Question

Are reactions with water to form a gas physical or chemical changes?

Answer 1

`"Chemical change"` is characterized by the formation of new substances and the making and breaking of strong chemical bonds.

Explanation:

In your scenario, a material reacts with water to form a gas. I can think of a couple of actual examples which are manifestly examples of chemical change:

`Na(s) + H_2O(l) rarr NaOH(aq) + 1/2H_2(g)uarr`

So here strongish metal-metal and stronger `H-O` bonds have been broken, and new substances, a salt, and dihydrogen gas have been formed. So this is clearly an examples of chemical change.

Physical changes are largely changes of state, i.e. solid to liquid to gas, in which the phase of the material has changed, but the intrinsic chemical bond have is unaltered: cf water to ice, the `HOH` molecule remains intact but the molecule now resides in a crystalline lattice.