# Melting Ice: Designing an Experiment

Suggested Grade Level(s): 8-12

Estimated class time: one 45-minute class period

# Summary

This lesson uses a discrepant event to help students realize that a
carefully designed experiment may yield unexpected results, due to
unseen events, even though the experiment is precisely planned and
executed. The addition of a new technology may clarify factors in the
experiment which were previously unknown.

# Objectives

-   Students will design an experiment to determine whether ice melts
    faster in pure water or salt water

-   Students will understand that the design of an experiment can affect
    the outcome

-   Students will understand that new technology can reveal processes
    that were previously unseen

# National Science Standards

-   NS.9-12.1 SCIENCE AS INQUIRY

> As a result of activities in grades 9-12, all students should develop

-   Abilities necessary to do scientific inquiry

-   Understandings about scientific inquiry

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-   NS.9-12.4 EARTH AND SPACE SCIENCE

As a result of their activities in grades 9-12, all students should
develop understanding of

-   Origin and evolution of the earth system

-   Origin and evolution of the universe

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-   NS.9-12.7 HISTORY AND NATURE OF SCIENCE

As a result of activities in grades 9-12, all students should develop
understanding of

-   Science as a human endeavor

-   Nature of scientific knowledge

-   Historical perspectives

# Knowledge Prerequisite

It is helpful if students have had experience with winter conditions of
snow and ice on pavement and the use of de-icing agents such as salt to
remove the ice.

# Materials

-   Clear plastic party cups with smooth sides (best for observation)

-   Ice cubes of uniform size

-   Bucket of fresh tap water at room temperature

-   Bucket of very salty water at room temperature

-   Graduated cylinders at least 100 ml

-   Clock

# Preparation

The day before the experiment prepare one large bucket of plain tap
water and another bucket of warm tap water with **as much salt dissolved
in it as possible**. Lab grade salt is nice, but expensive and not
really necessary. Un-iodized salt from the grocery is fine, but the
water will be a little cloudy due to non-clumping additive. If the
students ask why one container is not clear, explain the additive and
that it is not likely to have an effect on the outcome. **Allow both
buckets to sit in the classroom overnight so that they are the same
temperature and at equilibrium with the air.** Check the temperature of
the buckets before you begin the experiment.

Use ice cubes from a refrigerator icemaker if possible because they are
very uniform. If you make them in ice cube trays, each cube should be
only about1/2 to ¾ of the compartment. Use a syringe to fill them if
necessary to make them uniform. (Cubes made from a full tray will not be
melted in a class period and the students will need to make a judgment
about which is melting faster. Using smaller cubes will result in one
cube being totally gone before the other and the results won't be as
fuzzy. A cube containing about 15ml of water is about right. They do not
need to be "cubes" but they should be as uniform as possible, whatever
the actual shape.)

Students will work in teams of two. Each team has two ice cubes and two
cups, one with **100ml** of fresh water and one with **100ml** of salt
water. Ask them to watch very closely and make careful observations of
the time they begin and as the ice is melting.

# Procedure:

As scientists search deeper into the origin of the universe and the
secrets of galaxies in deep space they become more and more dependent on
remote technology to collect data. The outcome of the experiment is
dependent on the ability of the experimental design to answer the actual
question it is asking. Sometimes the design itself can influence the
result, even when the experimenter makes every effort to remain
unbiased.

The precision and accuracy of the sensors themselves can determine the
result of an experiment. The original detection of the cosmic microwave
background seemed extremely uniform throughout the universe. The
addition of NASA's COBE satellite, launched in 1989, showed that there
were slight variations, tiny lumps, in this earliest remnant of the Big
Bang.

### I. Engagement 

Have your students imagine that it is a snowy winter day and the highway
department is working hard to clear the roads. There have been few "snow
days" this year and they find themselves wishing the highway guys were
not quite so efficient. Sensing that the class is less than enthusiastic
for school today, you decide to begin with an easy question: *Will an
ice cube melt faster in fresh water or in salt water?*

(**Teacher note**: Most students who have experienced icy winter
pavements will expect the salty water to melt the ice faster because
salt is used on sidewalks to melt ice. Pouring fresh water on an icy
pavement would just make it even icier.)

### II. Exploration

(**Teacher note**: This activity has more "punch" if it is introduced
without the summary and objectives listed above. It is also good to do
this activity during the winter when snow removal and icy pavements are
fresh in everyone's mind.)

The students should be told something like this "The best science
experiments are carefully designed to be a fair test. We try to be sure
that we do not bias the outcome by doing something which gives one side
or the other an edge. Today we are going to design an experiment and we
will ask a question which most of you can probably answer already.
*[Will an ice cube melt faster in fresh water or in salt
water?"]{.underline}*

Ask each student to write the question on her paper and then to write a
Hypothesis that explains what she thinks will happen and why.

Begin a list on the board of the things that will be necessary to insure
that the experiment is designed as a "Fair test". Students will suggest
using the same size containers, same shape and volume of ice cube, same
temperature and volume of melting water, same environmental conditions
in the room etc. When they have volunteered as many factors as they can,
ask if they should stir the melting cubes or not? Mention that stirring
would be hard to make exactly the same in both containers and wait for
them to tell you that neither container should be stirred. It **is very
important that neither container be stirred during the experiment!**

It will become obvious very shortly that the cube in fresh water is
melting much faster than the expected one in salt water. If one of the
cubes rolls over, don't disturb it, but continue making observations.
Most students do not watch very closely, but at least remind them
several times. Very careful observation indicates the motion of density
currents in the fresh water container, but if they don't see them, don't
mention it yet.

Remind the students that it is very important to label and document all
samples, to measure carefully and follow all of the steps for a "fair
test". They will assure you that they have done everything correctly and
it is still not coming out "right". Now is a good time to be very
dramatic about precise and careful procedure. When the cube in fresh
water has completely melted, the one in the salt water should still be
fairly large. Adjust the size of your ice cubes to fit your class
period. Have them clean up and put away materials, but do not discuss
the results until later. Ask them to write if they will be accepting or
rejecting their original hypothesis and why.)

### III. Explanation

Have the students repeat the experiment just as they did before, but
this time add five drops of food coloring to the top of each melting ice
cube. Ask them write down any new observations which are different from
those your have already recorded.

(**Teacher notes**: The next class period, set up two new containers
which are large enough for the students to see from their seats .Place a
thermometer in each container and record the temperature. Do not remove
the thermometer. Add the ice cubes as before.)

Explain that sometimes the same experiment done with the addition of a
new technology reveals something happening which was not apparent
before. Scientists do the best they can to collect data with the tools
and abilities they bring to the problem, but are always looking for
additional ways to find out what is really going on. After a few
minutes, record the temperature on the thermometers. The bulbs of the
thermometers are sitting on the bottoms of the containers and there is
beginning to be a difference between them. The thermometer in the fresh
water shows a decreasing temperature, but the one in salt water remains
about the same.

Have students carefully add 5 drops of food coloring to the top of each
melting ice cube without disturbing the container. The food coloring
will remain with the melt water as it flows off of the ice cube. Cold
fresh water is denser than the freshwater in the cup, but it is less
dense than the salty brine in the other cup. The melt water in the fresh
water cup is sinking to the bottom of the container. The cube in the
fresh water is always bathed in the warmer water of the cup while the
cold water from the melting cube sinks away from it. The cube in salt
water is surrounded by its own very cold melt water which is still less
dense than the very salty brine. The cold melt water insulates the cube
from the room temperature water around it. The cube in the coldest water
melts slower! A band of colored melt water will form at the bottom of
the fresh water cup and at the top of the slat water cup. The food
coloring "technology" showed that the cubes were not actually in the
same environment. The temperatures on the thermometers confirm it.
Originally we thought that stirring the containers would lead to an
unfair test because it would be hard to do it uniformly. Now the
students will ask what would have happened it they were stirred? This is
a good place to remind the students that the results of one experiment
are frequently the beginning of a new question and encourage them to
find out for themselves.)

### IV. Extension

Next, have the students design a new experiment to answer additional
questions they may have. Remind them to be sure to control the variables
so that they conduct a Fair Test.

(**Teacher note**: Students can repeat the experiment trying to
incorporate some type of uniform stirring. If the cups of water were at
the same temperature originally and the ice cubes the same size and
temperature, the ice should melt at about the same rate in both cups as
they are stirred.)

Given the unexpected outcome of this experiment, someone will ask why we
use salt to melt ice on sidewalks. Depending on the time available you
can discuss that solutions have lower freezing points than pure water
and that salt is only applied to ice if the temperature is fairly close
to the freezing point of water or slightly above it. The salt dissolves
in the liquid, which makes it more difficult to freeze. At temperatures
close to freezing the pressure of traffic rolling over the pavement
raises the surface temperature just enough to melt the ice and the salt
dissolving into it keeps it from refreezing. The ice skater's blade
concentrates the pressure of his body weight so that he glides on a thin
layer of liquid water. If the temperature is already very cold, the
highway department spreads cinders or sand to improve traction. Salt is
not very effective at low temperatures.)

### V. Evaluation 

Finally, ask the students to write down their answers to the following
questions:

1.  What you learned from doing this experiment.

2.  Are you accepting or rejecting your hypothesis and why?

3.  This experiment is about melting ice. Why has your teacher asked you
    to do it now as part of an exploration of galaxies and deep space?

(**Teacher note**: Each new development in technology may explain a past
conclusion in a totally different light. This lesson uses a "new"
technology (in the form of food coloring) to help show us what's going
on in a physical process. In doing so, we are able to discard old
hypotheses and understand the phenomenon better. Likewise, the answers
to our questions are right in front of us, but we need an open mind and
new tools to see them.

The cosmic microwave background was predicted by the Big Bang theory to
be an extremely uniform radiation emitted from the Big Bang. It would be
the remains of radiation from the original Big Bang. The wavelengths
would have lengthened as space expanded, so that today we can detect the
remnants of that expansion as microwaves. The discovery of actual CMB
was a great boost to the Big Bang Theory. The CMB was initially found to
be very smooth. The question arose as to where do the galaxies then come
from. There needed to be some irregularities in the background from
which galaxies could form. The data discovered by the new technology on
the COBE satellite showed that the CMB, although generally smooth,
actually did contain the slightest universal non-uniformities. It is
those very slight "lumps" that gravity to pulled together to form the
galaxies we see today.)