What is the reason for salt dissolving quicker in hot water than it does in cold water?
Standards related to matter and interactions are addressed in Unit 2.The unit starts with identifying the types of particles.Developing a model to describe that matter is made of particles too small to be seen is covered in standard 5-PS1-1.The same amount of matter still exists despite the physical appearance being different.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 be using a variety of properties to identify matter through standard 5-PS1-3: Make observations and measurements.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 are trying to prove that it is still there.If you measure the mass of both separately, they should be the same when combined.
The goal of this lesson is for students to see if the mass of the water changes when salt is dissolved.
Students will collect evidence to support their findings in order to demonstrate an understanding.
I drew a picture of a cup of hot and cold water on a white board.The particles themselves will look very similar because they are both water in the liquid state.The hot water will have a little more energy, so I added some lines behind the particles to show that they will be moving a bit faster.Adding dots to both cups of water illustrates the salt that was added and is dissolving in the water.In order to record their answer on a white board, I ask students to discuss in their small groups which water the salt would dissolving in fastest and why.
I listen to students discuss their ideas.All groups think it will be quicker to get rid of it in the hot water.I want the group to record their thoughts on their boards.I shared a video of a group explaining why salt gets dissolved faster in hot water.They said that because the particles are moving faster in the hot water, they are likely bumping into the salt crystals, making them break into smaller pieces faster.Some of the other groups were changing the science vocabulary of melting and dissolving which I had to correct, and others couldn't explain their thinking in a clear way.
I take a cup of water to the microwave and heat it for 45 seconds after we talked about salt dissolution in hot and cold water.When it is dissolved, I add some salt.I circulate so students can see the cup of water after dissolving the salt in warm water.Even though they can't see salt in the water, I ask if there is still salt.Students say yes.They think they can prove to me that there is still salt in there.Students are clear on what I want to see on their whiteboards if I rephrase it in a couple of different ways.I want to know what tests can be done to prove that dissolving salt doesn't make it disappear.What can we do to show that the same amount of salt is in this cup?
While I circulate to listen to responses, I give students time to discuss and record their answers on whiteboards.One thing I have never tried or considered doing is volume.
Student responses can be seen more closely at these links.
Tasting it was one of the things that the groups had on their boards.Students are asked how many have been gargled with salt water.Students raise their hands.I asked them if they could taste the salt in the water.I remind them of the rule that we don't ever eat or drink anything in science and that if we tasted it, we would taste the salt water.I tell them that I reuse cups year after year and they don't always get cleaned so there could be other chemicals in that cup at some point.
One of the boards said to freeze the water.When we freeze the water, what will happen to this group?They thought the salt would be at the bottom of the cup when the water cooled.I told the class that I have never tested it, but we will do it to find out.I put half of the salt water mixture in the freezer.We will check it the next day.
Another group believed that we could show that there was salt in the water.If we put the cup in the window for a few days, would the water or salt evaporate or not?The majority of the class thinks the water will evaporate, but some think the salt may evaporate with it.I add green food coloring to the salt water mixture to make it easier to see when the water is gone.I put the cup in the window.
One of the last tests we need to do is to measure the water level and see if the salt caused it to rise.While I fill a graduated cylinder with water, I have this student elaborate a little more.The group says that since salt is a solid, it should cause the water level to rise just like marble did in the past.The group pulled in some knowledge from activities we have already done in class.The group was asked to read the new level of the water after I added salt to the graduated cylinder.It is possible to prove that there is still salt in the water because the volume has increased and remains at the new level even when dissolved completely.
I ask students how adding salt has affected the mass after finding that volume is one way to prove that there is still salt in the mixture.We changed the state of matter to see how it affected mass.I told them that the butter, ice, and chocolate did not change mass.I asked if dissolving the salt in the water would change the mass.Most of the students believe that it will support their thinking by saying that when we melted the items in the previous lesson we were just changing them.We are adding something when we add salt to the water.
Students are told to draw a 3 column chart on their whiteboards and label the columns with the mass of water, salt, and mixture.Each group has a balance and 100 grams of extra weight.I got extra weights out because the 100 grams of weight that is provided is not enough for each group.As soon as I set down the balances, groups began moving the slide on the back to get it balanced.They are familiarizing themselves with the steps to measure without having to remind them.
I teach students how to measure the mass of the water in a cup by showing them the cup.I told them that they would need to measure the mass of the salt in the smaller cup.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.They will need to find the mass of the salt water mixture and both cups after the dissolved salt.
I heat the cups of water in the microwave for 2 minutes.I asked students why I was heating the water.I can review what we said about salt dissolving faster in hot water.Students tell me that it will dissolving quicker.I need a couple of groups to repeat the directions of the activity before I give them materials to do it.
I give each group a cup of water so they can start measuring.I give each group a cup of salt and spoon as they measure.I listen to conversations.Science talk about the investigation taking place is allowed during these activities.I am checking to make sure that all students are participating and that each group writes down the unit of measure in their charts.
You can see in the video that the group has already recorded the mass of water and is now measuring their salt.They place the weights in chronological order, beginning with the largest weight and moving down to the smallest.
We share the results after students have recorded their 3 measurements.All groups found that the mass of the water and salt was the same as the total mass after dissolving.I always tell them that 1 or 2 grams is so minor that it could be due to the balance not being perfectly balanced when they think it is.It is difficult to tell if it is balanced with or without the last 1 gram weight.
Each student is given a Dissolving Salt Exit Ticket and time to complete it.We were able to find two ways to prove that there is still salt in the water.I want to make sure they understand that the salt has not disappeared.The particles are too small for us to see.