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  H 2 O 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 energy Exothermic: 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. H 2 O 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, Fe 2 O 3 ?

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

1.Synthesis 2 or more simple substances combine to form a new complex substance –Example: 2H + O H 2 O

2. Decomposition A complex substance breaks down into 2 or more simpler substances. –Example: H 2 0 2H + O

3.Single Replacement An uncombined element replaces an element that is part of a compound. –Example: Zn + CuSO 4 ZnSO 4 + Cu

4. Double Replacement Different atoms in two different compounds replace each other. Example: NaCl + 2AgNO 3 2AgCl + NaNO 3