Because of the surface tention.
Something that melts, something that changes shapes (for instance, play dough being squished), something that boils, something being mixed or dissolved (but only if it doesn't chemically react), etc. A physical change is a change of state of matter.
Explanation:
Start with a balanced equation.
2H2 + O2 → 2H2O
Assuming that H2 is in excess, multiply the given moles H2O by the mole ratio between O2 and H2O in the balanced equation so that moles H2O cancel.
5 mol H2O × (1 mol O2/2 mol H2O) = 2.5 mol O2
Answer: 2.5 mol O2 are needed to make 5 mol H2O, assuming H2 is in excess.
One lead atom, two nitrogen atoms, four oxygen atoms.
