Answer:
True.
Explanation:
Hello!
In this case, since the law of conservation of mass and energy states that both mass and energy cannot be neither created nor destroyed, if we want to analyze it in a chemical reaction, it is necessary to realize that the initial reactants are composed by a specific amount of atoms of each element and therefore the products must have that equal amount. For instance, going over the formation of water from hydrogen and oxygen:

We can see hydrogen are equal at both sides whereas oxygen do not; that is why we balance it by putting a coefficient of 2 just before hydrogen and water in order to equal the new 4 atoms of H and 2 atoms of O at each side.

That is why this statement is true.
Physical change there is no energy lost or gain. Basically the physical change is the change of state like melting
Answer:
4
Explanation:
The balanced equation for the reaction is as follows:
4Al + 3O2 --> 2(Al2O3)
Solids, liquids and gases all take up volume.
Answer:
Explanation:
Magnesium when it oxidizes has a valence of 2.
Oxygen, when it mixes with something, has a valence of - 2
So Mg and O2 will form something, but what? The answer is MgO
Mg + O2 ===> MgO
To balance this equation, you need 2 Magnesiums on the right and 2 Oxygens also on the right. The left will need a two Magnesiums.
The balanced equation will be
2Mg + O2 ====> 2MgO