[Gromek]

Can somebody (anybody) explain what happens if water(let's say in a tin can) is dropped into laddle filled with liquid metal (90tonnes of it).

I know there is an explosion, but what I am asking why does it explode. Is that due to rapid pressure change (liquid H2O to gas H20) or is it because water falls apart on the mollecular level dividing into O2 and H2 (and thus forming somekind explosive reaction like H-bomb)

I heard both versions, that's why I am asking.

[Scleros]

why does it explode. Is that due to rapid pressure change (liquid H2O to gas H20)

Yes as well as volume change as the water rapidly changes to steam.

...water falls apart on the mollecular level dividing into O2 and H2...

Maybe, I'm undecided how much this would be a factor in a foundry environment. The thermal decomposition of water absent a catalyst at high temperatures(1) (> ~2800 °F) could liberate combustible hydrogen and steam has been documented chemically reacting with certain substances under specific environmental conditions producing hydrogen in sufficient quantities explosive in air(2).

...and thus forming somekind explosive reaction like H-bomb

No, H-bombs rely on nuclear fusion of isotopes of hydrogen at the atomic level. Decomposition of water into hydrogen and oxygen is a chemical reaction at the molecular level. If fusion was as easy as water + hot metal, humanity would have exterminated itself by now.

References:
1 - Direct Solar-Thermal Hydrogen Production from Water...
2 - Evaluations of Graphite/Steam Interactions...

[Perito]

Scleros is pretty much correct that the immediate vaporization of the water is responsible for most of the explosive force. However, most liquid metals are active enough to react with the water to form hydrogen and the metal oxide. The hydrogen will also most likely explode under these conditions. Water won't "fall apart" into H2 and O2, but it will react with the metal to release H2.

[pramod awasthi]

Just due to formation of hydrogen gas

[Scleros]

The hydrogen will also most likely explode under these conditions.

How big of an explosion will a tin can's worth of water amount to?

[Perito]

How big of an explosion will a tin can's worth of water amount to?

I'm not sure how to describe magnitudes of explosions. Probably not a huge explosion. Enough to do some damage, however. Remember the hydrogen will also explode (probably).

[Scleros]

Remember the hydrogen will also explode (probably).

My prior post was badly worded. I was wondering about the hydrogen specifically. Compared to the steam, how much of a factor would a tin can's worth of hydrogen be in the resulting overall calamity?