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State the difference between a physical change and a chemical change, and list three indications that a chemical change has occurred. Which indication is most suggestive of a chemical reaction?
A chemical change produces a new substance that wasn't there before, but a physical change only changes the form of a substance.
If I have a sheet of paper I can tear it up, or put it through a shredder.
I end up with lots of small bits that look very different from a single sheet. However, the bits are still paper. I haven't produced anything new. If I was really clever and patient, I might even be able to stick the bits back together to make a single sheet of paper again. The sheet of paper has changed, but it is has undergone a PHYSICAL CHANGE.
However, I can also set light to the sheet of paper. I get some smoke and ash as a result, and we see hot flames too. The ash is different stuff, not paper at all. If it was the same I'd be able to burn it again! The ash and smoke are the result of a chemical reaction, so we say that the paper has undergone a CHEMICAL CHANGE.
Three good indicators of a chemical change are these.
- You get a new substance formed.
- The temperature changes. It could go up or down.
- The process is hard to reverse, and may even be impossible to reverse.
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