When oxygen gas and hydrogen gas combine, they form water. What does this indicate about the properties of new?

New compounds, because of the way the atoms that make them up are bonded, show new (emergent) properties. The convalent bonds that hold the oxygen and hydrogen atoms together give the molecule new properties, much like the ionic bonds that hold chlorine (a green, poisonous gas) and sodium (an explosive silvery metal) together to form salt crystals.

Mixtures are just that - mixtures. The components are just thrown together. Compounds are when atoms are joined together by bonds to form a discrete entity. An example - a balloon filled with hydrogen and oxygen is just a mixture. Ignite it - bang - now it is water. It is a compound of hydrogen and oxygen.