Hi - first, scientists are always honest (or supposed to be, anyway).

1. Antimatter is just matter with opposite quantum numbers and force charges. It can be in any state that it wants, be it solid, liquid, gas, hot plasma, or whatever.

2. The explosive properties of antimatter come from the fact that when it combines with regular matter, it annihilates to become pure energy. It doesn't matter what the phase of the antimatter is that's doing the annihilation, or the phase of the matter that it's annihilating with; all that matters is the mass.

3. The mass-energy conversion equation is the famous E = mc², where E is energy, m is mass, and c is the speed of light, which is 3 x 10⁸ meters/second. The more mass, the more energy. Of course, because any amount of antimatter consumes an equal amount of matter when it annihilates, you have to multiply the mass of the antimatter by two in order to get the correct energy released from the annihilation.

4. One gram of mass annihilated (half a gram of matter and half of antimatter, for example) will release an amount of energy equal to approximately 21 kilotons of TNT - roughly the size of the atomic bomb that destroyed Nagasaki in 1945. Release of energy from annihilating antimatter with matter will be very similar in terms of destructive power as a nuclear weapon of the same yield, so if your antimatter is half a kilogram instead of half a gram, then the explosion will be about 21 megatons instead of kilotons. Of course, there's no limit in theory to how big a blob of antimatter you can throw at something, allowing you to make an arbitrarily powerful explosion using antimatter. By contrast, the largest nuclear bomb ever detonated was the Soviet Tsar Bomba, which had a yield of approaching 50 megatons - roughly what you would get with one kilogram of antimatter.