becomes greater and greater. But when you try to cool a gas, its volume becomes smaller and smaller until it reaches to a point (- 273,16º C) where its volume is negligible. This theoretical temperature, which is the same for every gas, no matter its composition, is the lowest temperature we can reach. As we prefer to measure positive temperatures, we use the absolute scale to measure temperatures and Kelvin is the unit accepted by the International System. Therefore Charles law can be stated as Then converting temperatures from Celsius scale to the absolute scale is quite simple: you just need to add 273,16 to Celsius temperature to get Kelvin temperature T = 273,16 + t In order to explain Charles law according to the kinetic theory we have to remember that the temperature measures the average kinetic energy of the gas molecules. As the temperature increases, they move faster and faster, so they can fill more space. V = a. T Gay Lussac's Law It was discovered by the French physicist Gay-Lussac in 1809. It states that if volume remains constant, the pressure of a gas is directly proportional to its absolute temperature. In other words, the greater the temperature, the greater the pressure. Mathematically, it could be state like this p = b. T where temperature is measured in Kelvin, pressure in atmosphere and b is a constant As we have said before, the average speed of the molecules of a gas increase when temperature is greater. Therefore, the collisions against the walls of the container will be greater and the pressure will be increased as well We can summarize the three gases laws using a relationship of pressure, volume and temperature p.v T = p.v T
Activity 8: Try to explain why a ballon bursts when it is placed on a heater Activity 9: The tyres of a Formula 1 car must be heated before filling. Can you imagine what could happen if they don t do that? Activity 9: Why does a pressure cooker make meals faster than a cooking pan?