Summary

In this demonstration, students will recognize that there is a maximum amount of salt that can be dissolved into a specified amount of water at a given temperature. Students will be asked to make observations about two different saltwater 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.

Grade Level

Elementary School

NGSS Alignment

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

Objectives

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 saltwater solution with undissolved salt is still classified as a solution.

Chemistry Topics

This demonstration supports students’ understanding of

• Solutions
• Solute & Solvent
• Mixtures

Time

Teacher Preparation: 10 minutes to gather materials

Lesson: 45 minutes

Materials

• 2 Beakers or Cups
• Water
• Salt, about 1 cup
• Spoon/stirrer
• Student handouts

Safety

• Always wear safety goggles when handling chemicals in the lab

Teacher Notes

• 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 everything they know about mixtures and solutions prior to beginning the 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. 
  • 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. Also, be sure to tell students that the water in both beakers is the same temperature.
  • Start by adding a small amount, about a 1/2 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.