Question

A metal surface becomes dull on exposure to air. phyiscal or chemical change?

Answer 1

chemical.

Answer 2

chemical..form 4 or 5

Answer 3

This is a chemical change.

Want to know how I determined that it is a chemical change? Think of it this way, a chemical change can not be reversed by physical means. In other words, I can not reverse the effects air has had on the metal surface physically, instead, it would require chemical means to reverse the chemical change!

In reverse, a physical change IS something that can be reversed by physical means. For example, if I take a teaspoon of sugar and dissolve it into a hot cup of water (Btw: This creates a homogeneous mixture), I would have just performed a physical change. I know that this is a physical change because I CAN reverse this and separate the sugar from the water by physical means. An example of a physical mean here could be evaporation. I could evaporate the water so that only the sugar remains in my cup- hence, separating the sugar from the water!

It is that simple! I hope I have helped you differentiate between chemical changes and physical changes!