You can't easily estimate an object's density by simply looking at it. You have to think about the relationship between two of its properties: mass and volume. In these activities, students will learn about mass , volume and density and their relationship to each other by conducting investigations into buoyancy : floating and sinking objects used in their everyday lives. Dense, Denser, Densest! Up or Down? Demonstrate how the distribution of molecules in a substance determines its density. Predict, test and explain relative density by investigating the interactions of liquids and solids.
For example, a suitcase jam-packed with clothes and souvenirs has a high density, while the same suitcase containing two pairs of underwear has low density.
Size-wise, both suitcases look the same, but their density depends on the relationship between their mass and volume. Mass is the amount of matter in an object. Volume is the amount of space that an object takes up in three dimensions. If we take the same volume one cubic centimetre of foam, wood and concrete, we can see that each has a different mass. Less Dense, More Dense If something is heavy for its size, it has a high density.
If an object is light for its size it has a low density. Imagine a big bowl of popcorn, compared to a big bowl of pebbles, which would feel heavier?
It is easy to estimate relative densities if you keep either the volume or the mass of two objects the same. If you filled one bag with a kg of feathers and another with a kg of lead you would see that the feathers take up much more room, even though both bags have the same mass.
This because feathers are less dense, they have less mass per volume. If you made a copper cube and an aluminum cube of the same volume and placed one in each hand, you would be able to feel that the copper cube would be heavier. Copper has more mass per volume than aluminum. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 3. Engage Do a demonstration to show that the wax is heavier than the clay but that the wax floats and the clay sinks.
Materials for the demonstration 1 tea light candle Clay Clear plastic container Water Large balance Teacher preparation Use a small enough piece of clay so that you are sure that the candle weighs more than the clay. Procedure Place a piece of clay that weighs less than a tea light candle on one end of a balance.
Remove the candle from its metal container and place the candle on the other end of the balance. Expected results Even though the candle weighs more than the clay, the candle floats and the clay sinks. Explore Have students compare the density of water, wax, and clay. Question to investigate Why does a heavier candle float and a lighter piece of clay sink?
Materials for each group 2 tea light candles in their metal containers Clay Water in cup Small balance Tape Dropper Procedure Compare the density of wax and water Roll two pieces of tape and stick them to the center of the pan at each end of the balance.
Attach each tea light candle to the tape so that each candle is in the center of the pan. Use the wick to pull one candle out of its container. Expected results The water has a greater mass than an equal volume of wax. Ask students: Which weighs more, wax or an equal volume of water? Water weighs more than an equal volume of wax.
Which is more dense, wax or water? Water is more dense. Compare the density of clay and water Make sure you have one piece of tape in the center of each pan on the balance. Fill one container with clay and place it on the tape so that it is in the center of the pan. Place an empty container on the tape at the opposite end of the balance. Slowly and carefully add water to the empty container until it is full.
Expected results The clay has a greater mass than an equal volume of water. Ask students: Which weighs more, the clay or an equal volume of water? However, pupils faced with a heavy block of wood, perhaps too heavy for them to lift, and a lightweight paper clip, will make the wrong predictions.
They will predict that floating and sinking depends on the gravity force and not on an intrinsic property of the material, which is the density. Tangled up in this misunderstanding is the issue of density and also the language we use to compare things. What do larger or bigger mean? To say that a block of metal sinks in water because the mass is greater is only part of the explanation. Its mass is greater than the mass of the same volume of water. Density is the key concept behind these ideas and it is not an easy concept.
It involves the relationship between two quantities, mass and volume. Each of these is hard enough to understand on its own. The upwards push of the water increases with the volume of the object that is under water; it is not affected by the depth of the water or the amount of water.
If the weight force down is larger than the upward push of the water on the object then the object will sink. If the reverse is true then the object will rise — rising is the opposite of sinking. Different objects float at different levels in the water because as most regular objects are lowered into the surface of water, the upward push of the water steadily increases until it is in balance with the weight force of the object, and the object then continues floating at this level with the two forces in balance.
Many objects that are hollow and so generally contain air float because the hollow sections increase the volume of the object and so the upwards push for very little increase in weight force down. However, it is not necessary for an object to contain air in order to float.
Explore the relationships between ideas about floating and sinking in the Concept Development Maps : Laws of Motion. Students will be familiar with the idea that objects have weight and that the size of the weight force is determined by the type of material and how much of it is used to make up the object. Students should learn that when an object floats the water is pushing upwards on the object.
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