Density of Liquids and Gases

You can do some very rich math and science learning by examining the properties of fluids by doing some cool density and viscosity experiments! Guaranteed to amaze and delight!

Activity 1: Density Towers

You can see density towers on the internet, and all of them have rich learning in them. When I do them, I focus on learning how math will help us prove the science that we see! So fun!

Materials:

test tubes - enough for each pair to have one
test tube holders - enough for each pair to have one if possible
water - in beakers for pouring
oil - in one beaker for the teacher to pour - I use vegetable oil
molasses or corn syrup - in one beaker for the teacher to pour
dish soap - blue Dawn is fun
food colouring
paper towels
safety goggles

Safety Notes: Because of the shaking of test tubes, students should be wearing safety goggles. Also, oil can stain clothing as well as the food colouring, so warn students to try hard not to spill their test tubes on themselves or to not touch anything if their fingers get some of the solution on them. This is not edible science!

Set the stage! Activate prior learning by asking:
Conduct the experiment:

1. Have students pair up and put on their safety goggles. Discuss the safety precautions about the oil.

2. Have students get a journal or use See Saw or other app to take pictures with during the experiment. I usually get them to draw a test tube and then as we go through the experiment, they can add the various liquids to the test tube where they end up as a layer. They could also take pictures and record their observations that way.

3. Ask students to choose one of them to go and get one test tube and one test tube holder and return to their seat. The other student can take the test tube and fill it half full of water.

4. Ask the students to make a final prediction as to what will happen when you put some oil in their test tubes - by a show of hands, who thinks the oil will float, mix with the water, or sink to the bottom.

5. Go group to group with the beaker of oil and put about an eighth of a test tube of oil in.

6. Once all groups have seen what happens, ask them to say whether their predictions were correct or not. Discuss what happened. Say - Now we will test to see if it is just floating because it was the last liquid in or if it is truly lighter. Give all groups a piece of paper towel and ask them to choose one of the pair to lightly shake the test tube and invert it upside down (they can put the paper towel between their thumb and the test tube to help if needed).

7. Once all groups have finished shaking and inverting, have them examine what happened. Now can we say that oil floats on water? (yes)

8. Tell them that you will now be pouring some molasses or corn syrup in their test tube - show them the liquid in the beaker in case they have never seen it before. Ask them to predict whether the new liquid will float on the oil, float in between the oil and water, mix in with one of the liquids, or sink to the bottom and why they predict it? Listen for words such as thicker and write it down.

9. Go group to group and pour a small trickle of molasses or corn syrup in. Students will share their observations with the class once all groups have their third liquid. Ask them if their predictions were correct.

10. Next, ask them what they think will happen if they added food colouring. Ask them to predict with a show of hands if they think the food colouring will float on the oil, mix with the oil, float on the water, mix with the water, or sink to the bottom on or under the molasses or corn syrup. Ask why they predict what they do?

11. Allow students to put one drop of food colouring into their test tubes (remind them that food colouring can stain clothes). (They will notice that the food colouring will suspend itself in the oil in a droplet and then when it hits the water, it mixes with the water.)

12. Ask them why they think the food colouring mixed with the water. (Explain that food colouring is soluble in water, which means that the two liquids will mix together by dissolving together until they make a solution that is an even mix of the two and stay together as they are.)

13. The final step is to add the drop of dish soap. Ask students if they think the dish soap will float on the oil, mix with the oil, float between oil and water/food colouring solution, or sink to the bottom? Get a show of hands for each. Go around to each group and add a drop of food colouring to each test tube.

14. Ask them if their guess was correct. Why do they think it sank to the bottom? (answers will vary but will be similar to the molasses or corn syrup)

15. Ask them what dish soap is used for. I usually ask them which of them has done dishes before and what happens when you put an oily pan into the dish water. Tell them that they will see what dish soap does if they shake their test tubes like they did in the earlier part of the experiment. Let them shake their test tubes. (They will get lots of bubbles and the oil and water will mix together.)

16. Ask them what they think happened to the water and the oil particles when the soap was added. (The soap molecules attach to both the oil and the water particles and act like a bridge between the two so they can mix together for a short time.)

17. Have students complete their density tower observations.

18. Have students clean up by placing the soapy test tubes in a tub of warm water in a sink and you will be able to clean up easily thanks to the dish soap!

Extensions:

Most of the learning happens during the experiment itself. This experiment lends well to discussing the density of fluids, and can thus be applied to the density of gases. A lot of students don't believe that gases can be heavier or lighter than others! Use the following demonstrations to show them that gases can sink and float just like liquids!

Pouring Carbon Dioxide:

Check out these links and follow the directions to do an amazing demonstration that will show your students that you can pour carbon dioxide on a candle and put it out! Kids will be amazed when they realize that the invisible carbon dioxide gas is really there in a cup and because it is heavier than regular air, it can be poured from a cup on a candle and snuff it out. The main safety precaution is that you are using a lit candle, so be sure that your school will allow you to conduct this experiment. If not, just google it and there are plenty of videos of this demonstration. I will commonly take the students out on the school yard to do this on a day with no wind so that no fire alarms are set off! Materials you will need include a two beakers (or one), a candle, match, vinegar, and baking soda. One link is https://www.thoughtco.com/candle-science-magic-trick-607494 or you can try it the Steve Spangler way - https://www.stevespanglerscience.com/lab/experiments/co2-extinguisher/

Weighing Carbon Dioxide:

Another fun way to show that different gases can weigh more or less than others is by doing a demonstration using carbon dioxide gas versus air using some bucket balances.

Materials:

bucket balance - just one needed for your demonstration or enough for each pair of students in your class if they do this themselves (second option is always more fun for kids!)
vinegar
baking soda
beakers, one per pair or just one if it is a demonstration (100 or 200 mL beakers are good)
1 teaspoon or 5 mL measuring spoons, one per pair

Safety Notes: Use safety goggles for this experiment. This experiment can get messy so give students paper towel just in case they have a spill.

Prepare for and Conduct the Experiment:

1. Ask the students questions like:

Do you think that gases can sink and float like liquids in the density tower? Why yes? Why no?
How could we prove that some gases are heavier than others? (answers will vary)
Say - we are going to try to prove that some gases are heavier than others by making a heavy gas right here in our own classroom! That gas is a greenhouse gas called carbon dioxide. Does anyone know a way to make carbon dioxide? (some students may have seen this done when making volcanoes or other experiments)

2. Have students pair up or invite them to come to the demonstration area. They will need to use safety goggles if doing this experiment themselves (or you will need them if you do it yourself!)

3. Have them get the tools they will need that have been placed in the classroom (measuring spoon, beaker, bucket balance). Remind them not to start until you have discussed the steps with them. Have them make sure the bucket balance is even on both sides.

4. Ask them to get 10-20 mL of vinegar in the beaker, and 1 teaspoon (5 mL) of baking soda.

5. Have them mix the two ingredients and letting them bubble for a bit until all the bubbles have stopped or mostly stopped. Then have them pour the invisible carbon dioxide gas into one of the buckets, and watch their faces as the heavier gas pushes down the bucket balance! It won't be heavy enough to drop it completely but it should drop enough to show it. (If you are doing this as a demonstration, use more vinegar and baking soda and a bigger beaker so you can pour lots of the carbon dioxide into the balance, and you will see more of a drop!)

6. Ask them why they think this happened and how this proves that some gases are heavier than others - also ask, what gas is in the air of the classroom (mostly nitrogen and oxygen) and what this experiment tells us about those gases (they are lighter than carbon dioxide). Ask them if they know a gas that floats in our air (helium) and how they know (helium balloons float and rise in our air).

Calculating Density:

Now that students have seen that oil floats on water, why not have them prove it mathematically!

Materials:

a weigh scale, one for each pair
a graduated cylinder, one for each pair - 100 mL is best
water
oil
safety goggles

Safety Notes:

Be sure to have students wear safety goggles and remind them that oil can permanently stain clothing.

Prepare for and Conduct the Experiment:

Ask questions like:

1. You will need to give students some time to familiarize themselves with using a weigh scale. I usually give them a demonstration on how it works and let them weigh random things on it to practice for a while.

2. After they know how to use the scale, have them measure the weight of the empty graduated cylinder and record it.

3. Next, they will get exactly 50 mL of water in their graduated cylinder and weigh/record it.

4. They empty the graduated cylinder and dry it out thoroughly. Next, they will put exactly 50 mL of oil in their graduated cylinder and weigh/record it. (To save oil, I usually have students recycle it by pouring it into a large beaker for pouring and then they pour it back in when they are done.)

5. They do some math!

subtract the mass of the water in the cylinder from the mass of the cylinder =
subtract the mass of the oil in the cylinder from the mass of the cylinder =

(this will give them the masses of the oil and water that is in 50 mL of the liquid)

6. Compare the masses - which one was lighter? Does this make sense? Why? (the oil should be lighter)

7. Discuss why they had to keep the volume (50 mL) the same to compare the masses? (the volume has to same the same to see which one had more mass for the same amount of volume)

8. Calculate density of water and oil by dividing the mass by the volume - grams per millilitre. I usually google the exact density of water and vegetable oil to see which groups came the closest to the actual scientific calculation!

Overall Assessment for Math Expectations:

Math - estimate, measure, and record quantities, using the metric measurement system; solve problems using multiplication and division (and subtraction) using a variety of strategies; make connections among mathematical concepts and procedures, and relate mathematical ideas to situations drawn from other contexts; develop, select, and apply problem-solving strategies as they pose and solve problems and conduct investigations, to help deepen their mathematical understanding; develop and apply reasoning skills to make and investigate conjectures and construct and defend arguments; communicate mathematical thinking orally, visually, and in writing, using everyday language