Heating and Cooling
Physical changes involve re-arrangement of particles without altering any chemical bonds. Physical change is a change in state and there is no change in mass. 20g of ice melts into 20g of water and boils into 20g of steam.
Heating/cooling a substance normally increase/decrease its temperature. However, the temperature against time plots will show the heating and cooling curves have periods of constant temperature (i.e. the curve is flat and there is no change in temperature when heated/cooled). These constant temperature regions are when the substance is changing from one state to another. When melting or boiling, the heat energy supplied is used to break the 'force' holding the particles together (not the chemical bonds) rather than raising the temperature. When condensing or freezing, heat is given out but the particles fuse (join) together rather than lower the temperature.
Dissolving and Solubility
A substance (e.g. sugar) which dissolves to make a solution is said to be soluble. A substance which does not dissolve is said to be insoluble (e.g. sand). The substance which is dissolved is called the solute and the substance which does the dissolving is called the solvent. Water is the most common solvent but there are others (e.g. maths, hexane).
Dissolving is the breaking up of the solute (the solid being dissolved) and the particles of the solid filling the spaces between the solvent. The solute is in the solvent and it can be separated from the solvent by evaporation.
When dissolving, the solvent particles 'bump' into the lump of solute and break up the solute particles. The two different particles mix together to make the solution. At higher temperature, the solute will dissolve quicker and more solute can be dissolved because the particles move faster and get more mixed up. However, there is a limit of how much solute that a solvent will dissolve after which the solution gets saturated - called saturated solution.
The solubility of a solute is the number of grams of the solute which will dissolve in 100g of water at a particular temperateand the solubility changes with temperatue. If a hot saturated solition is cooled, the solute will crystallize out.
There are some rules about solubility:
- Most solids dissolve better in hot water than in cold water.
- Liquids either dissolve in water (miscible) or do not mix with water (immiscible). Changing the temperature does not affect this.
- Gases dissolve better in cold water than in hot water.
States of Matter
Different substances change state at differen temperatures. This is because the bonds between particles of different materials are different. Comparing water and iron, the bonds that hold the water particles together (not the chemical bonds between the elements) are weak while the bonds that hold iron particles together are strong. The melting and boiling points of iron are 1540°C and 2750°C respectively, much higher than water (0°C and 100°C) because it takes much more heat energy to lossen or break the iron bonds.
The melting/boiling points, which are the same as freezing/condensing points for a substance, defines the state of the substance. A substance is a solid up to its melting point. At that point it begins to melt and change into a liquid. Above its melting point it's still a liquid until its boiling point is reached, and then it becomes a gas. A gas just gets hotter (i.e. temperature gets higher) if it is heated further. Given the boiling and freezing temperatures of a substance, it should be able to determine whether that substance is a solid, liquid or a gas at a given temperature.
Expansion happens in all states of matter when heated. An increase in temperature causes particles to move around more and faster and this means they take up more space. Contraction is the opposite of expansion and there is less movement/vibration when cooled. When a liquid is heated its volume expands as the particles move apart with their extra energy. However, a sealed gas or liquid increase its pressure when heated because they cannot expand due to the fixed volume.
Geological Changes
Geological changes are changes to rocks both inside and outside the Earth. Weathering is the technical term meaning breaking down of rocks and there are physical weathering and chemical weathering.
Physical weathering can be due to either Onion Skin weathering or Freeze-Thaw weathering. Onion skin weathering is caused by the warming of the rock by day and cooling by night and the rock surface expand and contract repeatedly to eventual breaking away. Freeze-thaw is caused by water in the cracks freezes and expands to make the crack bigger and eventually bits break off.
Chemical weathering is due to pollution and acid rain break down rocks like limestone.
The weathering of rocks breaks them down into very small pieces which become soil. Different soil contains different grain size and a sandy soil drains well because it has large garins with big spaces between the them. A clay soil soon become waterlogged because of the small grain size. A fertile soil is good for gardening or farming and should contain a lot of rotted vegetation (humus) and be a loam soil (contains grains of different sizes).
The rock cycle is the process of changing the three types of rock from one form to another which takes millions of years to complete. The stages are:
Weathering (breaking down rocks) and Erosion (removal of rock)
Transportation - moving eroded bits of rock mostly by water but also by wind
Deposition - laying down sediment in lakes and seas
Burial/compression - squeezing and compressing the layers eventually forming sedimentary rocks
Heat/pressure - further compressing and heating turns the rocks into metamorphic rocks
Melting - intense heating melts the rocks into magma
Cooling - the molten magma turns into solid again - igneous rocks
Exposure - back to waethering and erosion
Rock Types
There are three different types of rock: sedimentary rocks, metamorphic rocks and igneous rocks.
Sedimentary rocks are formed from layers of sediment (rocks eroded or broken down in different ways) laid down in lakes or seas and compressed over millions of years. The sediment layers are 'stuck' together with salt crystals and fossils (dead remains of plants and animals) can form in the sediments. The type of fossil can be used to work out the age of the rock. Examples are limestone, chalk and sandstone.
Very high pressures and temperatures over long periods of time convert sedimentary rocks into metamorphic rocks by changing the structure. Marble is formed from limestone and slate is formed from mud. Schist is another metamorphic rock.
Igneous rocks are formed when molten magna pushed up to the surface of the crust (e.g. through volcanoes) is cooled. Sometimes it can solidifies below ground. Slowing cooling of the liquid magna produces large crystals while rapid cooling result in small crystals. Granite and basalt are igneous rocks.