Teacher Guide

Getting Salty

Experimenting with melting points

When different substances (table salt, rock salt, and sugar) are added to ice, which one will lower the melting point of the ice the most?

This resource was originally published in PhysicsQuest 2019: Heat.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Getting Salty

When different substances (table salt, rock salt, and sugar) are added to ice, which one will lower the melting point of the ice the most?

  • Table salt
  • Rock salt
  • Sugar
  • Cups for making ice (7)
  • Stopwatch
  • Freezer

Students start by predicting the outcome of their experiment. When they engage with the experiment of changing the melting point of ice, they will record their observations and use those observations to draw conclusions with their peers.

  • Total time
    45 - 60 Minutes
  • Education level
    Grades 5 - 9
  • Content Area
    Heat
  • Educational topic
    Thermodynamics, chemical reaction

Pure substances are substances that contain molecules of only one type. The molecules in a pure substance can be either elements or compounds.

Elements are one type of atom, like oxygen. A compound is made up of two or more elements that are chemically bonded together to make a new substance, like hydrogen and oxygen combining to make water. Mixtures are a combination of two or more substances that are not chemically bonded together, like salt in water or raisins in your trail mix.

Since these substances don’t react chemically, it’s possible to separate a mixture back to its original parts. Adding something to a pure substance to make it a mixture will change how that substance behaves. There are two basic types of mixtures: homogeneous and heterogeneous. Homogeneous mixtures are evenly mixed and appear the same throughout like saltwater. If properly dissolved, you wouldn’t be able to see the salt grains in a bucket of saltwater. In heterogeneous mixtures you can see the different substances making up the mixture, like in a trail mix, and they can easily be separated back into their original parts. Water is a pure substance when it contains only molecules of H²O. Pure water has a freezing point of 0°C, which means the melting point of ice made with pure water is also 0°C. When other substances like sugar or salt, are added to water, it becomes a mixture, and its freezing/melting point will change.

When salt (NaCl, sodium chloride) is added to water, it becomes a mixture, which changes the melting/ freezing point and the boiling point of the water. While water is freezing, the salt molecules disrupt the formation of ice crystals. This causes the liquid (saltwater) to drop to a temperature lower than 0°C before it reaches its freezing point and changes into a solid (ice). This also means that ice made with saltwater will melt at that same lower temperature. Other substances will have similar effects on water’s freezing/melting point.

Some chemicals, such as ethylene glycol, a compound in antifreeze, can drop the freezing point much lower than salt. It is commonly used to help de-ice planes or prevent freezing of pipes in RVs or boats. However, this chemical is poisonous to animals and humans.

Key terms

These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.

  • Compound: A substance that is composed of two or more separate elements chemically bonded together.
  • Element: A simple substance that cannot be broken down using a chemical reaction.
  • Mixture: Combination of two or more substances that are not chemically bonded together.
  • Molecule: Combination of one or two atoms that are chemically bonded together.
  • Pure Substance: A substance made up of only one type of molecule.
Before the experiment
  • Ask & Discuss

    How do you think the substances (table salt, rock salt, and sugar) will change the melting point of the ice?

  • Turn & Talk protocol

    Use the turn & talk protocol

    1. Pair students up
    2. Give them a minute to think quietly
    3. Give students 2 minutes to discuss their thinking
    4. Have students record their answers or share out to the whole group
Setting up
  • Fill the smaller cups with the substances

    Fill the smaller cups with the substances as follows and label as follows:

    • Cup 1 – 1/3 of the total table salt
    • Cup 2 – 2/3 of the total table salt
    • Cup 3 – 1/3 of the total rock salt
    • Cup 4 – 2/3 of the total rock salt
    • Cup 5 – 3 packets of sugar
    • Cup 6 – 6 packets of sugar

  • Label Cup 7 – water only (this cup will be your baseline)

  • Carefully pour 8 oz. (one cup) of water into each cup for the substances to dissolve. Make sure each cup is filled to the same level.

  • Place all seven cups in the freezer for at least two hours, or until the water has frozen.

  • Make a chart for recording the time it will take each cup of ice to melt.

During the experiment
Collecting data

  • Make sure students are put into intentional groups. See above.

  • Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.

Analyzing data

  • In the student guide, they will answer questions that help them understand what different compounds do to ice.

  • Continue to listen in on each group’s discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Teacher tip

Suggested STEP UP Everyday Actions to incorporate into activity:

  • When pairing students, try to have male/female partners and invite female students to share their ideas first.
  • As you put students into groups, consider having female or minority students take the leadership role.
  • Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
  • Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
  • As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.

Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.

Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives. Look for *** below to find suggested places to add.

Conclusion
  • Post the conclusion question

    Now that you have determined which substance works the best at lowering the melting temperature of the ice, how would you explain to a school that they should use it on icy sidewalks to keep people safe?

  • Use the All class science talk protocol
    1. Use the All class science talk protocol to have students share and refine their thinking.
    2. The teacher poses a question for students to answer.
    3. If necessary, give students time to think or write.
    4. The teacher asks students to turn their bodies toward the center of the room for a Science Talk.
    5. The teacher explains: “In a Science Talk, knowledge is held by your fellow scientists, and you should talk to each other. The goal of a Science Talk is to help each other understand a phenomenon. You can help your fellow scientists understand by: sharing results, using data, being as clear as possible, and listen carefully to deeply understand what your fellow scientists are saying.
    6. Teacher facilitates the student discussion by using Michaels and O’Connor’s Talk Science Primer and avoiding telling answers or asking closed-ended questions.
    7. Optional: At the end of the Science Talk, students can record their thinking.

  • After students have had a chance to discuss key ideas from the lesson and complete their student guides, you can now clarify and give concise definitions to the forces they experimented with.

Real world connections -

  • You can make homemade ice cream with students with this recipe

Suggestions for drawing, illustrating, presenting content in creative ways

Engineering and design challenges connected to the content

Credits

Credits:

Coordination, Research, Text, and Editorial Review Jon Anderson, Isabel Bishop, Randie Hovatter, Jamie Liu, Leah Poffenberger, James Roche, Natalie Ruiz, Laurie Tangren, Rose Villatoro, David Voss

Graphic Design and Production Meghan White

Illustration of Experiment Guides Isabel Bishop

Illustration of Dr. Chien-Shiung Wu Annamaria Ward

Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)

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