Investigating Temperature and Density
A Yakima WATERS Mini Lesson
Targets and Assessment
WA Science Standards Addressed:
 6-8 INQA: Scientific inquiry involves asking
and answering questions.
 6-8 PS3C – Heat consists of random motion
and vibrations of atoms and molecules. The
higher the temperature, the greater the
atomic or molecular motion.
Assessments:
 Formative – students are given three
statements about density and must choose
which they agree with.
 Summative – students write an explanation
of why density changes with temperature on
a molecular level.
Lesson Parameters
Content Area: Physical Science, Properties of Matter,
Chemistry
Overview: Students create a density column out of hot,
cold, and room temperature water.
Grade Level: 8th grade
Suggested Time: one 50 min class period
Special Materials:
 food coloring
 hot pot
 See details in Materials Section
*NOTE TO TEACHER/INSTRUCTOR: This activity is a modified version of a lesson from the American Chemical
Society found here: http://www.middleschoolchemistry.com/lessonplans/chapter3/lesson6
LEARNING OUTCOMES:
Knowledge:
 Student will be able to explain how and why molecules are more spread out at higher temperatures
and closer together at lower temperatures.
 Student will be able to explain that the way molecules respond to changes in temperature affects the
substance’s volume, which in turn causes a decrease in density (higher temperature) or an increase in
density (lower temperature).
SCIENCE CONCEPT BACKGROUND:
Different substances have different densities, but the same substance may also have different densities at
different temperatures. Remember that temperature is the measure of the average kinetic energy of the
molecules in a substance. When a substance is heated, the molecules move more rapidly. Conversely,
molecules move more slowly when a substance is cooled. This translates to an increase in volume when a
substance is heated and a decrease in volume when a substance is cooled. Since the mass of the heated or
cooled substance is the same as the room temperature mass (and only volume is changing), the effect of
heating or cooling a substance is a decrease or increase in its density, respectively.
This lesson builds on pre-existing knowledge about density. Before this lesson, students should know that all
substances have mass, and therefore density. They should know that density = mass / volume. They should
also know that less dense substances float on more dense substances.
MATERIALS:
Per lab group:
 pipette
 ice cold water with blue food coloring
 hot water with yellow food coloring
 room temperature water
 clear plastic cup
PROCEDURE:
Engagement & Formative Assessment
Engage students with a formative assessment. Put three statements about density on the board or overhead projector
and ask students to (1) write down which one they agree with and (2) explain why. See attached PowerPoint slide for an
example of this assessment. The purpose of this formative assessment is to check students’ knowledge about density so
this lesson may build on that prior knowledge. Thus, the statements in the assessment should touch on what students’
have learned about density from previous lessons. In this example, these are:
(1) All substances have mass (even air!) and density.
(2) Density = mass / volume
(3) Less dense substances are more buoyant (and float) compared to more dense substances.
When students have chosen, have them crumple up their papers and throw them at a neighbor. This makes
the next step anonymous. Now (after quieting down the class), ask several students to read what is on their
paper. This will give the instructor an idea of students’ knowledge moving into the lesson. If many students
choose the wrong answer, it may be necessary to re-teach certain concepts. Otherwise, it is fine to briefly
review the concepts in the assessment and move forward.
Activity Directions
1) Students are given the following lab procedure (steps 2-5 below) to read and become familiar with.
2) Students are asked to write a hypothesis explaining what will happen when the hot and cold water are
added and why.
3) Once a hypothesis is written, students in groups of 1-4 should collect the pre-prepared lab materials
including ice cold water with blue food coloring, boiling hot water with yellow food coloring, room
temperature water in a clear, plastic cup, and pipette.
4) Students add a few drops of blue cold water to the room temperature water in a clear, plastic cup. Water
should be added at mid-level in the cup (not at the top, not at the bottom). Students record observations.
5) Students add a few drops of yellow hot water to the room temperature water in the same clear, plastic cup.
Water should be added at mid-level in the cup (not at the top, not at the bottom). Students record
observations.
6) Students should observe that the yellow hot water rises and the cold blue water sinks. Students have in
effect created a density column. Remind students that this is different from previous density columns,
however, because this density column only contains one substance – water. The only manipulated variable in
this case is temperature.
Wrap Up
At this point the instructor should address students with regard to the last step above. Explain that this
density column is different from previous density columns because there is only one substance involved.
Point out that this necessitates some new explanation, which provides a nice segue to lecture-style instruction
on the board or overhead projector.
Explain how temperature affects substances at a molecular level. Start by explaining that temperature is a
measure of average kinetic energy, or how quickly the molecules are moving around. At higher temperatures,
molecules move around more quickly and become more spread out. This means an increase in volume, which
means a decrease in density (mass stays the same). At lower temperatures, molecules move around less
quickly and become more packed together. This means a decrease in volume, which means an increase in
density (again, mass stays the same).
Summative Assessment:
Have students re-examine their hypotheses and write new explanations of how and why temperature affects
density with reference to what is happening on a molecular level. The best answers will be well-written
explanations of how temperature affects how quickly molecules move, that this affects the volume of a
substance, and that the change in volume causes a change in density. These best explanations also link higher
temperatures to lower densities and vice versa. Good answers are those that make the correct connection
between high temperatures and low densities but fail to explain why on a molecular level. Poor answers do
not convey any of the above information or connect temperature and density incorrectly.
EXTENSION(S):
After 5 or 10 minutes, have students re-examine their density columns. Likely, the water has mixed and is all
roughly room temperature. The result is a green, homogenously mixed solution rather than a density column.
Ask students to explain why the water is now green instead of separated blue/ yellow.
There are also excellent videos on the website (included below) for this lesson that reinforce the explored
concepts.
TEACHING TIPS:
Not every density column will turn out right because some students add too much hot or cold water or they
add it at the wrong height in the cup. If students do not make a proper density column, have them repeat the
experiment or observe a neighbor’s cup.
SOURCES:
Attached documents: formative_assessment.ppt
For the original ACS lesson plan from which this one is modified:
http://www.middleschoolchemistry.com/lessonplans/chapter3/lesson6
For extension videos:
http://www.middleschoolchemistry.com/multimedia/chapter3/lesson6#hot_water_on_cold_water
Author: Andy (James) Menking, Yakima WATERS Project, CWU, Fall 2011