Which equation represents a physical change?
(1) h2o(s) + 6.01 kj ==> h2o(l)
(2) 2h2(g) + o2(g) ==> 2h2o(g) + 483.6 kj
(3) h2(g) + i2(g) + 53.0 kj ==> 2hi(g)
(4) n2(g) + 2o2(g) + 66.4 kj ==> 2no2(g)

The answer would be number one because notice the states of all the other answers. The numbers 2-4 are all adding energy to a gas and the products end up in the gas state as well which denotes no change in physical state. However if you look at number one it also has energy added but the reactant is originally a solid while the product is a liquid. Example- (s)= solid (l)= liquid (g)= gas

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).

Its strengthits conductivity of heat and electricity its shiny colourmalleable and ductility