1. Compounds
A compound is a pure substance composed of two or more ________ that are _________ combined.
elements
chemically
A compound has different properties than the ________ that make it up.
elements
Magnesium oxide is very different from the elements magnesium or oxygen, which make up MgO.
Example:

2. Compounds form from ________ reactions. chemical
The element sodium (solid metal) reacts violently with the element chlorine (nonmetal gas)
Example:
to produce the compound sodium chloride (table salt – a solid compound.

3. Some compounds can be broken down into the elements that make them up.
Many times heat energy or electric current is applied to the compound to break it down.
Heating mercury (II) oxide breaks it down into the elements mercury and oxygen.
Examples:

4. An electric current goes through water and breaks it down into hydrogen and oxygen gas.
This is called electrolysis.

5. Most substances in the World are __________.
compounds
Food – sugars, proteins, and fats
Fabric for clothes
Carbon dioxide
Silicon dioxide
All kinds of plastics