Notes #1 Introduction to
Chemical Reactions
Name
Date________
Block____
Chemical reactions involve the rearrangement of atoms and ions to form new substances.
Chemical reactions involve chemical changes. Substances that exist before a chemical reaction
are called reactants. The substances that exist after the chemical reaction are called the
products.
There are a few criteria used to determine whether a chemical reaction has occurred. These
include the formation of one or more new substances (that did not exist before the reaction), an
energy change (example: energy that is released as heat and/or visible light, or energy that is
absorbed); a change in color.
A) A new substance has been formed.
1) a gas is formed but none of the reactants had been a gas; Ex.: you would observe bubbles
rising in a mixture of liquids or a vapor condensing in the cooler air above two liquids that
you had mixed or a vapor is observed above a heated solid that is releasing a gas;
2) a solid is formed but none of the reactants had been a solid; Ex.: you mix two clear colorless
liquids and an opaque yellow solution forms, and upon standing, a yellow solid settles to the
bottom of the test tube and a clear colorless liquid remains on top;
3) a liquid is formed but none of the reactants had been a liquid; Ex.: a solid is placed in a small
open dish and a liquid is observed to form around the solid; or two gases react and a liquid
forms.
B) There is an energy change.
1) no heat is added but the temperature of the reactants increases or decreasesas the reaction
progresses;
2) the reactants are not refrigerated but the temperature of the reactants decreases as the
reaction progresses;
3) visible light energy is given off by the reactants as the reaction progresses;
Be VERY careful here –often we raise the temperature of the reactants to start the reaction by
heating the reactants. (The particles of the reactants must have enough kinetic energy for the
reaction to occur.) The addition of heat to start a reaction is not the same as the heat given off by
the reaction once it has started. Ex.: you heat charcoal until it starts “burning” (that is, reacting
with the oxygen in the air) on its own; then you remove the “addition of heat” such as the match,
electric starter, etc. The reaction of the carbon with the oxygen in the air continues and releases
enough heat energy to keep the reaction going (until one of the reactants - charcoal or oxygen - is
used up) and sometimes visible light is also given off. If you did not “help” the reaction get
started, it might take many years for the reaction to go to completion to produce carbon dioxide
gas and water vapor. Any heat release from this “unaided” reaction would occur so slowly so
you would not be aware that the reaction was actually exothermic.
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C) There is a change in color.
You have only the colors of the visible light spectrum, the intermediate colors of the visible
spectrum and white, black, gray and silver (actually tints of black), gold, copper, brown, from
which to choose. SHADES of a color mean that the observed color is DARKER than the pure
color; TINTS of a color mean that the observed color is lighter than the pure color.
Names of colors that are used to describe substances are given below. If two colors are
hyphenated, the first color is the more dominant color observed.
red
red-orange
orange-red
orange
orange-yellow
yellow
yellow-green
green-yellow
green
green-blue
blue-green
blue
blue-violet
gray (usually dull)
violet-blue
silver (usually shiny)
violet
gold (usually shiny)
white
copper (usually shiny)
black
brown (includes: cream, tan, and beige)
Deeper or darker shades of red and lighter or paler tints of red are still red. You should always
include “dark” or “light” if the adjective applies to an observed color. “Deep” and “pale” refer
to the color intensity (also called color saturation). Deep is NOT the same as Dark; light is not
the same as pale. A dark red solution may appear to be a pale red if the original solution is
diluted by the addition of water or other colorless solvent, or if viewed in a much smaller amount
such only a few drops.
Only a change from one color listed above to another is an actual color change.
IF YOU HAVE DIFFICULTY DISTINGUISHING BETWEEN COLORS, ASK FOR HELP.
IF YOU ARE COLOR-BLIND, LET ME KNOW.
Remember: CLEAR means that you can see through it. CLEAR is NOT a color! True solutions
and some solids (such as gem-quality crystals) may have a color and still be clear.
NOTE: Changes in physical state (solid ↔ liquid ↔ gas) are not always indications of
chemical reactions.
Ice is water in the solid state H2O(s); water vapor is water in the gas state, H2O(g).
Bubbles may be observed rising from within a liquid if the liquid is being heated and:
a) there had been gas dissolved in the liquid (carbonated beverages contain CO2 gas dissolved in
water; tap water and water obtained from oceans, rivers, lakes, streams, etc., contain
dissolved O2 gas as well as other atmospheric gases);
b) the liquid itself is undergoing a change in physical state from liquid to gas at its boiling
point..
A gas may be said to be a new substance if the reactants were both solids and there
was no evidence of melting, or one or both of the reactants were liquids and bubbles
are observed rising from within the solution and there is no addition of heat.
Remember: you must be aware of the conditions under which the reactants are put
together.
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