Chemical change The three states of matter
Change in Mass The Water Cycle
Heating copper sulphate
Heating magnesium ribbon
If a solid chemical is heated several things could happen:
It could melt (change from a solid to a liquid)
It could sublime (change from a solid to a gas)
It could burn or oxidise (react with oxygen)
It could decompose (split apart)
or….. nothing happens at all (eg copper oxide will do nothing when heated)
In a physical change no new substance is formed
Examples of physical change are: evaporation, melting, freezing and condensation.
Physical changes are usually temporary and can often be reversed
In a chemical change a chemical reaction is taking place and a new substance is always formed
Examples of chemical change are decompose, oxidise and combustion.
Chemical changes are usually permanent and cannot easily be reversed.
Table to show the effect of heat on some chemicals
| Chemical | Appearance | Effect of heat | Change in mass when heated. |
| Magnesium ribbon | Grey metal | Burns with an intense white flame to leave a white ash called magnesium oxide. | Gain because it combines with oxygen from the air |
| Copper foil | Pink metal | Oxidises. Forms a layer of black copper oxide on the surface. |
Gain because it combines with oxygen from the air. |
| Hydrated cobalt chloride | pink/purple crystals | Decomposes. Turned to a blue powder called anhydrous cobalt chloride and gave off water vapour. The blue powder gradually turned pink again when it was left standing in the room. |
Loss because it loses water vapour. |
| Potassium permanganate | dark purple/black crystals | Decomposes and produces oxygen gas | Loss because it loses water vapour |
| Calcium carbonate (chalk, limestone) | White solid | Decomposes, producing carbon dioxide gas and leaving a white solid called calcium oxide (lime) | Loss because it loses carbon dioxide |
| Hydrated copper sulphate | Blue crystals | Decomposes Turns to a white powder called anhydrous copper sulphate and produced water vapour. |
Loss because it loses water vapour. |
| Sulphur | Yellow solid | Melts then burns with a blue flame to producing choking fumes of sulphur dioxide gas. | The sulphur dioxide would weigh more than the original sulphur. |
| Zinc oxide | White powder | Turned yellow when hot and white when cold. | No change. |
| Iodine | Dark grey/purple crystals | Sublimes Forms a purple vapour which turns back to grey iodine crystals when it cools. |
No change because all of the iodine vapour turns back to solid iodine crystals. |
| Copper carbonate | Green Powder | Decomposes. Evolves carbon dioxide gas and leaves a black residue of copper oxide. | Loss because it gives out carbon dioxide gas. |
Types of change that could occur when a single substance is heated:
The change could either be PHYSICAL or CHEMICAL.
A chemical change always produces a new substance and is usually permanent.
A physical change forms no new chemical and is usually temporary.
The following are types of chemical change because in each case a new substance is formed
1. Decomposition
This is when a compound splits apart into two (or more) chemicals.
eg: copper carbonate decomposes when heated to form carbon dioxide gas and leave black copper oxide.
Word equation: Copper carbonate ———> copper oxide + carbon dioxide
2. Oxidation
: When a chemical combines (joins up with) oxygen to form an oxide.
eg copper foil will oxidise when heated strongly in air
Word equation: copper + oxygen ——-> copper oxide
3. Combustion (or burning) is a kind of oxidation where a flame is usually produced.
eg Magnesium will burn in air to form magnesium oxide:
Word equation: Magnesium + oxygen ——-> magnesium oxide
4. Reversible reactions
Heating copper sulphate
If some copper sulphate (blue crystals) is placed into a test tube and heated gently it is seen to go white and steam come out of the test tube. The steam condenses into little drops of water.
This happens because the copper sulphate is decomposing (splitting apart) It gives off water and turns into a new chemical called anhydrous copper sulphate which is a white powder.
The blue copper sulphate is sometimes called hydrated copper sulphate
Hydrated copper sulphate → anhydrous copper sulphate + water
If the white anhydrous copper sulphate is scraped into a dish and water is added the white copper sulphate gets hot (steams) and goes blue again
Anhydrous copper sulphate + water → hydrated copper sulphate
Putting water on to anydrous copper sulphate and seeing if it goes blue is sometimes used as a test for water
Signs for a chemical change are:
1. A change in colour ( eg changes from pink to blue)
2. A change in temperature (eg gets warm or hot)
The Change in Mass when a chemical is heated
Some chemicals (like copper carbonate or copper sulphate) will lose mass.
They do this because they are compounds that decompose and release a chemical to the air.
Some chemicals (like magnesium or copper ) will gain in mass.
They do this because they are elements and they combine with oxygen.
To find out if there is a change in mass the chemical needs to be weighed before and after heating.
Example 1: Copper foil will gain in mass when heated in air because the copper oxidises (combines with oxygen)
Word equation: copper + oxygen ——-> copper oxide
Example 2: Magnesium will gain in mass when it burns because the magnesium combines with oxygen however care must be taken to make sure that the smoke produced all gets weighed (See Burning Magnesium).
When compounds that decompose are heated they will often LOSE mass
Example 1: When copper sulphate is heated it will lose mass because it decompose and give off water vapour. (See experiment to heat copper sulphate)
Example 2: When Copper carbonate is heated it will lose mass because it decomposes and gives off carbon dioxide gas.
Physical changes
Melt: When a solid changes from being a solid state to a liquid when it is heated above it’s melting point (eg melting ice to form water)
Sublime: When a solid changes from a liquid state to a gas when heated (eg heating iodine)
Chemical changes
Burn: when a substance reacts with oxygen making a flame (eg burning nagnesium)
Oxidise: when a substance reacts with oxygen (eg heating copper or magnesium)
Decompose: When a solid compound splits apart (eg potassium permanganate forming oxygen when heated)
