# Understanding the difference between physical and chemical changes

Which of the following is a chemical change?

A. Hydrolysis of a dipeptide.

B. Stretching of a balloon.

C. Freezing of water.

D. Breaking a stick.

I'm pretty sure that the answer is (A), but I'm not 100% positive.

I know that, for (C), you can freeze water, and then you can melt it back to liquid water.

For (D), you can break a stick, and the stick is still now two sticks.

For (B), however, I think that it is a physical change, but I'm not entirely certain.

Can someone please explain this for me?

• It seems you're getting it more or less right. – Mithoron Oct 15 '16 at 22:57

## 2 Answers

Yes, (B) is a physical change. Stretching the balloon simply rearranges the polymer strands in the balloon material, making them straighten out instead of being kinked up. When you let the balloon go, they just pull back to their former kinked shape.

Your reasoning on (C) and (D) is solid.

The answer is indeed (A). See Wikipedia's page on hydrolysis for more information.

The question was upvoted because it has some interesting features. On the one hand, it suggests that the four answers have significant differences (which they do). But it also suggests that the world consists of two kinds of people: chemists and physicists.

Actually, there is a third kind: non-scientists.

But each of the changes has a component that could be of interest to a chemist, and also one that could be of interest to a physicist, hence the usefulness as a question. Here, one must put on the mathematical hat. Answer A changes the chemical composition of the starting material, so is certainly a chemical change, but the physical properties of the final product will be different (mass, density, melting point), so could possibly be of interest to a physicist, or, perhaps to a chemical engineer, who sort of resembles a physicist in some ways. The mathematician would rate the degree of chemicalness as about 99%.

Answer B would seem to be a physical change: we assume the balloon returns to its original shape when the stretching force is removed. But if that force is repeated, and/or increased, the balloon does not return to its original shape, and the process may be better explained by a chemist than a physicist. Hence the uncertainty of the OP. A serious mathematician could rate one small stretch to be about 99.99% physical while multiple large stretches could be as much as 2% chemical.

Answer C, the freezing of water, depends on the purity of the water. If it is distilled, air-free, then my mathematician would say 100% physical change, but also of considerable interest to chemists. But since the chemical composition does not change, we'll give it to the physicists.

Answer D, breaking a stick, must have some molecules sliding over some others, but surely some must break - perhaps a few thousand in $$10e^{23}$$. The mathematician says that's insignificant, and he's right: it's a physical change, at least to non-scientists, who make up the vast majority of people in the world.

And that's my point, after all the verbiage: the question requires a level of analysis which is not too deep, because nitpickers of the chemical or physical persuasion could probably find something of interest in all four answers and claim a different answer. But we hope to acquire a depth of reasoning and explanation that enables us to slice thru the question at a depth of reasoning that enables the quickest separation and answer. Otherwise our answer will be marked incorrect and require extensive verbal defense, which will still be regarded as inadequate.

Perhaps a dash of philosophy would help.