Determining Freezing Points Lab
Lesson 1 of 5
Objective: Students will be able to experience phase change, as they record the temperature of candle wax as it cools, which is technically referred to as freezing (liquid to solid).
This lesson is based on California's Middle School Integrated Model of NGSS.
NGSS Performance Expectation (PE): (MS-PS1-4) Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
Science and Engineering Practice 6: Constructing explanations and designing solutions.
Disciplinary Core Ideas (DCI): PS1.A: Structure and Properties of Matter - The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter.
Crosscutting Concepts (CCC): Cause and Effect - Cause and effect relationships may be used to predict phenomena in natural or designed systems.
Common Core ELS/Literacy Standard: (RST.6-8.3) Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.
Lab Set-Up (Pre-lab)
This lab will have the students collect multiple data points for approximately 20-25 minutes using a thermometer and candle wax. The teacher will need to keep the candle wax melted as a liquid until the students are ready. The students will watch the candle wax harden as they record the cooling temperature of the wax. By the end of this lab your students should have practical experience observing changes of state (freezing), which occur with variations in temperature and can be predicted by knowing the properties of the material (DCI - PS1.A: Structure and Properties of Matter).
This activity will provide a real world example of how a pure substance (candle wax) changes from one state (liquid) to another (solid). It is beneficial for the students to understand that the particles in the wax are slowing down and aligning into a solid, in a process referred to as freezing (MS-PS1-4). The students should also understand that they are creating a model to help them better understand the freezing process (S&E Prac - Constructing explanations and designing solutions).
Materials: safety goggles, large test-tubes, test-tube tongs, test tube rack, wire loop stirrer, thermometer, timer. Each student will need safety goggles, the remaining equipment will be one per group.
CAUTION: Always insist upon student eye protection. As part of my lab procedures I make each group sign-in for the equipment (see Laboratory Equipment Check List - Determine Freezing Points Lab). At the end of the lab I check to see if any of the equipment is broken/dirty/missing and initial if everything is okay. If at anytime a student in that group is not properly wearing their eye protection I draw a sad face over all the names in the group. The innocent kids get the message and self-police their group.
TIP: I made cheap wire loop stirrers by asking a local dry cleaner if they could donate wire coat hangers. I then cut the straight portion of the coat hangers (14 inches) and bend each end in a loop for stirring/safety.
I buy cheap non scented candles and use a sharp knife to cut in into pieces that will fit into a test tube. Remove any wicks and discard. [Need photo]
To liquify the wax I have a hot plate with a large beaker filled with water. I monitor the water's temperature to keep it a warm (not boiling). Place the wax filled test tube into the warm water and allow the wax to melt.
At the beginning of the lab, a student from each group will bring a test-tube tong to retrieve a test-tube with liquid wax inside, carefully carry it back to their group and place it into the test-tube rack.
REFLECTION: I stess that the wax is hot and they will feel it if any lands on their skin. However the temperatures do not produce any visible burns. The most dangerous time happens when a student is carrying the hot wax back to their group. Timidness can be dangerous. I teach my kids to use one had and hold the test-tube with the clamp and use the either hand to hold the top of the test-tube. I have carried many test-tubes to the groups directly if no on win the grow is confident enough.
Insert the thermometer into the test-tube and record the initial temperature. Make sure not to touch the thermometer to the side of the test-tube. Record the temperature every minute and note the physical state of the wax. After the wax has become a solid for several minutes and the temperature has stabilized the students can stop taking measurements. The results will be graphed on the included graph.
This activity is about students recording measurements accurately. It is also practical experience that liquid wax freezes into solid wax. Each student would receive a copy of the lab and a copy of the graph.
NOTE: Most students assume that freezing is a cold condition, in reality it is the change of state from a liquid to a solid.
1) Use the test tube tongs to obtain a test tube filled with liquid candle wax (C25H52). Use caution as the candle wax (C25H52) will be hot. Place the test tube in the test-tube rack.
2) Use the wire loop stirrer and stir the liquid to make sure that it is the same temperature throughout (if the candle wax (C25H52) appears to be a liquid you do not need to stir it).
3) Place the thermometer into the liquid candle wax (C25H52) and record the initial temperature. Hold the thermometer so that it does not touch the sides of the test tubes. Leave the thermometer in the test-tube for the entire experiment.
4) Wait until the liquid in the thermometer has stopped rising. Record this temperature.
5) Start the timer.
6) Continue recording the temperature of the liquid candle wax (C25H52) every minute until it has formed a solid.
7) Record the temperature the candle wax (C25H52) became a solid (box ‘AD’) and indicate how many minutes that took.
8) Graph your results. Indicate at what point the liquid became a solid.
In order to complete this lab the students have to perform precise multistep procedures to get the desired results, while taking measurements (CCLS - WHST.6-8.3).
One of the themes to this unit is that phase changes, such as boiling and freezing, have nothing to do with temperature. Boiling is not hot and freezing is not cold, even though we use these definitions in everyday language. I stress that boiling is the change of state from liquid to gas and freezing (this activity) is the change of state from liquid to solid.
By the end of this lab the students should also understand that in science a "cause" will lead to an "effect". In this case the liquid candle wax, when removed from a heat source (water bath) the wax froze into a solid. (CCC - Cause and Effect).
As a wrap-up to this lesson, students draw four pictures in their interactive notebooks that describe the phase change (freezing) experienced in this activity.
TIP: As the students are working on their drawings it might be fun to play one of the many popular songs from the movie soundtrack 'Frozen".
I have included a PowerPoint that details what the phase is called when matter changes from one state to another i.e. - SOLID to LIQUID (melting)
I also teach a mnemonic sentence to remember all the phases.
My sick bat eats canned food daily.
M - melting
S - sublimation
B - boiling
E - evaporation
C - condensation
F - freezing
D - deposition