If someone takes a material that looks pure, and then they do something to it that results in two separate materials, how can they know if they originally had a pure-looking
mixture of two things that they simply separated into its parts, or if they originally had a
pure material that was a compound that was chemically broken down into new
compounds or elements?
lile

The determination of the nature of the combination of the two separate materials in the original material that looks pure is based on the characteristic of the change process they undergo to become two separate materials.

Therefore, they can know if they initially had a mixture or a compound by the following steps;

1) If the two separated materials can be easily remixed to form the original material, then they likely have a mixture of the two separate materials

2) If the process that results in the formation of the two separate materials, involves the absorption or evolves heat or other forms of energy, then the original substance was a compound

3) If the two newly formed materials is seen to be a permanent change, then the original material was a compound

Explanation:

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.