Science Puzzle
Can You Mix It Back?
Stir a spoonful of salt into warm water and it vanishes. Boring, and easily undone: leave the water to evaporate and every grain of salt comes back.
Now pour vinegar onto a spoonful of bicarbonate of soda. It erupts into froth. When the fizzing stops, can you get your vinegar and your bicarbonate back the way you got the salt back?
The Answer
No. They are gone, and the fizz is the evidence. Dissolving salt is a physical change: the salt is still salt, just hidden among the water molecules, which is why evaporation recovers it perfectly.
Vinegar and bicarbonate of soda do something far more drastic. Their atoms rearrange into entirely new substances: carbon dioxide gas, which bubbles away into the room, water, and a salt called sodium ethanoate. The original ingredients no longer exist. You cannot un-fizz the mixture any more than you can un-fry an egg or un-burn a match.
The quick test for a chemical change: has something genuinely new appeared? A gas, a colour change, heat given off, a solid forming in a clear liquid. Any of those means the atoms have rearranged.
The principle: Physical changes rearrange substances and can usually be reversed. Chemical changes rearrange atoms into new substances and usually cannot.