Week 6-What happens when a candle burns? 


Burning

Burning is an example of an irreversible change. When you burn wood you get ash and smoke. You cannot change the ash and smoke back to wood again.

For a material to burn, three things are needed; fuel, oxygen and heat.

Burning is an example of an ireversible change

The Fire Triangle

To produce any fire three things are always needed

FUEL: something to burn

OXYGEN: this comes from the air. More air = more oxygen.

HEAT: Nothing will burn if the temperature is not high enough

This week we will prove that oxygen is needed to make a candle burn. We will place a candle in a jam jar with the lid on and time how long the flame takes to go out. We will repeat the experiment using different sized jam jars.


Test your knowledge: Watch this clip and decide which changes are reversible or irreversible. www.bbc.co.uk/education/clips/zc84d2p

Below is a brief summary of all that we have covered recently on reversible changes in Marvellous Mixtures and irreversible changes in All Change.

Reversible changes are temporary

Examples of reversible changes:

  • Evaporating : eg water from a puddle evaporating on warm day
  • Dissolving eg sugar dissolving in hot tea
  • Melting eg chocolate melting when it gets warm
  • Freezing eg water freezing when it cools down below 0C
  • Boiling eg water boiling when it is heated to 100C
  • Condensing eg steam condensing on a cold bathroom mirror

Revise about reversible changes, including dissolving, melting, boiling, condensing, freezing and evaporating and the part played by evaporation and condensation in the water cycle.

All the changes above are temporary can be easily reversed.

They are sometimes called physical changes and no new chemicals are formed.

Several of the changes above take place in the Water Cycle

Irreversible changes are Permanent

Examples of irreversible changes:

  • Making toast
  • Cooking an egg
  • Rusting
  • Burning (eg burning wood)
  • Reacting baking soda with vinegar

The changes above are permanent and cannot be reversed .

They are sometimes called chemical changes and always result in a new chemical substance being formed.

When a new substance is formed it means that a chemical reaction has taken place.

Signs that a chemical change is taking place:

A change in temperature (it will usually get hot)

A colour change (eg a blue chemical turns white)

A gas is produced.

Homework: Design a poster illustrating the 3 things required to produce fire. Remember to include a sentence explaining what happens to the fire if one of these things is missing.