The Why Behind Teaching This:
Unit 2 addresses standards related to matter and it's interactions. The unit begins with identifying types of matter and the particles that make it up. This is covered in standard 5-PS1-1: Developing a model to describe that matter is made of particles too small to be seen. We will be changing matter by melting, evaporating, and dissolving to prove that although the physical appearance has changed, the same amount of matter still exists. This is covered in standard 5-PS1-2: Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved. We will also be using a variety of properties to identify matter through standard 5-PS1-3: Make observations and measurements to identify materials based on their properties. The investigations and experiments during this unit will focus on physical and chemical changes that occur when mixing matter which addressed in standard 5-PS1-4: Conduct an investigation to determine whether the mixing of two or more substances results in a new substance.
This specific lesson addresses standard 5-PS1-2: Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved. In this lesson, we are dissolving salt in water and students try to prove that although you cannot see the salt, it is still there. One way of doing this is to measure the mass of both separately and they should be equal to the mass of both when combined.
The goal of this lesson is for students to determine if the mass of the water changes once salt is dissolved in it.
Students will demonstrate an understanding for this by collecting evidence to support their findings to share with the class.
Preparing for Lesson:
Dissolving Salt in Water:
I draw a picture on the front white board of a cup of hot water and a cup of cold water. As I draw them, I explain that they are both water in the liquid state so the particles themselves will look very similar. One difference is that the hot water will have a little more energy, so I add some little lines behind these particles to illustrate that they will be moving a little faster. I then, add in little dots to both cups of water to illustrate the salt that was added and is dissolving in the water. I ask students to discuss in their small groups, which water the salt would dissolve in fastest and why, and record their answer on a white board.
As students discuss their thinking in groups, I circulate to listen to their ideas. All groups believe that it will dissolve faster in the hot water. I ask group to share their thinking that they have recorded on their boards. I have shared a video of one group sharing their response, Explanation of why salt dissolves faster in hot water. They said that because the particles are moving faster in the hot water, that they are probably bumping into the salt crystals making them break into even smaller pieces faster. I shared this response because some of the other groups were interchanging the science vocabulary of melting and dissolving which I had to correct, and others could not really explain their thinking in a clear way.
Prove To Me The Salt Is Still In The Water?
After our discussion on whether salt will dissolve faster in hot water or cold water, I take a cup of water to the microwave and heat it for 45 seconds. I add some salt and stir until it is dissolved. After dissolving the salt in warm water, I circulate so students can see the cup of water. I ask them if there is still salt in the water, even though they cannot see grains of salt in there. Students tell me yes. I ask them to discuss with their group ways they think they could prove to me there is still salt in there. I rephrase it in a couple of different ways so students are clear on what I want to see on their whiteboards. I say, what tests could we do to prove to others that dissolving salt doesn't make the salt disappear, that it is still there, we just can't see it. I also say, what could we do, to show that the same amount of salt is still in this cup?
I give students time to discuss and record answers on whiteboards while I circulate to listen to responses. I hear some great conversations about volume, evaporating, and even freezing which is one thing I have never tried or considered doing.
Testing Some Of the Ideas:
One of the things that many of the groups had on their boards was taste it. I ask students how many of them have ever gargled with salt water. Most students raise their hands. I ask them if they can taste the salt in the water and they respond yes. I tell them that if we tasted it, we would taste the salt water, however, I remind them of the rule that we don't ever eat or drink anything in science. I tell them that I reuse cups year after year and they don't always get cleaned so there could have been other chemicals in that cup at some point.
Another thing listed on one of the boards was to freeze the water. I asked this group what they believe will happen when we freeze the water. They said they thought the salt would settle out of the water as it cooled and the salt would be at the bottom of the cup with the ice on top. I told the class that I have never tested this and I am honestly not sure what would happen so we will test it to find out. I pour about half of the salt water mixture into a cup and place it in the freezer. I tell them we will check it the following day.
Another groups believed that we could prove there was still salt in the water by evaporating the water. I ask the class if we place the cup in the window for a few days, would just the water evaporate or would the salt evaporate with it? The majority of the class believes just the water will evaporate but some think the salt may evaporate with it, and a few say they aren't sure. I add some green food coloring to the salt water mixture so that the salt will be colored and easier to see when the water does evaporate. I place the cup of the salt water mixture in the window.
The last test that I point out on a board for us to test is one that says to measure the water level and see if the salt caused it to raise up. I have this student elaborate a little more while I go fill a graduated cylinder with 50mL of water. They group explains that since salt is a solid, when it is added to the water it should cause the water level to rise just like the marble we have done in the past. I compliment this group for pulling in some previous knowledge from activities we have already done in class. I add some salt to the graduated cylinder and ask the group to read the new level of the water and they say it is now 56mL. That is one way to prove there is still salt in the water because the volume has increased and remains at the new level even when dissolved completely.
After finding that volume is one way to prove that there is still salt in the mixture, I ask students how they think adding the salt has affected the mass. I refer back to another lesson we did on changing the state of matter to see how it affected mass. I reminded them that when we changed the butter, ice, and chocolate, that it did not change mass. I asked again, if they think dissolving the salt in the water will change the mass. Most of the students believe it will and support their thinking by saying that when we melted the items in the previous lesson we weren't adding anything, we were just changing it. Now, when we dissolve the salt in the water, we are adding something.
I ask students to draw a 3 column chart on their whiteboards and label the columns with "mass of water, mass of salt, and mass of mixture". I provide each group with a balance and 100 grams of extra weight. The 100 grams of weight that is provided with the balance is not enough for each group so instead of having to share I got extra weights out. I was very pleased to see that as soon as I set down the balances groups began moving the slide on the back to get it balanced. This means that they are familiarizing themselves with the appropriate steps to measure without having to be reminded.
I show students the cup of water they will be using and tell them they will measure the mass of the water in the cup. I then hold up the smaller cup of salt and tell them they will then need to measure the mass of the salt in the smaller cup. I explain that after they find the mass of both, they will pour the salt into the water, place the smaller cup under the larger cup and stir the mixture until there is no more salt visible. Once the salt has all dissolved, they will need to find the mass of the salt water mixture, and both cups together.
I take the cups of water over to the microwave and heat them all at once for approximately 2 minutes. I ask students why I am heating the water before I pass it out to them. This allows me to review what we discussed earlier about salt dissolving faster in hot water. Students are able to tell me that it will dissolve faster. I then ask a couple of groups to repeat the directions of the activity before I provide them with materials so I can check for understanding.
I provide each group with their cup of water so they can begin measuring that first. As they begin measuring, I pass out the cup of salt and spoon to each group. I circulate to observe and listen to conversations. The only conversation that is allowed during these activities is science talk about the investigation taking place. I am also checking to make sure that all students are participating and that each group is remembering to write down the unit of measure in their charts.
In this video of student measuring mass of salt you can see that this group has already recorded the mass of the water and is now measuring their salt. They correctly place the weights, beginning with the largest weight, and moving down to the smallest.
After students have recorded their 3 measurements, we share results. All groups found that the mass of the water + the mass of the salt was equal to the total mass after dissolving. A couple of groups were off by 1 or 2 grams but I always tell them, 1 or 2 grams is so minor that it could be due to the balance not being perfectly balanced when they think it is. Sometimes it is difficult to tell if it is more balanced with or without that last 1 gram weight on so it is a judgment call.
I provide each student with a Dissolving Salt Exit Ticket and provide time for them to complete. We actually found two ways to prove that there is still salt in the water, finding the volume before and after, and finding the mass before and after. I am checking for understanding that they understand the salt has not disappeared. Although the particles are too small for us to see, they are still there.
Evaporating and Freezing:
After a few days, I show them the cup of water set out in the window to evaporate and they can see that we are left with just the salt. This is another way to prove that the salt is still in the water.
After a couple of days, we also checked the water in the freezer and we could not see the salt separated from the ice so this was not a good way to prove there is still salt in the water.