Physical and chemical changes

Chemical and Physical changes can come in many forms.

One might be ice cubes melting, propane being lit ablaze to cook, or iron chains rusting.

So if we wanted to write the ice cubes melting in the chemical form you could write it as H2O(s) > H2O(l), you would see the water molecules change, this is considered a Physical change, along with boiling it into vapour, that is also a physical change. 

However, a general rule of thumb is that when you are talking about appearance or overcoming intermolecular change, it becomes a physical change.

However, chemical bonds either mean that molecules are breaking or forming.

Propane is burnt and turned into fire to light stoves, and when propane is lit on fire, it starts to form molecules of carbon dioxide.

Chemical Reactions happen when bonds are formed are broken between atoms, for water 2H2O, one of the 2 is molecules of molecular Hydrogen and the other 2 are telling us that for the O2 reaction, you need 2 Hydrogen.  

Molecular oxygen and molecular Hydrogen are extremely reactive, and they are even used in rocket fuel because of how reactive they are.
And if we use the 2H2O it makes 2 molecules of water, combining them all together you get the H2O to have 4 molecules of Hydrogen and 2 Oxygen molecules.

The reason why these two molecules combine is because they have lots of energy, and they bounce around a lot to the point they collide with other molecules and leave their old bonds to make new bonds.

But since a 2H2O molecule is a combination of two molecules it is considered a Compound. 

All Chemistry follows a sort of formula, Ractiants and then the Products, starting off with the formula:

Ca(s) + Ag2SO4(aq)      ->          2Ag(s) + CuSO4(aq).

    Reactants                                 Products

Coeffeciants are the large numbers that come before a Chemical Formula for example the element above is Ag so a 2Ag means that 2 silver atoms are needed.
Subscripts are the smaller numbers that indicate there are multiple instances of the same element within a substance, in the example above, two silver atoms are inside the silver sulfate compound.

the (aq), (l), s() (g), mean what state of matter, the (l) means liquid, (s) means solid, (g) for gas and finally (aq) means aqueous which dissolves in water.