8.P.1.4

Students know:

· that when materials react with each other, many changes can take place, but that in every case the total amount of matter afterward is the same as before.

· that a balanced chemical equation supports the law of conservation of matter.

· how to identify examples that support the law of conservation of matter and can explain the conservation of matter using the idea of atoms. In chemical reactions, the number of atoms stays the same no matter how they are arranged, and the mass of atoms does not change significantly in chemical reactions, so their total mass stays the same.

Example: Hydrogen gas (H₂) reacts with oxygen gas (O₂) to form water (H₂O).

One molecule of hydrogen has two atoms and one molecule of oxygen has two atoms. A molecule of water has one oxygen and two hydrogen atoms. So, the two hydrogen atoms bond with one of the oxygen atoms to form a water molecule. There is an atom of oxygen left over, so another hydrogen molecule is needed in order to make a second water molecule.

The balanced chemical equation would be 2H₂ + O₂ à 2H₂0. Two molecules of water are formed from four hydrogen and two oxygen atoms.