In this lab you will explore concepts of density and solubility by mixing three liquids in a specific order. You will continue your work at home by comparing how a can of diet soda and a can of regular soda float in water.
Materials
School Lab
vegetable or olive oil
rubbing alcohol, 91%
water
glass vial with screw top,
total volume of 100 mL or less
Density is a physical property of all matter. It is defined as the amount of mass per unit volume. To calculate density we use the formula D = m/V, where D stands for density, m for mass and V for volume. In the chemistry lab the usual units are grams for mass and milliliters for volume. Such scientific definitions are fine but it is important to build a clear idea of what density really means. First of all, density is not the same thing as weight. One pound of water and one pound of lead both have the same mass. But because lead is 11 times more dense it is 11 times smaller in volume. The water has a volume of about 1 pint while one pound of lead is smaller than one serving of 5-Hour Energy™. We say that lead feels ‘heavy’ but when we say that we’re not saying ‘heavy for its size’ when that is what we really mean.
Density also determines whether or not objects will float. Water has a density of 1 gram per milliliter (1 g/mL). When an object has a density that is less than 1 g/mL then it will float. If an object’s density is greater than 1 g/mL then it will sink. This is simple enough. For solid pieces of material and for liquids that don’t dissolve in water it is easy to predict what will sink and what will float. A solid piece of lead sinks because its density is much greater than the density of water. Solid pieces of iron, most rocks, and glass all sink in water. Except not always. A bowl made of glass will float in the dishwater. A ship made of iron will float in the ocean. Even a giant container ship which can carry over 52,000 tons. These objects can float because the density of the whole object is less than the density of water. Hollow spaces plus the volume of the material add up while the mass of the material stays the same and as a result, the ship floats. Increasing volume while keeping the same mass causes density to decrease proportionally.
When it comes to liquids another factor plays an important role in determining whether they will float in water. If they dissolve in water then they will only float or sink depending on whether they are mixed with the water. If poured carefully they may float or sink and form a layer for a while but eventually they will dissolve. Liquids that can dissolve in water are called miscible. Miscible liquids include most types of alcohol and liquid soaps. Solutions, too, are able to mix with water but sometimes may be made to form a lower layer when water is poured carefully on top of them. A solution is a mixture of a solute, such as salt or sugar, and water. Some solutions have the same density as water: adding a single drop of food coloring will not change the density of the liquid very much. Other solutions may have a much higher density than water. Usually a solid does not add much volume to the water it’s dissolved in. With more mass in an unchanged volume a solution will always have a higher density than pure water, if there is a significant amount of the solute in the water. Adding mass while keeping the same volume causes density to increase proportionally.
Immiscible liquids, on the other hand, will always form a separate layer from the water due to the fact that they do not dissolve in water. Vegetable oil is the classic example of an immiscible liquid that has a lower density than water.
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In-class Lab Procedure
Safety
The following list does not cover all possible hazards, just the ones that can be anticipated. Move slowly and carefully in the lab: haste and impatience have caused more than one accident.
If you choose not to wear safety glasses you are choosing to sit out the lab.
Isopropyl alcohol is volatile and its vapors can be irritating; use with adequate ventilation
Isopropyl alcohol is flammable; keep away from fire and sources of ignition
Mixtures of oil, water, and alcohol should be treated as hazardous waste and may not be disposed of down the drain; see your instructor for information about how to handle these materials when your work is complete
The Challenge
The challenge is to make a mixture in which there is a layer of liquid floating on top of the oil. Next, by adding a third liquid, to make the oil move up to be the top layer.
In a small, narrow container with a tight-fitting screw-top lid add a small amount of the isopropyl alcohol. Use no more than 10 mL.
To the same container add about the same amount of the vegetable oil. Make sure to leave room to add a third addition of liquid.
Put the cap on the container and give it a shake. Observe what happens and what the mixture looks like after it settles down. Record your observations in your lab notebook. Be thorough in your description and include a note about what you learn about the miscibility of these two liquids and their relative densities.
Check with your instructor before moving on to the next steps so you can get feedback on your written observations.
Take off the cap and use a pipet to add water to the bottom of the mixture. Do not let the water mix with the top layer of alcohol. Add it slowly and carefully.
Record your observations in your notebook paying careful attention to the relative density of the oil, alcohol, and water.
Check with your instructor for feedback on your observations before moving on.
Put the cap on the container and screw it down tightly. Mix the contents thoroughly and record your observations in your lab notebook.
Devise a technique to make just two measurements in order to determine the density of the alcohol you used in the lab today. Record your procedure and result in your lab notebook.
At Home Lab Procedure
Safety
There are no safety concerns for this activity as long as students stick to the recommended actions.
Solutions and Density
Water is the original beverage. All other drinks are mixtures of water with solutes and or suspended solids or gases. Cola is no exception. For this activity you need a can of diet cola and a can of regular cola.
Stop up a sink and fill it about 3/4 full with water. Or use a small wash basin.
Look carefully at each of the two cans you will use. What makes them different? Describe them. Read the labels. Heft them in your hand: can you tell whether there is a difference in their weights? If you have a kitchen scale, weigh them and record your resutls. Do they differ in volume?
Put both cans into the water and observe what happens. Record your observations in your lab notebook.
If you do not drink soda or for some reason cannot get cans of your own then you can watch the video at this link: https://youtu.be/MzsORE0ae10. This is definitely not as fun as doing it yourself but in a pinch will at least give you the main idea.
Report
Answer the following questions in a typed report. Use a numbered question and answer format.
Make a list of the liquids in the classroom lab in order from least to most dense. Justify your answer by explaining the observations which led to your conclusion.
Use the nutrition information on the oil bottle to calculate its density (hint, the serving size is given in mL and the total grams of fat will give the mass). Record the result in a small data table along with the density of water and the density of the alcohol as you determined it in the lab. Do the numbers match your conclusions from the previous question?
If your numbers do not match the qualitative results from the first question then one or more numbers must be wrong. Look into this for your numbers and explain what you find about exactly why your numbers don’t match your results.
What liquids in the experiment were miscible? Which were immiscible. Again, justify your response using observations you made in the lab.
Why did the floating alcohol layer end up on the bottom after you added the water. Consider factors of density and miscibility in your answer.
Before putting the cans of cola into the water did you discover any significant differences between the two cans? If so, what?
How did the cans behave when they were put in the water? What does it tell you about the densities of the cola in the cans?
Look carefully at the nutrition information and note any differences between the diet cola and the regular cola which you could use to explain why the two cans behaved differently in the water. Why did one float and the other not?
A can of Coca-cola™ has 39 grams of sugar per 12 oz. serving. A Snickers™ bar has only 27 grams of sugar. Knowing this can you still drink cola casually without thought of the consequences?
Grading
Answer the questions in the analysis section of this lab handout (above) in a typed document.
