I believe the correct answer from the choices listed above is option C. The ratio of Al3+ ions to S2- ions in a neutral compound would be 2:3. When it is a neutral compound the chemical formula would be Al2S3. As you can see in every 2 ions of Al you need 3 ions of S to maintain neutrality. Hope this answers the question.
Physical! Because they do not modify the nature of the substance ( if you look at the chemical level) and it is reversible
Vapor is H20
Water is H20
Ice is H20
A chemical changes is not reversible
Burning wood for example! And the reactants interact together to form a new product.
