How can you tell a chemical change has happened?

You can tell when a chemical change has happened when the chemical changes in appearance such as color or physical state.

Explanation:

If a change happens it can change in any way shape or form.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.