Anything that's made up of just one kind of molecule:
water, a gold ring (24 carrot of course).
A chemical element is pure.
99,99999 % purity
Yes - but commercially always contains traces of impurities.
Bleach is a mixture, typically made up of sodium hypochlorite and water. It is not a pure substance because it contains more than one type of chemical component.
A pure substance contains only one kind of matter. It cannot be separated into other substances by physical means.
compound
energy
Temperature.
Calcium
chemical change
the product(s)
Generally, the carbohydrates.