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Diffusion is faster in gases than liquids where there is more space for them to move and diffusion is negligible in solids due to the close packing of the particles.
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Diffusion is responsible for the spread of odours even without any air disturbance e.g. use of perfume, opening a jar of coffee or the smell of petrol around a garage.
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The rate of diffusion increases with increase in temperature as the particles gain kinetic energy and move faster.
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Other evidence for random particle movement including diffusion:
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When smoke particles are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction. This is because the smoke particles show up by reflected light and 'dance' due to the millions of random hits from the fast moving air molecules. This is called 'Brownian motion' (see below in liquids). At any given instant of time, the hits will not be even, so the smoke particle get a greater bashing in a random direction.
1b. The particle model of a Liquid
- A liquid has a fixed volume at a given temperature but its shape is that of the container which holds the liquid.
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There are much greater forces of attraction between the particles in a liquid compared to gases, but not quite as much as in solids.
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Particles quite close together but still arranged at random throughout the container, there is a little close range order as you can get clumps of particles clinging together temporarily.
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Particles moving rapidly in all directions but more frequently collisions with each other than in gases due to shorter distances between particles.
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With increase in temperature, the particles move faster as they gain kinetic energy, so increased collision rates, increased collision energy and increased rate of diffusion.
Using the particle model to explain the properties of a Liquid
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Liquids have a much greater density than gases (‘heavier’) because the particles are much closer together because of the attractive forces.
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Liquids usually flow freely despite the forces of attraction between the particles but liquids are not as ‘fluid’ as gases.
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Note 'sticky' or viscous liquids have much stronger attractive forces between the molecules BUT not strong enough to form a solid.
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Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction, but the shape is not fixed and is merely that of the container itself.
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Liquids are not readily compressed because of the lack of ‘empty’ space between the particles.
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Liquids will expand on heating but nothing like as much as gases because of the greater particle attraction restricting the expansion (will contract on cooling).
- Note: When heated, the liquid particles gain kinetic energy and hit the sides of the container more frequently, and more significantly, they hit with a greater force, so in a sealed container the pressure produced can be considerable!
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The natural rapid and random movement of the particles means that liquids ‘spread’ or diffuse. Diffusion is much slower in liquids compared to gases because there is less space for the particles to move in and more ‘blocking’ collisions happen.
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Evidence for random particle movement in liquids:
- If coloured crystals of e.g. the highly coloured salt crystals of potassium manganate(VII) are dropped into a beaker of water and covered at room temperature. Despite the lack of mixing, convection etc. the bright purple colour of the dissolving salt slowly spreads throughout all of the liquid but it is much slower than gas diffusion.
- The same thing happens with dropping copper sulphate crystals or coffee granules into water and just leaving the mixture to stand.
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When pollen grains are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction. This is because the pollen grains show up by reflected light and 'dance' due to the millions of random hits from the fast moving water molecules. This is called 'Brownian motion' after a botanist called Brown first described the effect (see gases above). At any given instant of time, the hits will not be even all round the pollen grain, so they get a greater number of hits in a random direction.
1c. The particle model of a Solid
- A solid has a fixed volume and shape at a particular temperature unless physically subjected to some force.
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The greatest forces of attraction are between the particles in a solid and they pack together as tightly as possible in a neat and ordered arrangement.
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The particles are too strongly held together to allow movement from place to place but the particles vibrate about their position in the structure.
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With increase in temperature, the particles vibrate faster and more strongly as they gain kinetic energy.
Using the particle model to explain the properties of a Solid
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Solids have the greatest density (‘heaviest’) because the particles are closest together.
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Solids cannot flow freely like gases or liquids because the particles are strongly held in fixed positions.
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Solids have a fixed surface and volume (at a particular temperature) because of the strong particle attraction.
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Solids are extremely difficult to compress because there is no real ‘empty’ space between the particles.
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Solids will expand a little on heating but nothing like as much as liquids because of the greater particle attraction restricting the expansion and contraction occurs on cooling.
- The expansion is caused by the increased energy of particle vibration, forcing them further apart causing an increase in volume and corresponding decrease in density.
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Diffusion is almost impossible in solids because the particles are too closely packed and strongly held together with no ‘empty space’ for particles to move through.
2. Changes of State for gas <=> liquid <=> solid
We can use the state particle models and diagrams to explain changes of state and the energy changes involved.
These are NOT chemical changes BUT PHYSICAL CHANGES, e.g. the water molecules H2O are just the same in ice, liquid water, steam or water vapour. What is different, is how they are arranged, and how strongly they are held together by intermolecular forces in the solid, liquid and gaseous states.
2a. Evaporation and Boiling (liquid to gas)
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On heating particles gain kinetic energy and move faster.
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In evaporation* and boiling the highest kinetic energy molecules can ‘escape’ from the attractive forces of the other liquid particles.
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The particles lose any order and become completely free to form a gas or vapour.
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Energy is needed to overcome the attractive forces in the liquid and is taken in from the surroundings.
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This means heat is taken in, so evaporation and boiling are endothermic processes (ΔH +ve).
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If the temperature is high enough boiling takes place.
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Boiling is rapid evaporation anywhere in the bulk liquid and at a fixed temperature called the boiling point and requires continuous addition of heat.
- The rate of boiling is limited by the rate of heat transfer into the liquid.
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* Evaporation takes place more slowly than boiling at any temperature between the melting point and boiling point, and only from the surface, and results in the liquid becoming cooler due to loss of higher kinetic energy particles.
2b. Condensing (gas to liquid)
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On cooling, gas particles lose kinetic energy and eventually become attracted together to form a liquid.
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There is an increase in order as the particles are much closer together and can form clumps of molecules.
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The process requires heat to be lost to the surroundings i.e. heat given out, so condensation is exothermic (ΔH -ve).
- This is why steam has such a scalding effect, its not just hot, but you get extra heat transfer to your skin due to the exothermic condensation on your surface!
2d. Melting (solid to liquid)
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When a solid is heated the particles vibrate more strongly as they gain kinetic energy and the particle attractive forces are weakened.
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Eventually, at the melting point, the attractive forces are too weak to hold the particles in the structure together in an ordered way and so the solid melts.
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The particles become free to move around and lose their ordered arrangement.
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Energy is needed to overcome the attractive forces and give the particles increased kinetic energy of vibration.
2e. Freezing (liquid to solid)
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On cooling, liquid particles lose kinetic energy and so can become more strongly attracted to each other.
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Eventually at the freezing point the forces of attraction are sufficient to remove any remaining freedom and the particles come together to form the ordered solid arrangement.
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Since heat must be removed to the surroundings, so strange as it may seem, freezing is an exothermic process (ΔH -ve).
2g. Sublimation
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This is when a solid, on heating, directly changes into a gas without melting, AND the gas on cooling re-forms a solid directly without condensing to a liquid. They usually involve just a physical change BUT its not always that simple!
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Theory in terms of particles:
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When the solid is heated the particles vibrate with increasing force from the added thermal energy.
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If the particles have enough kinetic energy of vibration to partially overcome the particle-particle attractive forces you would expect the solid to melt.
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HOWEVER, if the particles at this point have enough energy at this point that would have led to boiling, the liquid will NOT form and the solid turns directly into a gas.
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Overall endothermic change, energy absorbed and 'taken in' to the system.
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On cooling, the particles move slower and have less kinetic energy.
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Eventually, when the particle kinetic energy is low enough, it will allow the particle-particle attractive forces to produce a liquid.
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BUT the energy may be low enough to permit direct formation of the solid, i.e. the particles do NOT have enough kinetic energy to maintain a liquid state!
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Overall exothermic change, energy released and 'given out' to the surroundings.