When solid (S1) is heated, it breaks up into a gas (G) and another solid (S2). All three of these chemicals are pure substances. On the basis of this information alone, which of these must be compounds? Why?

S1

Explanation:

Has to be S1. This is pretty glaring to me. It's said from the question that the chemical reaction that took place is given by the following

S1 + G = S2, with S2 being an offspring of the first S1. When S1 got reacted, it broke down into S2, so, this only confirms that both S1 and E2 are S3 are compounds.

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s