Question

Which would show an example of how physical changes are reversible? Cutting a gold bar in two peices and then putting the pieces next to eachother. Or burning a match and then relighting it. Or melting tin and then cooling it into a mold. Or combining sodium and chlorine to make the new material of table salt

Answer 1

Answer: melting tin and then cooling it into a mold

Explanation: Physical change is a change in which shape, size or state of the substance is changed and no new substance is formed. Physical changes can be reversed by

Chemical change: When there is a change in rearrangement of atoms  which leads to the formation of the new substance.

a) Cutting a gold bar in two pieces and then putting the pieces next to each other: It is a physical change but it is not reversible as original state is not achieved again.

b) burning a match and then relighting it: This is a chemical change as new substances are formed.

c) melting tin and then cooling it into a mold: This is a physical change and is reversible as the original state has been achieved again.

d) combining sodium and chlorine to make the new material of table salt: It is a chemical change.$2Na+Cl_2\rightarrow 2NaCl$

Answer 2

Melting tin, then cooling it into a mold.

You're only melting a solid into a liquid, then back into a solid. You're not making a completely new substance.