There are two types of changes of matter: Physical change, and Chemical change. Thermal energy is always involved in chemical changes either by giving out or taking in heat from the surroundings.

Some physical changes do not involve heat such as changes in physical states. Other physical changes do not involve heat such as crushing metal cans, or cutting objects into smaller pieces.

• A change in physical state of matter

• Expansion and contraction of matter

Heat is taken in from the surroundings when substances undergo melting and boiling and evaporation. Heat is given out to the surroundings when substances undergo freezing and condensation.

New substances are not formed after the change. The change is only temporary and can be easily reversed.

Example:

• Water vapour condenses, where gas changes to liquid. Heat is given out ot the surroundings.

• Ice melts and changes back into water. Water can then change back to ice if placed in a freezer.

• Photosynthesis

• Rusting or iron

• Food decaying

Chemical changes produce new substances, which usually involves chemical reactions.

• Burning wood is considered as a chemical change as thermal decomposition breaks down wood into different chemicals, water and carbon dioxide, as long as there is a presence of oxygen.

• Word equation: Wood + Oxygen > Carbon dioxide + water

In a chemical change, neither mass or energy is created. Hence the total mass and energy before and after a chemical change is remains the same. In chemical changes, there are four types:

• Decomposition, where a substance is broken down into two or more simpler substances.

• Combination, two or more substances chemically combine to form new substances.

• Combustion, where a substance combines with oxygen gas in the air when heated to form new substances and give out light.

• Oxidation, oxygen is added to a substance after a reaction.

In a word equation, the elements in the reactants rearrange themselves to form new products. A word equation can be used to represent the changes in a chemical reaction.

Example:

A reactant between nitric acid and sodium hydroxide produces sodium nitrate and water.

Word equation: Nitric acid + Sodium hydroxide > Sodium nitrate + water

• Photosynthesis

• Film photography

Photosynthesis is a process where chlorophyll absorb light energy and convert it to carbon dioxide and water into glucose.

• Reactants: Carbon dioxide and water

• Products: Glucose and oxygen

Word equation: Carbon dioxide + water > Glucose + oxygen

Film photography is a process where film contains a chemical compound called silver bromine. Upon exposure to light, it decomposes to the elements silver and bromine.

• Reactants: Silver bromine

• Products: Silver and Bromine

Word equations: Silver bromine > Silver + Bromine

Examples:

• Calcium carbonate > Calcium oxide + Carbon dioxide

• Zinc carbonate > Zinc oxide + Carbon dioxide

Combustion is when a substance is burnt into the air and it combines with oxygen to form new substances. Light energy is also given out.

Examples:

• Magnesium +oxygen > Magnesium Oxide

• Methane gas + oxygen > carbon dioxide + water

• Wood + oxygen > carbon dioxide + water

Combination is when two or more substances chemically combine to form new complex substances

Examples:

• Sulfur + Iron > Iron(II) sulfide

Oxidation is when oxygen is added to a substance after a reaction such as in the rusting of iron

Examples:

• Iron + oxygen > Iron(III)oxide

Example:

• Water can be decomposed into elements hydrogen and oxygen

Substances can be mixed together to form new substances. Heat energy will be given out or taken in after the chemical change.

Examples:

• Nitric acid + sodium hydroxide>sodium nitrate + water

• Ammonium chloride + calcium hydroxide > Calcium chloride + water + ammonia