Do you think your results would change if the corn syrup was replaced with water why or why not

Right now, as you read this, there are millions of things happening throughout your body. The food you ate just a bit ago is making its way through a watery slurry inside your stomach and small intestines. Your kidneys are working hard to excrete waste and extra water. The lacrimal glands near your eyes are secreting tears, which allow your eyelids to close without damaging your eyeballs. What’s one thing that all of these processes have in common? They all rely on osmosis: the diffusion of water from one place to another.

Osmosis factors heavily in each of these processes and is an important force for keeping every single cell in your body healthy. Osmosis is hard to see without a microscope. But if we create our very own model of a cell, using a shell-less chicken egg, we can see what happens when we manipulate the osmotic balance in the “cell”!

Materials

• 3 eggs
• 3 glasses (large enough to fit the egg plus liquid)
• 3 butter knives
• White vinegar (about 3 cups)
• Distilled water (about 2 cups)
• Light corn syrup (about 1 ¼ cups)
• Slotted spoon
• Measuring cup (1 cup)
• Measuring spoons (1 tablespoon and ½ tablespoon)
• Sticky notes and marker
• Scale (optional)

Procedure

Note: It’s okay to touch the eggs, but remember to wash your hands afterwards to avoid any nasty surprises!

1. Place one egg in each glass. Pour in enough vinegar to cover each egg. Bubbles will start to form around the egg, and it’ll float up. To keep it submerged, put a butter knife in the glass to hold it down.

2. Put the three glasses in the refrigerator and allow to sit for 24 hours.

3. Gently holding the egg in the glass, pour out the old vinegar. Replace with fresh vinegar, and let sit in the refrigerator for another 24 hours. Repeat this process until the shells are fully dissolved and only the membrane remains. This should take about 2-3 days.

4. Gently remove the eggs using the slotted spoon and rinse with tap water in the sink. Rinse out the empty glasses as well.

5. Gently put the shell-less eggs aside for a moment on a plate.

6. Prepare three different sugar-water solutions as follows, labeling with sticky notes:

Glass 1: Label “hypertonic”. Pour in one cup of corn syrup.

Glass 2: Label “isotonic”. Add 1 ½ tablespoons corn syrup to the one cup measuring cup, and fill the remainder with distilled water. Pour into glass (make sure you get all the corn syrup out!) and stir to dissolve.

Glass 3: Label “hypotonic”. Pour in one cup of distilled water.Gently put one shell-less egg in each of the glasses, and let sit in the refrigerator for another 24 hours.

7. Remove the glasses from the refrigerator, and gently put the eggs on a plate. If you weighed the eggs before putting them in each solution, weigh them again. What happened to each of the eggs?

How does osmosis work?

Osmosis is the scientific term that describes how water flows to different places depending on certain conditions. In this case, water moves around to different areas based on a concentration gradient, i.e. solutions which have different concentrations of dissolved particles (solutes) in them. Water always flows to the area with the most dissolved solutes, so that in the end both solutions have an equal concentration of solutes. Think about if you added a drop of food dye to a cup of water – even if you didn’t stir it, it would eventually dissolve on its own into the water.

In biological systems, the different solutions are usually separated by a semipermeable membrane, like cell membranes or kidney tubules. These act sort of like a net that keeps solutes trapped, but they still allow water to pass through freely. In this way, cells can keep all of their “guts” contained but still exchange water.

Now, think about the inside of an egg. There’s a lot of water inside of the egg, but a lot of other things (i.e. solutes) too, like protein and fat. When you placed the egg in the three solutions, how do you think the concentration of solutes differed between the inside of the egg and outside of the egg? The egg membrane acts as a semipermeable membrane and keeps all of the dissolved solutes separated but allows the water to pass through.

How did osmosis make the eggs change size (or not)?

If the steps above work out properly, the results should be as follows.

In the case of the hypertonic solution, there were more solutes in the corn syrup than there were in the egg. So, water flowed out of the egg and into the corn syrup, and as a result the egg shriveled up.

In the case of the isotonic solution, there was roughly an equal amount of solutes in the corn syrup/water solution than there was in the egg, so there was no net movement in or out of the egg. It stayed the same size.

In the case of the hypotonic solution, there were more solutes in the egg than in the pure water. So, water flowed into the egg, and as a result, it grew in size.

Osmosis and You

Every cell in your body needs the right amount of water inside of it to keep its shape, produce energy, get rid of wastes, and other functions that keep you healthy.

This is why medicines that are injected into patients need to be carefully designed so that the solution has the same concentration of solutes as their cells (i.e. isotonic). If you were sick and became dehydrated, for example, you would get a 0.90% saline IV drip. If it were too far off from this mark it wouldn’t be isotonic anymore, and your blood cells might shrivel up or even explode, depending on the concentration of dissolved solutes in the water.

Osmosis works just the same way in your cells as it does in our egg “cell” model. Thankfully, though, the semipermeable membrane of the egg is much stronger, so you don’t have to worry about the egg exploding as well!

View the video below to see what you and your students will do in this lesson. 

Students will be able to explain that the density of a liquid has to do with how heavy it is for the sample size. Students will also be able to explain that if a liquid is more dense than water, it will sink when added to water, and if it is less dense than water, it will float.

Key Concepts

• A liquid, just like a solid, has its own characteristic density.
  
• The density of a liquid is a measure of how heavy it is for the amount measured. If you weigh equal amounts or volumes of two different liquids, the liquid that weighs more is more dense.
• If a liquid that is less dense than water is gently added to the surface of the water, it will float on the water. If a liquid that is more dense than water is added to the surface of the water, it will sink.

Note: We are purposely using the terms “size” and “amount” instead of “volume” in this lesson about density. We are also using “heavy”, “light”, and “weight” instead of “mass”. If your students have already learned the meanings of volume and mass, you can easily use those terms to define density (Density = mass/volume), and then use those terms in this lesson. 

NGSS Alignment

• NGSS 5-PS1-3: Make observations and measurements to identify materials based on their properties.

Summary

In the previous lesson, students learned that density has to do with how heavy an object or substance is relative to its size, and that density determines whether an object sinks or floats. Students also observed that you can compare the density of a substance to the density of water by comparing the weights of equal amounts of the substance and water using a balance.  

In this lesson: 

• As a demonstration, the teacher will compare the weight of an equal amount or volume of water and corn syrup so students can observe that corn syrup is more dense than water and sinks.
  
• Students will compare the weight of an equal amount or volume of water and vegetable oil and see that vegetable oil is less dense than water and floats.
• Students add corn syrup to layered oil and water and see the corn syrup sinks below both the oil and water.

Evaluation

Download the student activity sheet (PDF) and distribute one per student when specified in the activity. The activity sheet will serve as the Evaluate component of the 5-E lesson plan.  

Safety

Make sure you and your students wear properly fitting safety goggles. Isopropyl “rubbing” alcohol is a flammable liquid. Keep away from heat, sparks, open flames, and hot surfaces. Isopropyl alcohol is also irritating to eyes and skin, and may cause drowsiness or dizziness if inhaled. Work with isopropyl alcohol in a well-ventilated room. Read and follow all warnings on the label.  

Clean-up and Disposal

Remind students to wash their hands after completing the activity. All common household or classroom materials can be saved or disposed of in the usual manner. 

Materials

• Water
  
• 2 Clear plastic cups
• Corn syrup (Karo syrup), 1 cup
• Food coloring
• Popsicle stick or plastic spoon
• Vegetable oil
• Isopropyl “rubbing” alcohol (70%)
• Ice cubes
• Balance

Teacher Preparation

Pour 50 mL of corn syrup, 50 mL of water, and 50 mL of vegetable oil into three plastic cups for each group.  

Note: Corn syrup and vegetable oil can be difficult to clean out of graduated cylinders. To avoid this mess, measure and pour 50 mL of water into each of three plastic cups. Then mark the outside of each cup to indicate the level of the liquid in each cup. Pour out the water from two of the cups and dry the inside with a paper towel. Next, use those cups to measure the amount of corn syrup and vegetable oil for each group. Add 1 drop of food coloring to the corn syrup. 

Each group will need 50 mL of corn syrup, 50 mL of water, and 50 mL of vegetable oil in separate cups.  

For the demonstration, you will need 50mL of water and 50 mL of corn syrup (colored with 1 drop of food coloring) in separate cups.  

• Since we weighed equal amounts and the corn syrup was heavier, is water or corn syrup more dense?
  Corn syrup is more dense than water.

• Predict what will happen if we pour the corn syrup into the water. Will the corn syrup float or sink when added to the water?
  The corn syrup should sink in the water.

1. Pour the colored corn syrup into the cup containing water to see if the corn syrup floats or sinks in the water.

It will sink. The corn syrup sinks in the water. 

Give each student an Activity Sheet (PDF).
Students will record their observations, and answer questions about the activity on the activity sheet. 

Explore

2. Have students compare equal volumes of water and vegetable oil and test whether the oil floats or sinks when added to water.

Question to investigate: Is vegetable oil more or less dense than water?

Materials for each group

• 50 mL of water in cup
  
• 50 mL of vegetable oil in cup
• 50 mL of corn syrup in cup
• Balance

1. Place the cups of water and vegetable oil on opposite ends

The oil weighs less (is lighter) than an equal volume of water.

Ask students: 

• Which is less dense, water or vegetable oil?
  The vegetable oil is less dense than water because it weighs less than an equal volume of water.

• Predict what will happen when you pour the vegetable oil into the water. Will the oil sink or float?
  The oil will float on the water.
   

1. Pour the vegetable oil onto the water to see if it sinks or floats.

The oil floats in a separate layer on the water.

Explain

4. Use an animation to review and explain student observations.

Show the Animation Density of Liquids.

Explain that to compare the density of corn syrup and water you can compare the weight of equal volumes of water and corn syrup. Since the same volume of corn syrup is heavier than water, it is more dense and sinks in water. Explain that to compare the density of oil and water you need to compare the weight of equal volumes of water and oil. Since the oil is lighter, it is less dense than water and floats on water.  

Extend

5. Do a demonstration to compare the density of water and isopropyl alcohol. 

Materials for the demonstration

• 2 clear plastic cups
• Water
• Isopropyl “rubbing” alcohol (70%)
• 2 ice cubes

1. Label one cup Water and the other Alcohol. Pour water and isopropyl alcohol into their labeled cups until each is about ½ full.
   Show students the two liquids and point out that they look very similar.
2. As students watch, place an ice cube in each liquid.

An ice cube floats in water but sinks in alcohol. 

Ask students: 

• Do you think water and isopropyl alcohol have the same density or different densities?
  The liquids must have different densities because the ice cube floats in one but sinks in the other. Explain that since ice floats in water, liquid water must be more dense than ice. Since ice sinks in isopropyl alcohol, alcohol must be less dense than ice. This means that water and isopropyl alcohol must have different densities and that the water is more dense than isopropyl alcohol.

  You could check this by comparing the mass of 50 mL of water and 50 mL of isopropyl alcohol on a balance.

Expected results 

The water will weigh more than the same volume of isopropyl alcohol (but not by much).