Photosynthesis and respiration
Mass and weight    Force and pressure
Melting and dissolving   Decomposing and burning


1. Photosynthesis and Respiration

Photosynthesis is the name of the process by which the plant manufactures sugars.
Word Equation:
Carbon dioxide  + water  + energy from the sun  ——->      sugar   +    oxygen

Respiration is a chemical reaction that provides all life with energy
Word equation: sugar + oxygen  —–> carbon dioxide + water + energy

Photosynthesis  is MAKING sugar, using up energy
Respiration is USING UP sugar and RELEASING energy

Photosynthesis takes place only in plants and only in the light
Respiration happens in all animals and plants and happens all the time.



1.  Mass and weight

Remember:    Mass is measured in grams (or kilograms) and weight in newtons.

The weight of an object is caused by the pull of gravity.
No gravity = no weight
The pull of gravity on Earth is 10N on every kilogram
……so a girl with a mass of 35kg will have a weight of 35kg x 10 = 350 N

You may well see on a cereal packet Weight = 500g. This is WRONG!
Weight is measured in NEWTONS and mass measured in grams

It should say Mass= 500g  (or weight = 5000N)

2. Force and pressure

What’s the same: Both force and pressure are a kind of push

What’s different:
Pressure shows how concentrated or spread out the force is.

Example:The diagram shows 2 boys. Each has a mass of 40kg
The weight of each boy is 40kg x 10 = 400N

The first boy is wearing trainers and second  boy is wearing football boots with studs.

They are both going to produce exactly the same downward force on the floor
however, with the boy wearing studs the force is
concentrated onto a smaller area. This makes the pressure greater.

Pressure = Force ÷ Area

Boy wearing trainers
Estimated area of both feet = 400 cm²
Pressure = 400 ÷ 400 = 1 N/cm²

Boy wearing trainers
Estimated area of studs = 20cm²
Pressure = 400 ÷ 20 = 20 N/cm²

Even though the total downward force is the same with each boy, the studs have a smaller area so the pressure is greater.
This is why the studs sink into the soil (or mark the floor) and the trainers do not.

What happens if the first boy stands on one foot?


If you were standing on sand you are more likely to sink in on one foot. Why?

The weight of the boy has not changed so the downward force is still 400N
the area in contact with the floor has halved so the pressure will have doubled.

Same force. Smaller area so greater pressure

If you wanted to make the pressure less you have to INCREASE the area.
If you found yourself on thin ice it would be best to lie down Why?

The weight of the boy has not changed so the downward force is still 400N
the area in contact with the floor increased so the pressure will be less.

Same force. Larger area so less pressure


Other examples


1. Melting and dissolving

When sugar is stirred into hot tea it dissolves NOT melts

If mixed with water (the solvent) sugar will dissove. It does not even need to get hot although hot water will make the sugar dissolve quicker.

In order to melt the sugar MUST be warmed up to a temperature above its melting point (which is 160°C)

A substance will melt (turn into a liquid) when it is above a temperature known as its melting point.
In order to melt it MUST get warmed
It does NOT need to be added to a liquid.

A substance will dissolve when it is added to a suitable solvent (liquid that dissolves it).
It does NOT need to be heated but it DOES need to be added to another liquid.
After it has dissolved it will have formed a solution.

2. Decomposing and Burning

When something burns it combines with oxygen
It increases in mass and you always end up with a compound called an oxide

Magnesium ribbon burns to form a white compound called magnesium oxide,

word equation
magnesium + oxygen → magnesium oxide

When something decomboses it splits apart
Decomposition often happen when a compund is heated (but not all compounds decompose)

eg If hydrated copper sulphate is heated it decomposes to form anhydrous copper sulphate and water

Hydrated copper sulphate  (blue)   →    anhydrous copper sulphate (white) + water

Chemicals you may have heard of that decompose
(for more information on these, see heating chemicals)

Hydrated copper sulphate (see above)
Hydrated cobalt chloride (pink)
Copper carbonate (green powder)
Calcium carbonate (White)
Potassium permanganate (Black)

  • Both burning and decomposition are examples of chemical change
  • Elements can never decompose
  • When a chemical burns to form a solid, there will normally be an increase in mass
  • When a chemical decomposes and forms a solid there will normally be a decrease in mass

For scholarship students
Burning is an exothothermic reaction and heat will always be evolved.
(burning magnesium is very exothermic, giving out lots of heat)

Decomposition is usually endothermic and heat energy is used up.
(Limestone has to be heated strongly to make it decompose)