1. Compounds What is a compound?

2. Compounds A compound is a substance made of two or more elements. Water is made of hydrogen and oxygen. A compound always has the same ratio (a fixed ratio) of elements. Water always has two atoms of hydrogen for each atom of oxygen. The elements are not just mixed together—they are combined through a chemical reaction.

3. Writing Compounds Some compounds, like water and sugar, have there own names. Most do not. All compounds have names that tell what they are made of. Salt is made of a sodium ion and a chloride ion, so we call it sodium chloride. Often, we will just write the symbols. NaCl If a compound needs more than one of an atom, sometimes its name can tell us. Carbon dioxide has one carbon atom and two oxygen atoms.

4. Writing Compounds When we write the symbols the number of atoms needed goes after the symbol as a subscript (below the line). Carbon dioxide is written CO 2. The way we write down the symbols for a compound tells us how it is made, so we call it the chemical formula for that compound. Carbon dioxide: CO 2 Water: H 2 O Sodium chloride (salt): NaCl Hydroxide: OH - Glucose: (sugar) C 6 H 12 O 6 Ammonia: NH 3 Iron oxide (rust): Fe 2 O 3 Hydrochloric acid: HCl Octane (in petrol) C 8 H 18 Ethanol (alcohol) C 2 H 6 O Methane (natural gas) CH 4 Propane (cooking gas) C 3 H 8

5. Molecules Molecules are groups of atoms. Since compounds are made of more than one element, the smallest piece of a compound is a molecule. Some non-metals form molecules made of only that element.

6. Chemical Reactions Chemical reaction happen when two or more atoms come together to form a new substance. Oxidation is one of the most common chemical reactions. A burning piece of wood, a piece of apple turning brown, and a piece of iron rusting are all examples of oxidation.

7. Writing Chemical Reactions When we write down a chemical reaction, we write the substances we start with (the reactants) on the left, with “+” between each. Usually we write only the chemical symbols. The substances we finish with (the products) go on the right, with plusses in between. The reactants and products are separated by a “→”. We say “yields” when we read the reaction. Na + + Cl - → NaCl Sodium plus chloride yields sodium chloride (salt).

8. Writing Chemical Reactions Anytime we write a chemical formula, the number of atoms of an element is written as a subscript after the symbol for that element. NaOH + HCl → NaCl + H 2 O Sodium hydroxide plus hydrochloric acid yields sodium chloride (salt) and water.

9. Writing Chemical Reactions When we write a chemical reaction, it is like writing the recipe for a cake. We must tell how much of each thing we need. We write the number of atoms or molecules of each substance in front of the symbol, unless we only need one of that atom or molecule. Ca 2+ + 2F - → CaF 2 Calcium plus two fluoride ions yields calcium fluoride.

10. Writing Chemical Reactions Sometimes we want to keep track of certain molecules even after the chemical reaction. We do this by putting parentheses “( )” around the formula for that molecule. Ca 2+ + 2(OH - ) → Ca(OH) 2 Calcium plus two hydroxide ions yields calcium hydroxide.

11. Conservation of Mass Matter can not be create or destroyed in a chemical reaction. When a chemical reaction happens there is no change in mass. The mass of all the reactants equals the mass of all the products. 2g H 2 + 16g O 2 → 18g H 2 O 40g NaOH + 36g HCl → 58g NaCl + 18g H 2 O