Physical vs. Chemical changes?

Question

I am not good at science so I need some help! I had my elementary students list examples of physical changes. Their book listed water changing to ice or steam as one, or cutting or shaving wood. I read somewhere that physical changes can be reversed... but if I cut up a piece of paper, I can't reverse it without using tape or something. So what am I missing here? And as far as the responses I got, there were some clever/slightly deranged kids. One said animals, when they die. They're still animals. Or if you cut up a dog, it's still a dog. Or if you add salt to water, the salt is still there. Which confuses me- if you add kool aid mix to water, you dont have water anymore, you have kool aid. Can it be reversed? And lastly, someone said growing, like a baby growing to be an adult. Does that make sense? But the change from a seed to a flower would be chemical change, yes? Thanks for your help science people.

Answer 1

A physical change is a change that occurs to a substance's size, shape or state. Cutting up paper is a physical change because, in theory, you can still rejoin the edges of that paper to make it just as it was before (but I know this is almost impossible without the proper machinery). Adding Kool Aid (cherry for example) to water creates a solution (a homogenous mixture of a solid dispersed in a liquid). You simply have millions of tiny red solid state Kool Aid granules evenly dispersed in water so well that it looks like a chemical reaction took place BUT IT DID NOT. The key is that the ingredients themselves (the water and the Kool Aid) have not been changed, only displaced. By boiling the water completely you would be left with dry Kool Aid but again this is only possible with hi-tech machines found in labs. BUT IT IS POSSIBLE.

In contrast, a chemical reaction actually changes the chemical makeup of the substances involved. New molecules are formed and energy is either given off or absorbed in the process.

Living and growing (people, animals and plants) is an EXTREMELY complex example of both chemical and physical processses. Being alive you are able to chemically stimulate physical processes. When death occurs these chemical processes stop and chemical decomposition takes place.

Answer 2

Paper can be recycled and wood can be made into particle board, plywood, veneer, etc. There may be a chemical change in severing some of the constituent fibers of the wood/paper, but the process is at least somewhat reversible, though more easily for paper. When organisms die, the chemical processes keeping them alive stop and they decompose, all chemical changes. Salt water can be distilled or evaporated and the salt is no longer in the water: reversible. Likewise with the kool aid. And, finally, organisms' growth is most definitely the consequence of chemical changes. Hope that covers everything.

Answer 3

How about this? A physical change is something that occurs in a substance without it changing into or interacting with another substance. It still has it's own composition like water, steam and ice are all H2O! Water and Kool-aid are not. Physical properties are color ( snowshoe rabbits are brown in summer, white in winter but they are always snowshoe rabbits), density, melting point, etc.
Chemical changes are the conversion of substance(s) into other substance(s)- this is a chemical reaction and something has to be added or taken away to produce a chemical change. Like our friend water; add an electric current to water and it breaks down into hydrogen and oxygen-2 gases, so we have changed the physical and chemical properties.
And no, a physical change does not have to be reversible. Growing up is a physical change, but chemically we remain the same.

Answer 4

Physical change: Changes to physical properties like shape, size, states of matter, reversible.
All conditions need not exist at the same time!

Chemical change: Can include above but MUST have heat or light exchange, non-reversible

1) cutting up paper (changing size)
2) Add salt to water - they can be separated by evaporating water

I hope it helps.

Answer 5

Chemical... except of direction... (there is often a yet, isn't there)... If the exchange from good to gasoline or liquid to gasoline (the two actual variations) is what's occuring to allow the cloth to grow to be airborne (and then smell), then that's a actual exchange. however the way you have pronounced it, it relatively is probable chemical which you're searching for for...

Answer 6

Try this site. I think it explains things rather simply and it even has a worksheet link.

When kids say things about cutting up dogs, please tell them that it is an inappropriate answer. If you do this in a calm, matter-of-fact manner, they will soon learn that it's not funny and it's not acceptable.

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson15.htm