Question

A light green powder is heated in a test tube. A gas is given off, and the solid becomes black. Was the change physical or chemical, and is the green powder an element, a compound, or a mixture?

Answer 1

See explanation

Explanation:

The question describes an experiment similar to the decomposition of copper(II) carbonate (`CuCO_3`) which is when heated in decomposes into carbon dioxide gas (`CO_2`) and copper(II) oxide (`CuO`) according to the following decomposition reaction:

`color(green)(CuCO_3(g))->color(gray)(CO_2(g))+CuO(s)`

Therefore, the green substance is a compound and the change is a chemical change since giving off a gas is one of the several indication of a chemical change that include:

1. Gas evolution
2. Formation of a precipitate
3. Evolution of heat
4. Odor
5. Color change
6. etc.

Here is a video on physical and chemical change that describes the aforementioned experiment:

Lab Experiment #2: Physical & Chemical Changes.