Answer:
Well we know that matter is neither created nor destroyed by chemical reactions or physical transformations. It is not destroyed it simply was converted to gas. =D
Here are some examples for those type of reactions.
<span>
Combustion reaction: CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(l)
</span><span>
Decomposition reaction: CaCO3(s) ---> CaO(s) + CO2(g)
</span><span>Double replacement: AgNO3(aq) + NaCl(aq) ---> AgCl(s) + NaNO3(aq)
</span>One common thing in all is that they are reactions. They have reactants to form new substances called product.
Explanation:
(a) potassium oxide with water

According to reaction,1 mole of potassium oxide reacts with 1 mole of water to give 1 mole of potassium hydroxide.
(b) diphosphorus trioxide with water

According to reaction,1 mole of diphosphorus trioxide reacts with 2 moles of water to give 2 moles of phosphorus acid.
(c) chromium(III) oxide with dilute hydrochloric acid,

According to reaction,1 mole of chromium(III) oxide reacts with 6 moles of hydrochloric acid to give 2 moles of chromium(III) chloride and 3 moles of water.
(d) selenium dioxide with aqueous potassium hydroxide

According to reaction,1 mole of selenium dioxide reacts with 2 moles of potassium hydroxide to give 1 mole of potassium selenite and 1 mole of water.
You have to do it to see the results