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Objective: Define Density.
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MatchCard Information Pieces are used to create the density formula (mass divided by volume) and identify which objects have greater density.
Projects: Given a set of irregular objects, students arrange them by their estimates of most to least dense. Calculate the density of the objects using a scale and water displacement. Learn a mnemonic for the density formula.
Here's Your Copy of the Density Formula MatchCard
This is MatchCard #10 of the Chemistry Unit Study. Find more information on MatchCard Science below.
Let's Learn the Density Formula
Density equals Mass divided by Volume
Have a large object and a small object on hand to show the student(s). The large item should have less density by greater weight because of its size.
For instance, you could have a small rock, gold ring, or quarter for the little object with greater density. Then you might use a large bouncing ball or plastic dinner plate as the larger item with less density.
Ask the students which item has greater mass. The answer should be the larger item.
Then ask which would have greater mass if they both were the same size. The answer would be the smaller item.
Then ask: "How can we compare the mass of these two objects of different sizes?"
Give the student time to think about and compose the answer to this question.
The Density Formula
Explain that chemists use the density formula to compare the mass of objects. The formula is listed above: mass divided by volume.
So what two things do we need to determine density?
Review the difference between mass and weight. Mass stays the same and is not afected by gravity. Weight changes with gravity. Your mass would be the same on the moon or on earth; whereas your weight would be different.
Hands On Learning With the Density Formula
Students benefit from practice using density. First, give them a set of relatively small objects with various densities. These objects could include:
Basically, any items that will fit in a cup and do not dissolve in water will work. Try to get five to eight different items to work with.
- Different coins
- Several small or medium rocks
- Marble, lego, small toys
- Paper clip
Let them make a list of the items: from most to least dense. Have them guess while handling the objects.
Write it down, so they can compare it later.
Mass is determined by a scientific scale. If you don't have a scientific scale, you can use a small scale like a postal or diet scale to measure weight. You do want to use metric so convert to grams.
If you do not have an accurate enough scale, you may need to measure a number of items. For instance, when measuring the density of pennies, it helps to get the mass and volume of four or five pennies rather than just one, unless you have a scientific scale with precision for tenths of grams.
Let them try to figure out how they might measure volume of irregularly shaped items.
You may be able to measure by dropping the object into a beaker or cylinder and determining how much the volume in the container increases by milileters.
The best way is to use water displacement. It would be great if you happened to have a water displacement measurer. But in the very likely case you do not have one, here is how to make one:
- Puncture a hole in a disposable or Styrofoam cup.
- Place the cup inside a measuring cup that has metric measurements (usually milliliters).
- Fill the cup to the level of the hole.
- Drop the object into the cup. It will displace the volume of the object so that the same volume of water spills out of the hole and into the measuring cup.
- Measure the volume of water in the cup.
The fact that some water will stick to the sides of the cup makes this less accurate than if you had an official water displacement cup. However, it does give an estimate of the volume.
Liquid volume of milliliters (ml) can be converted to solid volume of cubic centimeters (cc).
Now use the density formula of mass divided by volume to determine the density of each of your objects. List them from most to least dense.
Now compare the list of actual densities to their list of estimates before the activity. How close were they?
Trick for Remembering the Density Formula
It’s an old mnemonic, but it works! Students can remember the formula by thinking of the phrase “dearly beloved”. (Some people use the phrarse “dumb love” or even “dense love” but I think “dearly beloved” is a little nicer, even if a bit archaic sounding.).
First, write down the density formula using only the first letter of the three words. It would be D = M/V. (Don't make the division sign at a slant but straight across, parallel to the bottom of the paper.)
The "D" stands for "dearly" or "dear." Then draw the M and V to look like a heart.
The "M" forms the top of the heart. Draw it so it looks like the famous golden arches of a fast food restaurant.
Then draw the "V" as the bottom of the heart.
Now you have the density formula in a picture they will never forget!