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Physics - The Kinetic Theory of Matter (AQA Syllabus A)

In GCSE Science students will look at the transfer of heat energy. This is the first of eight quizzes on that topic and it looks at the kinetic theory of matter and how the particles in solids, liquids and gases are affected by heat energy.

The kinetic theory of matter is one of the models that scientists use to help them to make sense of the world around them. The word kinetic always suggests something is moving and this theory is about the movement of particles in solids, liquids and gases. Particles transfer heat energy into kinetic energy. The kinetic theory can explain many properties of the three states of matter and is related to the amount of thermal energy distributed through the particles of an object. The particles in solids, liquids and gases have different amounts of energy, are arranged differently and move differently too.

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The kinetic theory explains why substances freeze and melt, why they boil and condense, and why evaporation causes cooling. It helps to explain why heat can be conducted by solids, how convection occurs in fluids and much more... including why solids cannot be compressed but gases can. In solids the particles are close together and fixed in place. Their only movement is to vibrate. Because of this, solids have definite shapes which they will keep - unless they are broken or cut. It is very difficult to push the particles in solids any closer together and so they can't be compressed.

In liquids the particles are only a little further apart than in solids, so liquids can't be compressed either. The main difference is that the particles in a liquid are free to move past each other, which is why liquids flow and take up the shape of the container in which they are placed. This movement was something that was first noted in 1827 by botanist Robert Brown. Brown observed that pollen grains floating on water moved around in random directions. It then took about 75 years until Albert Einstein came up with an explanation - water molecules were colliding with and pushing the pollen grains around.

Finally, in gases the particles are far apart and moving extremely rapidly in random directions. Because the particles are so widely spaced, gases are easily compressed.

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  1. What will a substance do when it is heated?
    Whilst some substances may well burn or explode, thankfully there aren't many of them!
  2. According to the kinetic theory, what are the particles in a heated solid doing?
    As any state of matter is heated, the movements of the particles increase
  3. Why does hot air rise?
    This causes a convection current as cold air moves in to take its place
  4. When a liquid is evaporating it cools down. Why?
    Slower moving particles have less thermal energy so the temperature of the liquid is lower. The cooling effect is more noticeable the faster that something evaporates
  5. Which of the following would increase the speed at which a liquid evaporates?
    Answers one and two have exactly the opposite effect - they slow down the rate of evaporation. Not on the above list is temperature. Raising the ambient temperature (temperature of the surroundings) increases the rate of evaporation
  6. Some types of thermometer are filled with mercury, a liquid metal. Why do they work?
    Answer three may have been quite appealing to you but you should never talk about the particles expanding. They remain the same size, whether a substance is hot, cold or at room temperature; it is the speed at which they move and the space that they occupy that changes. Mercury is toxic and was replaced in later thermometers by a mixture of alcohol and water which works in the same way. Digital thermometers work in an entirely different way using a device called a thermocouple
  7. What happens to the particles of water vapour as it changes from a gas into a liquid to form a film of water on the inside surface of a cold window?
    When any gas is cooled its particles slow down and move closer together forming a liquid - they condense. Some gases, like water vapour, condense easily but other gases, like oxygen for example, need to be cooled well below 0oC before their particles slow down enough to become a liquid
  8. What happens to a gas in a sealed container when it is heated?
    The pressure increases because the particles have more thermal energy so they hit the walls of the container with greater force
  9. Why are warm fluids less dense than their cold equivalent?
    More thermal energy means that the particles of the fluid move faster and take up more space. This means that there are fewer particles in the same volume of warm areas of the fluid than in the cooler areas of the fluid, making it less dense
  10. Which states of matter contract when you cool them?
    As the thermal energy is removed, the particles move less and can pack together more closely. Water is a bit odd - it contracts until you get to 4oC then it starts to expand. As it forms ice, it expands a lot, then, when you cool ice down, it contracts exactly as you would expect

Author: Kev Woodward

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