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- Describe qualitatively the molecular structure of solids, liquids and gases.
The molecules in a solid are close together and arranged in a regular pattern.
The molecules in a liquid are close together and are irregularly arranged.
Gas molecules are far apart and randomly arranged.
- Relate the properties of solids, liquids and gases to the forces and distances between molecules and to the motion of the molecules.
Solids have very strong intermolecular forces of attraction. This causes their molecules to be tightly bound (solids have the smallest distance between molecules). They are also arranged regularly, or in other words, arranged in a lattice, as this minimises the distance between particles.
The strong forces of attraction prevent the molecules from moving, so they vibrate around a fixed point.
The strong intermolecular force also means that the solid maintains a fixed volume and shape (all molecules are held tightly in place). Solids cannot be compressed because their molecules are already packed close together.
Liquids have a moderately strong intermolecular force of attraction. This means that the molecules are held close together, but not so tightly that they can’t move. This allows liquids to rotate and flow over one another. Due to the movement of molecules in a liquid (the molecules in a liquid are constantly in motion, even if the liquid as a whole is stationary), the arrangement of molecules in the liquid is irregular.
Because liquid molecules can move, their shape is not fixed – they tend to assume the shape of whatever container they are in. This is due to gravity – it forces the liquid to settle at the bottom of the container, making it fill out the container’s shape.
The intermolecular force of attraction in liquids are not as strong as the forces in solids, so the molecules are usually further apart. This means that liquids can be compressed slightly (and thus, their volume can change), but this change is negligible, and not usually worth taking into account in the exam.
Gas molecules have so much kinetic energy that they are free to move in any direction, and the molecules are very far apart. The kinetic energy of molecules overcome any forces of attraction, making those forces negligible. The movement of gas molecules also means that they tend to fill out their container, taking its shape. As gas molecules are very far apart, they can be compressed, forcing the molecules closer together. This expansion of gases to fill out its container and the ability to compress gases means that the volume of gases is not fixed.
- Interpret the temperature of a gas in terms of the motion of its molecules.
The higher the temperature, the more kinetic energy the gas molecules have (heat energy is converted to kinetic energy), and so the faster they move. There will also be more collisions between molecules, making their motion more erratic. The molecules also move further apart, expanding the gas.
The lower the temperature, the less kinetic energy the gas molecules have (kinetic energy of the gas is lost as heat to the surroundings), and so they move slower. This may mean the intermolecular forces of attraction become stronger, bringing the molecules closer together.
- Describe qualitatively the pressure of a gas in terms of the motion of its molecules.
When a higher pressure is exerted on the gas, the gas molecules gain kinetic energy, causing them to move faster and more erratically (due to more collisions).
- Describe qualitatively the effect of a change of temperature on the pressure of a gas at constant volume.
A higher temperature means the gas molecules move faster and have more collisions.
As the gas molecules will collide with the surface of their container more and at a higher speed, the total force they apply on the container will increase. Pressure is the force applied on a surface per unit area, so the gas pressure increases. Note that this is true when the volume is constant – if the volume increases, then the molecules have to travel further to hit the container, reducing the number of collisions. This could nullify the effect of the temperature on pressure.
The opposite is true when the temperature falls at a constant volume – molecules move slower, so fewer collisions and the collisions are less forceful, so the pressure falls.
Notes submitted by Sarah.
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