•Chemical Reactions: is the process by which one or more substances change to produce one or more different substances.

Chemical Equation: a shorter, easier way to show a chemical reaction.

2H + O H2O

The arrow means “yields” or “makes”

Reactant – substance that enters into a chemical reaction (these are on the left of the arrow)

Product – the new substance (these are on the right of the arrow)

Ways to Identify a Chemical Reaction

1. Change in color

Examples: a)Rust on

iron b)bleach on

blue jeans

2. Forms a gas (you can see bubbles)

Example: vinegar and baking soda mixed together

forms bubbles.

3. Change in energyExothermic: reactions in which heat or light are released (container will feel hot)

Endothermic: reactions in which heat is absorbed (when heat is absorbed, the container will feel cold.)

4. Produces a precipitate

Example: Lead nitrate + potassium iodine yields lead iodide, which sinks to the bottom of the beaker.

The solid that appears in a solution during a chemical reaction is called a precipitate

A chemical reaction…• Is indicated by the formation of new

substances with properties that are different from those of the starting substances.

• Ex: Chlorine is a green, toxic gas and Sodium is a highly reactive, shiny metal. When they chemically combine to form Sodium Chloride, they form ordinary table salt with properties very different from the reactants (Chlorine & Sodium).

• If a substance releases bubbles, it may be undergoing a chemical reaction, but it is not for sure…

• A boiling pot of water releases bubbles, is that a chemical reaction?

• Sometimes, we may need to look for more evidence of a chemical change occuring.

Writing chemical formulas

• Elements are represented by symbols, right?

• Well, when we put elements together to form compounds, we can use a formula to represent the compound

• Ex: NaCl is the formula for sodium chloride

Subscripts• The subscripts show how many atoms

are in a chemical formula.

• H2O The 2 in this formula is a subscript, (written below the line) that tells us there are 2 Hydrogen atoms in this formula. How many oxygen atoms are in a water molecule?

How many Iron atoms are in rust, Fe2O3?

Law of conservation of mass

• When a reaction occurs, atoms are not destroyed, they are just rearranged.

• The law of conservation of mass says the amount of matter does not change in a reaction, and the total mass of the reactants must equal the total mass of the products.

• To demonstrate this law, we balance equations to show that atoms don’t

disappear or appear, they are simply rearranged.

Four types of Chemical

Reactions

Four types of Chemical

Reactions

1. Synthesis• 2 or more simple substances

combine to form a new complex substance

– Example: 2H + O H2O

2. Decomposition• A complex substance breaks down

into 2 or more simpler substances.

– Example: H20 2H + O

3. Single Replacement• An uncombined element replaces

an element that is part of a compound.

– Example: Zn + CuSO4 ZnSO4 + Cu

4. Double Replacement

• Different atoms in two different compounds replace each other.

Example: NaCl + 2AgNO3 2AgCl + NaNO3