Disclaimer: Due to unforeseen difficulties, we have had to take down the images on this notes page. They will be replaced shortly. We apologise for the inconvenience, but hope that the new images will provide you with an even better learning experience.
- State the distinguishing properties of solids, liquids and gases
- Describe the structure of solids, liquids and gases in terms of particle separation, arrangement and types of motion
This is basically the same thing as the previous point.
- Describe the changes of state in terms of melting, boiling, evaporation, freezing and condensation
Melting: The substance changes state from solid to liquid.
Boiling: The substance changes state from liquid to gas, at a fixed temperature (when the liquid reaches its boiling point, i.e. the temperature it boils at, it will boil).
Evaporation: The substance changes phase from liquid to gas over a wide range of temperatures. Evaporation can occur at any temperature as long as the substance is a liquid at that temperature. It is a superficial process (only particles at the surface of the liquid escape as a gas).
Freezing: The substance changes state from liquid to solid.
Condensation: The substance changes state from gas to liquid.
- Demonstrate understanding of the terms atom, molecule and ion
Atoms are the building block for everything we see. They possess a nucleus made of protons (tiny positively charged particles) and neutrons (tiny neutral particles), and electrons (even tinier, practically weightless, negatively charged particles, orbit the nucleus. These three particles make up a single atom.
Molecules are structures made of multiple atoms (at least two atoms) chemically bonded together. A simple mixture of atoms, where they are not chemically bonded, is not a molecule. In the image shown, each sphere shows an atom, and each line is used to represent the bond between the atoms – they don’t literally look like that, but drawing molecules in this way helps us understand their 3D shape.
Ions are atoms or molecules with an overall positive or overall negative charge – this happens when the number of protons and electrons in the atom or molecule are not equal. If there are more protons than electrons, the ion will be positively charged. E.g. if the ion has 18 protons but only 15 electrons, it will have a charge of +3. Likewise, if there are more electrons than protons, the ion will be negatively charged. E.g. if the ion has 8 protons and 10 electrons, then it will have a charge of -2.
- Explain changes of state in terms of particle theory and the energy changes involved
When a substance is heated, we are essentially providing it with heat energy. As the particles of the substance absorb this energy, they move faster. When a substance is cooled, it loses its energy to its surroundings, so the particles move less.
In a solid, the strong attraction between particles holds them together. Even though the particles vibrate about a fixed position, it is not enough to disrupt the structure. However, as a solid is heated, the particles gain energy and vibrate more vigorously. Eventually, they vibrate so vigorously that the particles are forced apart, disrupting the structure. The temperature that this occurs at is called the melting point. As the particles are further apart, the attraction between them is weaker, allowing them to move over one another instead of being held in place. The substance is now a liquid. This process is called melting.
As the liquid is heated, the particles gain energy until they all move so vigorously that they have the exact amount of energy required to break free of the bonds holding the particles together. The temperature that this happens at is called the boiling point of that liquid. The particles can now move randomly in any direction at high speeds, making the substance a gas. This process is called boiling.
As a gas is cooled, its particles lose energy to their surroundings. This means they move less vigorously, allowing the attraction between the particles to have a stronger effect. This forces the particles closer together and limits their motion to sliding over each other. This makes the substance a liquid. This process is called condensation.
As a liquid is cooled, its particles lose energy to its surroundings. The particles move less vigorously, allowing the attraction between them to have a stronger effect, meaning they are held closer together. As a result, the particles become packed tightly together in a regular arrangement and can only vibrate around a fixed point. As a result, this substance is now a solid. This process is called freezing or solidification.
Evaporation is similar to boiling in that it describes the change of state from liquid to gas, however, unlike boiling, evaporation is a superficial process and can occur at any temperature between the melting and boiling point of the liquid. In a liquid, particles gain energy from their surroundings, allowing them to move more vigorously, until the particles at the surface of the liquid gain enough energy to break away as a gas.
- Describe and explain diffusion in terms of the movement of particles (atoms, molecules or ions)
Due to the random motion of particles, fluid substances (gases or liquids), tend to spread out as much as they can. So if there’s more of one type of particle in one area, it will tend to spread out until the particle is evenly distributed throughout the area (it achieves equilibrium).
Concentration is a measure of how much of a substance there is in a single area, in other words, the amount of substance per unit volume.
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down the concentration gradient.
- Describe and explain dependence of rate of diffusion on molecular mass
The smaller the molecular mass, the less energy it needs to move. So, at the same energy, molecules with lower mass will move faster than molecules with more mass. This allows diffusion to occur faster.
To sum up, the smaller the molecular mass, the greater the rate of diffusion.
Notes submitted by Sarah
Click here to go to the next topic.
Click here to go back to the Science menu.
IGCSE AID 