Melting point of a substance is what kind of property?

for the first question, Hypochloride is being released from your bleach to oxidize your fabric. As you know oxidation simply mean addition of oxygen, so when hypochloride (ClO-) comes in contact with fabric oxygen is added and some organic bonds are broken and the dirt is washed out.

For the second question. The melting and boiling point of pure substance is sharp. Like water has 100oC boiling point and OoC melting point. So anything from that means it has impurities in it. Just like if you make tea, you have introduce something else into the water, if you should try boiling your tea, you'll find out that the boiling point is not going to 100oC anymore. Every other liquid have their boiling point and melting point. When you know them, you'll be able to determine the purity.