Physical Science
Period:
Name:
ANSWER KEY
Date:
Practice Test for Unit 3: Ch. 3, and some of 15 and 16: Kinetic Theory of Matter, States of matter,
and energy and thermodynamics, and gas laws.
1. The Kinetic Theory of Matter states that all matter is composed of atoms and molecules that
are in a constant state of constant random motion
2. What is Brownian motion, and why does it happen in terms of kinetic theory?
Small pieces of matter in a fluid – flecks of dust, pieces of pollen, smoke particles, etc. – seem
to vibrate randomly because they are being constantly hit from all sides by the molecules of the
fluid (water, air, etc.)
3. What is thermal expansion, and why does it happen in terms of kinetic theory?
Thermal expansion is the property of most matter that matter expands as heated and contracts as
cooled. All matter is in a constant state of motion, the faster they move, the more space they
need to move in and push the other molecules away. Thus as solids and liquids (phases of
matter with definite volumes) increase temperature, the molecules move more and take up more
space, so the substance increases in size.
4. What is diffusion, and why does it happen in terms of kinetic theory?
The quality of matter that substances in materials where atoms are free to flow (gases or
liquids), small particles put into the fluid tend to spread out randomly until they are evenly
mixed (go from areas of high concentration to low concentration). This happens because both
the fluid and the particles are constantly moving, so will be pushed and dragged around by other
molecules until they mix evenly.
5. According to the Kinetic Theory what does temperature measure? (don't say "heat")
The average kinetic energy of the atoms and molecules of a substance
6. How does a thermometer work?
When the molecules of the fluid change temperature, say upwards, they move more, this causes
them to take up more space (thermal expansion). The fluid in the thermometer has nowhere
else to go, so it rises up the tube. The reverse is true for falling temperatures
7. Why can nothing be cooled to below absolute zero?
At absolute zero all molecular motion stops. Since temperature is the average kinetic energy of
the atoms and molecules, if there is no motion, there is no kinetic energy. Something can’t
move at a slower speed than stopped.
8. How does the freezing point of a substance compare to the melting point of the same substance
in the same atmospheric conditions? Why is that?
The freezing point and melting point are the same. They are both changing between a solid and
a liquid, so it’s the same phase change in reverse. The temperature when the atoms have
enough energy such that any added energy will just make them flow past each other (melting)
must be the same as the temperature the just flowing liquid would need to such that any
removed energy would just lock the atoms in place rather than change their temperature
(freezing)
9. Consider the properties of solids, liquids, and gases we have covered in class.
a) Fill in the following table for the the common states of matter in terms of relative
intermolecular bond strength (small, medium, or large), particle movement (vibrating
only, flowing fast, or flowing slowly), particle organization (random or orderly), particle
spacing (in contact or not in contact), and type of shape and volume each has (variable
or definite).
State of Relative bond
Particle
Particle
Particle
Type of
Type of shape
mater
strength
movement
organization
spacing
volume
Solid
Large
Vibrate only
Orderly
In contact
Definite
Definite
Liquid
Medium
Flow slowly
Random
In contact
Variable
Definite
Gas
Small
Flowing fast
Random
Not in contact
Variable
Variable
b) Explain using the kinetic theory of matter.
Overview: The colder a particular substance is, the lower the speed it moves. The slower
the movement, the easier it is for the intermolecular forces to hold onto neighboring atoms
and molecules. The opposite is true for faster molecules, the faster they move the harder
they are to hold onto.
Detailed Explanation: Solids have slowed down enough that the neighboring molecules can
pull harder, keep them in contact, lock them in position (only allowing vibration) and since
they’re locked in position they have an orderly arrangement, and thus cannot change shape
or volume.
Liquids have heated up enough to space a little further apart and partially break free of their
neighboring molecules, but not enough to let them fully escape (they are still in contact –
like holding someone at arm’s length), because of the further spacing and weaker hold, they
are able to flow and thus take a random arrangement, but since they are still touching
(essentially at arm’s length) they remain in contact, so the volume doesn’t change, but the
shape can as the molecules flow.
Gases have added enough energy to fully break the attractive bonds of neighboring
molecules, thus the gases can go anywhere they want. Hence gases are spaced much further
apart, so far in fact that the bond is very weak, considered nonresistant in ideal cases.
Because of the not touching and much weaker hold, they are able to flow anywhere and are
randomly arranged, ad can take any shape or
10. Draw the phase change triangle, listing the 3 common phases of matter, naming the transitions
between them, if the changes are endothermic or exothermic changes and why that makes since.
Dashed  endo
Deposition
Solid  exo
Condensation
Gas
Vaporization
Sublimation
Makes since because solids have
the least energy while gases have
Solid
the most, so to change from a solid
Liquid
into either a liquid or gas, you need
Melting
to add energy. Likewise, changing
anything into a gas will also take energy. So the opposite is true for going into a solid or from a gas.
Freezing
11. Consider evaporation and boiling.
a) What are two main differences between evaporation and boiling?
Evaporation only occurs at the surface of a liquid, whereas boiling occurs throughout the
liquid. Evaporation occurs at temperatures below the boiling point of the substance,
whereas boiling occurs at the boiling point.
b) How is it similar to boiling?
Both evaporation and boiling are forms of vaporization, where a liquid turns into a gas
(the high energy particles leave to become gas, leaving the lower energy particles
behind)
12. What is the first law of thermodynamics, and what would that limit about a bouncy ball that is
dropped from a certain height? Why is that?
Energy cannot be created or destroyed, it can only transfer forms. If you drop (not throw) a
bouncy ball, the 1st law would say the total energy when you drop it and when it comes back up
to its highest point must be the same. So, the ball cannot bouncy higher than the distance it was
dropped from.
13. What is the second law of thermodynamics, and what would that limit about a bouncy ball that
is dropped from a certain height? Why is that?
All things tend to progress more towards disorder than to order. As a consequence, some
energy is always lost to thermal energy during an energy transformation. As such, the total
energy of the ball and area system when dropped must equal the total energy at the highest
point in the bounce. Because some of the energy is transferred to heat, there is less for
gravitational potential energy, so the ball will never bounce as high as it was dropped from (and
each subsequent bounce will be lower).
14. All forms of energy can be divided into one of two main categories of energies, what are those
two main energy types. Give at least three examples of forms of energy in each of the two
types.
Kinetic Energy (energy due to motion):
Potential Energy (stored energy):
 Transformational KE
 Elastic potential energy
 Solar (kinetic) energy
 Gravitational potential energy
 Sound (kinetic) energy
 Nuclear potential energy
 Electrical (kinetic) energy
 Chemical potential energy
 Thermal (kinetic) energy
 Electromagnetic potential energy
15. What happens to the temperature of a substance during a phase change? Why is that?
The temperature doesn’t change, because all the energy gained or lost by the substance just goes
to changing the state of matter, not the temperature (the energy gained breaks bonds of attraction
between neighboring molecules instead of changing their speed, or the energy lost goes to
forming bonds of attraction between neighboring molecules instead of changing their speed)
16. Draw a sketch of the temperature vs. energy graph for ice at -20ºC being heated to steam at 120ºC.
Label where the H2O is 1) a solid, liquid, or gas, 2) taking in or releasing energy, 3) going through
a phase change and what change that is called.
Temperature (C)
Temperature vs Energy
140 Melting & taking Vaporization &
taking in energy
in energy
120
100
80
60
40
Liquid & taking in
energy
20
0
-20
Solid & taking in
-40 energy
Gas and taking in
energy
17. What causes gas pressure (not what affects it, but what causes it in the first place)?
Collisions between the gas particles and other particles, the sides of the container, and any
objects inside the gas are what cause the gas pressure.
18. What happens to the pressure of a gas in a closed container if the volume decreases, but
temperature remains the same? Why?
Pressure increases (more collisions happen with walls of container if they are closer together)
19. What happens to the volume of a gas in a closed container if the temperature increases, but the
pressure remains the same? Why?
Volume increases (atoms hit the walls harder so push it outwards more until pressure is
equalized)
20. What happens to the temperature of a gas in a closed container if the pressure decreases but the
volume stays the same? Why?
Temperature decreases (atoms are moving slower so they hit the walls not as hard)
21. What happens to the volume of a gas in a closed container if the pressure decreases but the
temperature remains the same? Why?
Volume increases (particles are moving the same speed but don’t hit the walls as often when the
volume increases)
22. What happens to the pressure of a gas in a closed container if the speed of the gas increases, but
the volume and number of gas particles remains the same? Why?
Pressure increases, this is because they hit each other and the walls of the container harder.
23. What happens to the pressure of a gas in a closed container if the original gas was replaced by a
new gas that has more massive gas particles, but maintains the same number and speed of gas
particles as the original? Why?
Pressure increases, this is because since they are larger, they hit each other and the walls of the
container with more force than lighter particles when hitting at the same speed (their KE is
related to their mass and speed, with the same speed, a larger mass means more KE to give).
24. Consider 0°C water and the same mass of 0°C ice
a) What has more internal energy, the water or the ice? Why is that?
0°C water has more internal energy because melting is an endothermic process (the
molecules had to gain energy to partially break the attractive bonds)
b) What has more average kinetic energy the water or the ice? Why is that?
Temperature is related to the average kinetic energy, since they both have the same
temperature they must also have the same average kinetic energy.
c) What has more stored energy, the water or the ice? Why is that?
Since the water has more overall energy, but the same amount of average KE, the stored
energy (PE) must be greater for the water. Think of it as lifting a block up on Earth the
further they are lifted against the gravitational force, the more energy is stored, the same
goes for the further the molecules are moved away from each other against the forces of
attraction between them.
25. How if at all would your answers to question 24 change if we were considering 100°C water
and 100°C steam instead of ice and water.
The only difference would be the words liquid would change to gas and solid would change to
liquid, the idea is exactly the same – Steam has more internal energy, the same average KE, and
more PE than water at the boiling point.
26. Which is usually larger, the amount of energy to turn a solid into a liquid at the m.p., or the
amount of energy to turn the same amount (in terms of mass) of a liquid into a gas at the b.p.?
Why is that?
Energy needed to change the liquid in to a gas at the m.p. is typically greater, because it takes
way more energy to fully separate the molecules from each other to form a gas than it does just
to get the molecules able to move past each other when going from a solid to a liquid.
27. Below is Temperature vs. Time graph of a certain substance being heated and then cooled.
Assume the same amount of energy is added or removed each second.
160
Gas and taking in
energy
140
Gas and releasing
energy
Condensation &
releasing energy
120
Temperature (degrees C)
100
Vaporization &
taking in energy
liquid & releasing
energy
80
60
Melting & taking
in energy
Liquid & taking in
energy
40
freezing &
releasing energy
20
0
0
100
200
300
400
500
600
700
800
900
1000
-20
-40
Solid & taking in
energy
solid & releasing
energy
-60
time (s)
a) On the graph, label where, if anywhere, the substance is: 1) solid, liquid, or gas; 2)
melting, freezing, vaporizing, or condensing; 3) taking in energy or releasing energy
b) What are the melting, boiling, and freezing temperature of the substance. How do you
know?
i. Melting point =
-20°C, because: temperature doesn’t change during a
phase change, s the temperature is constant there as all energy is going to turn
the substance from a solid into a liquid
ii. Freezing point =
-20°C, because: : temperature doesn’t change during a
phase change, and the freezing point is the same temperature as the melting
point.
iii. Boiling point =
130°C, because: temperature doesn’t change during a
phase change, s the temperature is constant there as all energy is going to turn
the substance from a liquid into a gas
c) At what time was the substance removed from the heat?
450s How do you know?
That’s when the temperature starts to drop again, meaning it was releasing energy rather
than gaining energy.
d) What took a longer time, the object to melt or the object to vaporize? vaporize
Assuming the same amount of energy is added each second, why is that? It takes more
energy to fully separate the molecules rather than just to partially break the bonds.
28. Use the information below to answer questions A - H
Substance
Melting Point
Boiling Point
Water
0
100
Iron
1535
2750
Mercury
-38.8
356
Lead
327
1751
Ammonia
-78
-33
Ethyl acetylene -126
8
a) What temperature scale is being used here? How do you know?
Celsius, because the melting point and boiling point of water match those of the Celsius
scale.
b) At what temperature would ammonia change from a gas to a liquid? How do you know?
Gas to a liquid is condensation, which is the reverse of vaporization, so the highest
temperature it will occur at is -33°C
c) In what phase is iron at a temperature of 2600 °C? How do you know?
A liquid, because 2600°C is above the melting point (already melted) and below the boiling
point (not yet boiled)
d) At what temperature (in Kelvin) will lead melt? How do you know?
Lead melts at 327°C and to convert to K, we need to add 273, so the melting point is 600K
e) Is it possible to boil mercury in a lead container? Why?
No, the melting point of lead is less than the boiling point of mercury, so the lead container
will melt before boiling the mercury.
f) What, if anything, happens to iron when the temperature drops from 1800o to 1500?
Why is that?
The iron freezes into a solid, because it’s a liquid at 1800°C (higher than the melting point
of 1535°C) and lower than the boiling point, but 1500°C is less than the melting point of
iron, so it will be a solid again.
g) At what temperature does lead freeze? How do you know?
327°C because the freezing and melting points are the same temperature.
h) Which of the substances would be a solid, which would be a liquid, and which would be
a gas in a standard refrigerator (temp of about 3.3 °C)? How do you know?
Solids (3.3°C must be below the melting point): iron and lead
Liquids (3.3 °C must be more than melting point but less than boiling pt): water, mercury
and Ethyl acetylene
Gas (3.3°C must be above the boiling point): ammonia
29. Circle the two temperatures below that are not possible?
7,000,000 K
- 25 K
-400 °F
-279 °C
5,000,000 °C
30. Why are those temperatures circled in #29 not possible?
The lowest temperature possible is absolute 0 (-273°C, 0K, or -459.7°F), so there can be no
temperatures less than those.
31. Why is it much better to understand a process or concept than to memorize different aspects?
If you understand, you can figure things out based on your knowledge, if you memorize, you are
depending on one of the answers always being something you memorized, and it is very easy to
memorize things wrong.