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Observing Signs of a Chemical Reaction

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									Observing Signs of a Chemical
          Reaction




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Definitions

l   Physical Changes – do not alter the
    chemical composition of the components
    –   Example – change of state (boiling, evaporating,
        sublimation, condensing, etc), dissolving, etc.
l   Chemical Changes – do alter the chemical
    composition
Observing Signs of a Chemical
Reaction

Evidence that a chemical change has taken place
1.  Formation of a Precipitate – solid formed when 2
    liquids are added together
l   Some precipitates will be very obvious and sink to
    the bottom while other precipitates may only turn a
    clear solution cloudy
Observing a Chemical Reaction

2. Color Change
l Make sure that there is a definite color change
l If the color of a solution fades as more liquid is
   added – it is not a definite color change
l If 2 different colored solutions are mixed the color
   change can simply be the result of mixing
Yellow and Blue make green – not a chemical change
l Example Answer : Solution turned from colorless to
   yellow
Observing Signs of a Chemical
Reaction

3. Temperature Change
l All chemical changes result in a change in energy.
   The temperature changes indicate the direction of
   the energy – released or absorbed.

l   Sometimes the temperature changes are too small to
    be easily noticed by touch, then thermometers can
    be used to accurately measure temperature changes
    and to calculate the energy change
Observing Signs of a Chemical
Reaction

l   Exothermic Reactions give off energy and
    generally cause the surroundings to get
    hotter (feels hotter)

l   Endothermic Reactions require energy and
    generally cause the surroundings to become
    cooler (feels cooler)
Observing Signs of a Chemical
Reaction

4.Formation of a Gas

Bubbling and/or fizzing without the addition of heat
l Sometimes bubbles are slow to appear
l Do not confuse trapped air bubbles with the production of a gas
l Trapped air bubbles can be tapped away but the bubbles will
  reappear if a gas is being produced

Example Answer: bubbling occurred so gas given off
Observing Signs of a Chemical
Reaction

5. Odor Change
The production of a distinctly different odor
  than the original materials is an indication of
  a chemical change.

								
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