Balancing means that the same number of elements on one side of the equation must be the same as the number of elements on the other side.
You need 3 carbons on the right side because right now there is only one. Try putting a 3 in front of the CO2 and see if the oxygens come into balance as well.
Al2(CO3)3 ⟶ Al2O3 + ___CO2
Al2(CO3)3 ⟶ Al2O3 + 3CO2
Balance check
Element Left Right
Al 2 2
Carbon(C) 3 3
Oxygen 9 3 + 6 = 9
And it is balanced.
Part B
It's a decomposition of some kind. Two smaller molecules are created from one larger one.
C. Its density is less than water This is why. The denser the object is the farther it sinks. Since crystallized ice floats it is less dense than the surrounding liquid.
