Download Physical and Chemical Changes

Physical and
Chemical
Changes
Let’s Review:
In A Physical Change....
• A substance is changed physically,
but not chemically. It is still the
same substance.
• Ex: Ice melts into water. It is still H2O, just in a
different physical form.
• It changes shape or size
• It dissolves.
• It changes state of matter (freezes, melts,
evaporates, condenses)
In A Chemical
Change....
• A substance’s chemical formula
is changed and the substance is
changed into something
completely new.
• It no longer has the physical or chemical
properties it had before.
Examples of what I
mean….
SIGNS OF A
CHEMICAL
CHANGE
It’s a chemical
change if...
• A new substance
is formed
• If the combination of two
substances bubbles,
(makes a gas) it’s a
chemical change
• The gas is a new
substance.
For Example....
Example: Alka-Seltzer
Place some water in a test tube.
Put an Alka-Seltzer tablet into the
water and IMMEDIATELY place a
balloon over the test tube
What Happened?
What did you observe happening?
Was a new substance created?
How do you know?
Oxygen was created
The chemical reaction that just
occurred was:
C6H8O7 + 3NaHCO3 → 3H2O + 3CO2 + Na3C6H5O7
citric acid + baking soda → water + carbon dioxide + sodium citrate
carbon dioxide is released as a gas.
Carbon Dioxide was one of the new
substances being created.
It’s a chemical Change if....
An explosion occurs.
In a chemical reaction, gas is formed as a new
substance. Sometimes, the gas that forms is
highly combustible and an explosion occurs! An
explosion is evidence a chemical reaction did
occur.
Calcium Carbide explosions
It’s a chemical change if....
A precipitate is formed.
A precipitate is a solid that
forms and separates out
from a liquid mixture.
This goes along with the “a
new substance is formed”
idea.
For example...
Limewater reaction:
Using a straw, blow a couple of breaths into
the limewater solution. What happens?
Was there a chemical
change?
Which caused a chemical change? Air or your
breath?
Breath reacted with limewater, but room air
didn’t. What do you think is different about
exhaled air that caused a chemical change?
Here is the reaction:
Ca(OH)2 + CO2 → CaCO3 + H2O
It’s a chemical
change if...
• There is a color
change.
• A color change
often indicates a
new substance was
formed. (remember
that it is possible to have
a color change in a
physical change too!)
For Example....
Add 3ml of the sodium iodate solution (solution
A) to a test tube.
Add 3ml of the sodium sulfate, citric acid and
starch solution (solution B) to the test tube. Swirl
the test tube around.
DON’T TAKE YOUR EYES
OFF YOUR TEST TUBE!!!!!!
AND THERE YOU HAVE A
COLOR CHANGE!
Now add 1-2 crystals of sodium thiosulfate to
your test tube and swirl it around.
Did another chemical reaction just occur?
How do you know?
It’s a chemical change if...
• Temperature changes without
YOU heating or cooling the
substances. (Without you
putting it in ice or on a hot
plate)
• The substances reacting with
each other cause the
temperature to change.
Whenever chemical reactions occur, energy
is transferred to or from the surroundings.
An exothermic reaction is one which
transfers heat energy to the surroundings
An endothermic reaction is one which takes
heat energy from the surroundings
Exothermic
Exothermic reactions feel warm or hot because
they are releasing heat or light.
Example: Hand warmers are an exothermic
reaction.
Example: Lighting a match is an exothermic
reaction.
Exothermic reaction
Endothermic reaction
Endothermic reactions feel cold because
they are taking heat away from your
body.
Example: Vinegar and Sodium
bicarbonate
It’s a chemical
change if….
•It burns
• Example: A match burning
• Water burning
Physical or
Chemical change?
• Gas is created?
• Chemical change
• Mixing salt and water?
• Physical change
• Rust
• Chemical change
What kind of change
is it if you..
someone...
•Burn paper?
•Chemical change
•Evaporate salt water?
•Physical change