Our laboratory today was inspired by the large glass of soft drink featured in the beginning of Chapter 1 of the textbook. Keep in mind what you read about “Soda Pop Fizz” while conducting these observations and experiments.
For our first laboratory, I’ll help you fill out your notebook by writing out the sections you will need to add in purple. You don’t need to write everything out word for word, but make sure what you write is understandable to someone else who might read it.
Experimental Title: Lab 1. Density of Liquids: Soft Drinks and Water
Date of laboratory: To be performed June 3, 2014
Purpose: We are going to look at a common physical property of substances, density, and how it could be used to identify an unknown substance in the real world. At the same time, we will be learning how to use some common laboratory equipment and practice measuring volume of liquids accurately.
The purpose of this laboratory is to determine and compare the density of regular and diet soft drinks to water, as well as investigate how density could be used to identify an unknown substance.
Below is a video that explains density nicely. Before you watch:
Note 1: You will not be responsible for the terms intensive and extensive property in this course, because they are not used in the textbook. Just be aware that those are ways to categorize properties of matter based on whether they change with the amount of matter present or not.
Note 2: We will be using the method he demonstrates for finding the volume of an irregularly-shaped object in lab next week.
(There is a pop-up ad).
You may want to summarize what you have learned about density in the Introduction section of your notebook. At the very least include:
Density can be calculated using the formula:
density= mass (g)/volume(mL)
Because the volume of a substance changes with temperature, density also changes with temperature.
Special safety concerns for Lab 1:
- Although we will not need goggles or gloves today, please do not ever eat or drink anything during chemistry laboratories.
- If anything spills, please clean it up immediately with a paper towel and let your instructor know.
- If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
- Large plastic bin or sink
- Cans of regular soft drink
- Cans of diet soft drink
- Unknown soft drink labelled A
- Unknown soft drink labelled B
- Graduated cylinders
- Table top scales
- Transfer pipette
Part 1. Observe the density of soft drink cans placed in a large container filled with tap water.
Do you expect the cans of soft drink to float, sink, or maintain neutral buoyancy in tap water? Given that the cans likely contain an equal amount of aluminum and gases, do you expect any differences in how the cans will behave?
Fill a plastic storage bin with tap water. Place cans of regular soft drink and diet soft drink in the water. Observe whether the cans float or sink.
Leave room to record your observations in your notebook. Did what you observe match your predictions?
Part 2. Determine the density of unopened cans of soda
- Turn on the Salter kitchen scale.
- Make sure it reads zero.
- Place an unopened can of soda on the scale.
- Record the kind of soda in your notebook.
- Read and record the mass of the can in grams.
- Locate the volume of the can in ml and record it in your notebook.
- Repeat steps 2-6 until you’ve weighed 4 cans of regular soda and 4 cans of diet soda
- Using a calculator, calculate the density.
Include a table like this one in your notebook, so you’ll have room to record your data.
Soft drinks are actually complex mixtures containing a variety of substances such as colors, flavors, acids, sweeteners, preservatives, and caffeine. Remember from the life hacks video in the first lesson? It revealed that soft drinks contain phosphoric acid, which gives soft drinks a tangy taste. Phosphoric acid can also acts as a preservative, keeping the contents of the bottle fresh. Which of these ingredients, if any, might explain the densities we observed?
Read the nutrition facts of the a regular soda and a diet soda. Do you notice any large differences between diet and regular? How much of that substance is in regular soda? How much in diet? Is it safe to assume that the other ingredients will be roughly the same between diet and regular sodas of the same brand?
Part 3. Determine the density of tap water
In this part, you will be measuring the volume of water as well as weighing it.
Hint 1: When using a graduated cylinder, remember that water tends to creep up the sides of the container. This forms what is called a meniscus.
Hint 2: Because many people will be using the scales, go ahead and weigh your group’s graduated cylinder prior to use rather than using the tare feature on the scale. Remember to subtract that weight from your total each time.
Hint 3: There may be a residual of liquid in your graduated cylinder. If the liquid is another substance, other than the one you are measuring, that residual might interfere with your results. Rather than trying to wash and dry the graduated cylinder each time, an old chemistry trick is to simply rinse it twice with a small amount of the test liquid before proceeding.
For example, you just took the density of water and there is a few drops of water left in the bottom. Now you want to measure the mass of regular Pepsi®. Pour a small amount of Pepsi® into the cylinder, swirl it around and discard it (Never pour it back into the original container!) Repeat, and then you will be ready to proceed.
- Obtain a graduated cylinder, transfer pipette and beaker full of water.
- Weigh the graduated cylinder and record the mass in grams.
- Fill the graduated cylinder with 20 ml of water. Use the transfer pipette to fill to exactly 20 ml. Record the volume in your notebook.
- Weigh the cylinder plus the water and record the mass in grams.
- Subtract the weight of the graduated cylinder.
- Repeat steps 3 -5 with 40 ml of water and 60 ml of water.
- Calculate the density for each sample.
Example table for data:
Now graph your data. The volume goes on the x-axis and the mass on the y-axis. (If you don’t have a quadrille-ruled notebook, see you instructor for graph paper).
Water is known to have a density very close to 1 g/ml. Is this what you observed? What factors might have changed the densities? What errors might have occurred during measuring?
Check with the instructor for a density of water chart. If your values are significantly different, revise your techniques using the hints above and try again.
You might want to use this information to help you with the next part.
Part 4. Identify unknown samples.
Scenario: Imagine you are working in a restaurant. Someone has filled two taps, one with diet soft drink and one with regular soft drink. Unfortunately, no one knows which is which. The store manager doesn’t want to serve the public the wrong soft drink, and the two taste similar enough that there are some questions. Should she throw out the soft drinks, or can you tell which is regular and which is diet using chemistry?
You will be given:
- Recently-opened soft drink labelled “unknown A”
- Recently-opened soft drink labelled “unknown B”
- Previously-opened soft drink of type A labelled “flat unknown A”
- Previously-opened soft drink of type B labelled “flat unknown B”
- Equipment from previous parts of this lab
Use what you have learned in the previous parts of this lab to design a procedure to figure out which is regular soda and which is diet. Be sure to take careful notes of what you do and what your results are.
When you are finished, check with the instructor to see if your techniques correctly identified the types of soda and saved the restaurant some money.
Once you have completed the four parts, sit down and write a sentence or two to explain the results of each part.
Record any thoughts you have about the experiments, including:
- Possible improvements to the procedures or how to tweak techniques
- How the results differed from your expectations
- Suggestions for other experiments
- What key concepts you learned about density of liquids
We’ll go over the key concepts together at the end of lab.
Thank you for going over the lab ahead of time and getting your lab notebook ready. It will definitely help our first lab go more smoothly.
Please leave a comment or send an e-mail if you have any questions before our first meeting.