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Purpose
[stextbox id = “info”] To help the teacher and students become aware of their initial thinking about what happens to salt when it is stirred into water.[/stextbox]
Description
[stextbox id = “info”] Students are shown a cup of water (at least 12 ounces) and asked to draw pictures of what they would see in the cup of water if they had super strong glasses [2], more powerful than the most powerful microscope. Students can make their drawings using the “Before” cup on this task sheet [3]. The students can save these drawings in their science notebooks or they can be collected by the teacher. [Task instructions for pilot teachers. [4]]
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Next, the teacher and/or students weigh the water (using a sensitive scale [6]) and record the results. The teacher and/or students also weigh a small cup holding a tablespoon of salt (both with and without the salt in the cup, to determine the weight of the salt alone) and record those results. The teacher then pours the salt into the water, stirs the salt until it is no longer visible, and then asks students to make predictions about the current weight of the water. Ask for a sample of students with differing opinions to share their thinking with the class.
Finally, weigh the glass of saltwater and compare the weight to the separate weights of the water and salt.
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Safety Tip
[stextbox id = “info”]Do not let students eat the salt or drink the saltwater. As a rule, students should not eat or drink in a science activity.
[/stextbox]Questions to Ask Students
[stextbox id = “info”]- Describe your drawing of the cup of water. What does each part represent?
- Why did you predict that the weight of the water with salt stirred would be [greater than/less than/the same as] the sum of the salt and water separately?
- Where did the salt go when stirred into the water? Why do you think so?
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Student Thinking
[stextbox id = “info”] Take note of the student misconceptions in your class. Common ones include:- the salt disappeared—that is, it doesn’t exist anymore
- the salt melted
- the water absorbed the salt
- the salt evaporated
- the salt turned into water
If you record the students’ ideas (e.g., on a piece of chart paper), keep the record (e.g., save the paper, take a picture) to revisit later in the pathway.
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Implementation Tips
[stextbox id = “info”]- Consider beginning the Student Experience by having students write down the Driving Question in their science notebooks.
- A clear cup is best so students can observe the salt.
- Use room temperature (not cold) water.
- Discuss with students why it is important to subtract the weight of the cup when determining the weight of the salt.
- Don’t add too much salt to the water – you want the salt to completely dissolve.
- You’ll need to stir a little, and then let the water sit for a few minutes before having students examine it. If you use a spoon, some water may stick to it, so be sure to shake it off. You can also leave the spoon in the cup and subtract the weight of it from the total.
- If necessary, you can use a pan balance for this activity, with some modifications. Place a cup of water on either side of the balance. Make sure both cups have the same amount of water and that the balance remains level after the cups are added. Ask students to predict what will happen when you add a teaspoon of salt to one cup.