chemical reactions

[Scots-babe]

can anyone tell me which of the following are chemical reactons and why or why not.

thanx in advance. the sooner the better please!

a firework going off
starting a car engine
bursting a balloon
grilling bacon
grass growing

[jake-the-peg]

A chemical reaction occurs when there is a change at the molecular level. That is to say the chemicals change.

If you're stuck a good idea is to think whether or not you can run it backwards - it's not 100% reliable but a good indicatior at this level.

So in the first chemicals burn to set off a firework, in the second the petrol burns when the car starts - both burning both chemical reactions.

In the third one compressed air in the balloon escapes, you could catch the air, recompress it in another balloon - this is a physical process.

Grilling bacon causes chemical changes to the meat - you can't ungrill it and similarly with grass growing, energy and nutrients combine chemically and give the grass energy to grow.