In this demonstration, students will recognize that there is a threshold for the amount of salt that can be dissolved into a specified amount of water. Students will be asked to make observations about two different salt water samples to determine if a sample that contains undissolved salt is still considered a solution. The saltiness, or salinity, of the water samples will be used to help students make connections about how the melting of polar ice caps is changing the average salinity of the ocean.
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- 5-PS1-4: Conduct an investigation to determine whether the mixing of two or more substances results in new substances.
- Scientific and Engineering Practices:
- Developing and Using Models
- Constructing Explanations and Designing Solutions
By the end of this demonstration students should be able to
- Understand that a solution is formed when one substance dissolves in another.
- Describe what happens when salt “disappears” when mixed with water.
- Recognize that a salt water solution with undissolved salt is still classified as a solution.
This demonstration supports students’ understanding of
- Solute & Solvent
Teacher Preparation: 10 minutes to gather materials
Lesson: 45 minutes
- 2 Beakers or Cups
- Salt, about 1 cup
- Student handouts
- Always wear safety goggles when handling chemicals in the lab
- Prior knowledge: Students are expected to understand that all solutions are mixtures, but not all mixtures are solutions. To make this idea more concrete for younger students, it might be helpful to use the following chart to give students a visual representation of this idea:
- Students should already be familiar with the vocabulary associated with this activity, such as, mixture, solution, solute, and solvent.
- It might also be helpful to create a circle chart to track student understanding and misconceptions within the mixtures and solutions unit. For this graphic organizer:
- First, have students share out everything they know about mixtures and solutions prior to beginning content. Record everything, even if it is incorrect thinking, inside the circle on the chart using the same color marker.
- Create a color key in the rectangle surrounding the circle for reference. Repeat this process after each activity is completed within the unit using a different marker color. As students learn more information, they should be able to revisit possible misconceptions (ex: “creating something new”) and correct them in the chart. An example is shown below.
- To complete the chart, a third color would be used after a subsequent activity (e.g. lecture, reading, and video).
- Vocabulary terms that will be used/discussed are defined below.
- Mixture (Simple Mixture): A combination of two or more substances.
- Solution: A special type of mixture in which one substance is dissolved or evenly dispersed into another.
- Solute: The substance being dissolved.
- Solvent: The substance that does the dissolving (generally water).
- Carry out the demo directions below. Amounts are approximate and can be easily adjusted depending on the supplies available to you.
- Fill two large beakers (or cups) about ¾ full of water. It is important to make sure you have the same amount of water in each container.
- Start by adding a small amount, about a spoonful, of salt to the first beaker. Stir the contents to help the salt dissolve more quickly into the water. It is important that students can observe all of the salt dissolving into the water.
- Direct students to answer the questions about container one. These questions can be answered individually, with a partner, or with a small group.
- Once students have filled in the information about the first container, pour a significantly larger amount of salt into the second beaker (the goal is for there to be undissolved salt settled at the bottom). Once you have stirred the salt and water, and students have observed the excess salt settle to the bottom of the container, have students begin answering the questions about the second container.
- Possible Misconceptions
- It is very easy for students to confuse dissolving with melting at the elementary level. For this activity, make sure you use correct terminology and never refer to the salt as “melting” into the water.
- Students often think that when a solution is formed a new substance is created, but this is not correct.
- As a possible extension activity, have students complete the following sea salt quiz provided through NASA: https://climate.nasa.gov/quizzes/quiz-salinity/
For the Student
A solution is a special type of mixture in which one substance, the solute, dissolves into another substance, called the solvent.
Is there a limit to the amount of solute (salt) that can be dissolved in a given amount of solvent (water)?
- Based on your observations, is there a limit to the amount of solute (salt) that can be dissolved in a given amount of solvent (water)? What evidence do you have to support your thinking?
- With your group, discuss the “Read” information in the box below. How do you think this will affect the salinity of the ocean (do you think it will increase, decrease, or stay the same). Record your thinking below.
Read: Global warming has caused the Earth’s polar ice caps to melt faster each year. This is introducing more water into Earth’s oceans. Scientists who study the environment are concerned with how this will affect the salinity (the amount of salt in a specific amount of water) of the ocean.
- With your group, develop a model that could be used to show polar ice caps melting. Draw a picture or write an explanation for your model below.