A physical change is any change in a substances form that does not change its chemical makeup. Examples of physical changes are breaking a stick or melting ice. A chemical change occurs when atoms of a substance are rearranged, and the bonds between the atoms are broken or formed. HOPE THIS HELPS!!
Explanation:
For Mg, (1.2 g Mg/24 g Mg) = 0.05 mol Mg.
For Cl, (3.55 g Cl/35.5 g Cl) = 0.1 mol Cl
So the ratio now is
Mg:Cl = 0.05 : 0.1 = 1:2
I got the 1:2 ratio by dividing both by the smallest number, which is 0.05 mol. Therefore, the empirical for formula of the substance is 
Last option:
CO2 (g) + H2O (l) -> H2CO3 (aq)
In the brackets:
g = gas,
l = liquid,
s = solid,
aq = aqueous.
So,
CO2 (g) = carbon dioxide gas
H2O (l) = liquid water
H2CO3 (aq) = aqueous carbonic acid
By measuring them, by monitoring them, or by separating them.
