Why do things with less density float?

• Demonstrate how the distribution of molecules in a substance determines its density.

• Investigate the relative densities of liquids and the relative densities of solids.

• Predict, test and explain relative density by investigating the interactions of liquids and solids.

• Demonstrate understanding of the relationship between density and buoyancy by building a boat.

Materials

• see individual activities for materials.

Background

Density, Mass & Volume

Simply put, density is how tightly “stuff” is packed into a defined space.

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.

Density is calculated using the following equation: Density = mass/volume or D = m/v.

Let’s compare three familiar substances to explore the concept of density. 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.

A pebble is heavy for its size, compared to a piece of popcorn which is light for it’s size.  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.

How can one substance have more mass per volume than another? There are a few possibilities:

1. Atoms of one substance might be a similar size yet have more mass than the atoms of another substance.
2. Atoms of one substance might be a similar mass but be smaller, so more of them fit within the same volume.
3. Atoms of one substance might be arranged in a way that allows more of them to fit in the same volume.

Any one or a combination of these explanations could be the reason why one substance has a higher density than another. In the case of copper and aluminum, their atoms are arranged similarly, but copper atoms are smaller and have more mass than aluminum atoms, giving it a higher density.

Density, Sinking and Floating
Why do some things float, while others sink? You might expect heavier objects to sink and lighter ones to float, but sometimes the opposite is true. The relative densities of an object and the liquid it is placed in determine whether that object will sink or float. An object that has a higher density than the liquid it’s in will sink. An object that has a lower density than the liquid it’s in will float.

You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking. For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Even though it’s lighter, the piece of clay has a higher density than water and therefore sinks. Even though it’s heavier, wax has a lower density than water, so the big candle floats.

Sinking and floating applies to liquids too. For example, if you add vegetable oil to water, the oil floats on top of the water because the oil has a lower density than the water.

Buoyancy and Archimedes’ Principle
The ancient Greek philosopher Archimedes found that when an object is submerged in water, it pushes aside (or displaces) an amount of water with the same mass as the object.

The water pushes upward against the object with a force (buoyancy) equal to the weight of water that is displaced.

Let’s explore Archimedes’ principle by dropping a bowling ball into a tub of water. When the ball is submerged in the water, it displaces its volume in water. According to Archimedes’ principle, the water can “push back” with a force equal to the weight of the water that has been displaced.

A litre of water has a density of 1 kilogram per litre (1 kg/L), so a bowling ball’s worth of water (4.5 L) can push back on the bowling ball with a force equal to 45 newtons (N). That’s the weight of a 4.5 kg mass. However, the weight of the ball is more like 55 N. That’s more than the buoyant force of the water it displaced, so it sinks.

A beach ball may have the same volume as a bowling ball, but it has a much smaller mass. When you a beach ball in a tub of water, it displaces the mass of water equal to its own mass—about 0.01 kg. If you were to try to push the beach ball down and displace more water, the water would push back with a force greater than the weight of the beach ball. The push of the water keeps the beach ball afloat.

Buoyancy is the upward force we need from the water to stay afloat. Buoyant forces are why we feel so much lighter when we are in a swimming pool. Our bodies are mostly water, so our density is fairly close to that of water. Because of this, an average person needs only a little bit extra buoyancy to float. A life jacket provides this extra lift.

Changing Density
You can change the density of a substance by heating it, cooling it, or by adding something to it. If an object sinks in water, it’s because the object has a higher density than the water. There are two possible ways to make that object float, however:

1. Increase the density of the water so that the water becomes denser than the object. For example, an egg will usually sink in a glass of water, because it is denser than water. Adding salt to the water increases the density of the water, allowing the egg to float. This experiment also works with people, but you need a lot of salt (try the ocean, or even better, the Dead Sea !)
2. Increase the volume of the object so that the object becomes less dense than the water. A great example of this is ice floating in water. Ice is formed by freezing water. When it freezes, it increases in volume as the water molecules move farther apart to accommodate the lattice structure of ice. Because the ice is now less dense than water, it floats. This phenomenon also explains why ships float even though they are made of steel. A ship is built in such a way that it encloses large amounts of open space. The ship still displaces its weight in water, but because of the way the ship is constructed, it takes up more space than the volume of the water it displaces, so it floats.
   
   

Vocabulary

Archimedes: Greek mathematician, physicist, engineer, inventor and astronomer (c. 287 BC–c. 212 BC).
Archimedes’ principle: Any object wholly or partially immersed in a fluid is buoyed by a force equal to the weight of the fluid that is displaced by the object. In other words, the buoyancy is equal to the weight of the displaced fluid.
buoyancy: The upward force that a fluid exerts on an object less dense than itself; the ability to float.
density: How closely packed together the molecules of an object or substance are.
displace: To push out of the way. For example, when an object goes into water, it displaces the water.
immiscible: Unable to be mixed together, like oil and water.
ironwood: The name for a large number of woods that have a reputation for hardness and high density.
mass: The amount of matter in a given space.
matter: The substance that makes up all physical things.
pumice stone: Lava froth known for its small mass and low density, despite looking like a rock.
weight: A measure of the force of gravity on an object.
volume: The amount of space a substance or object takes up.

Other Resources

BrainPOP | Science | Matter & Chemistry | Measuring Matter

EDinformatics | Mass, Volume, Density

WatchKnowLearn.org | Buoyancy and Density

ProTeacher Collection | Density

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 substance has to do with how heavy it is compared to the size of the object. Students will also be able to explain that density is a characteristic property of a substance.

Key Concepts

• Density is a measure of how heavy something is compared to its size.
• If an object is more dense than water it will sink when placed in water, and if it is less dense than water it will float.
• Density is a characteristic property of a substance and doesn’t depend on the amount of substance.

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

NGSS Alignment

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

Note: The NGSS Structure and Properties of Matter for 5th grade, Performance Expectation 5-PS1-3 states “density is not intended as an identifiable property. Assessment does not include density or distinguishing mass and weight.”

Although the standard does not call for density to be used as a characteristic property to identify a substance, a basic introduction to density is included here as an optional element of a learning progression leading up to a middle school understanding of density.

Summary

• Students are introduced to the concept of density and that density has to do with how heavy something is relative to its size.
• Students are also introduced to the idea that whether a substance sinks or floats in water is a characteristic property of that substance and does not depend on the amount of the substance.
• Students also learn that if an object is more dense than water it will sink when placed in water, and if it is less dense than water it will float.

Evaluation

Download the student activity sheet 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 glasses or goggles.

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

• Clay (1 ball for each group)
• Clear plastic container of water
• Popsicle sticks (20)
• Rubber band

• Ball of clay
• Clear plastic container of water

1. Hold up a ball of clay about the size of a ping pong ball and ask students if they think it will sink or float in water.
2. Place the clay in the water.

The clay ball will sink.

• Do you think that if I use ½-the amount of clay that it will float or sink in water?

1. Pinch off about ½ the clay, form it into a ball, and then put it in the water.

It will sink.

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 put smaller and smaller pieces of clay in water.  

Question to investigate: Does changing the amount of material in an object change the object’s density?  

Materials for each group

• Clear plastic container of water
• Clay ball (about the size of a marble)

1.  Place your ball of clay in the water to see if it floats of sinks.
2.  Break off about ½ of the original clay ball, form it into a ball, and place it in the water. Does it float or sink?
3. Break off another ½ of this smaller ball, form it into a ball and test to see whether it sinks or floats.
4. Repeat this process two more times to make smaller and smaller clay balls. Test each one to see if it sinks or floats.

• Do you think clay is more dense than water or less dense than water?
  More dense

Note: If students try a very tiny piece of clay, it may stay on the surface of the water due to the water’s surface tension. If students dip the tiny piece of clay in the water beforehand and then put it back on the surface of the water, it should sink.

3. Demonstrate that wood is less dense than water.

Materials for the demonstration

• Popsicle sticks (20)
• Rubber band
• Clear plastic container
• Water

Procedure

1. Place 1 popsicle stick in the water to see if it sinks or floats.

Ask students:

• Do you think wood is more dense than water or less dense than water?
  Less dense
• What do you think will happen if we tie 20 popsicle sticks together – will the bundle of sticks sink or float?
  Float

Explain

4. Use an animation to explain some basics about density.

Show the Animation – Density: Clay and Water

Explain that density has to do with how heavy something is compared to its size. To compare the density of two substances like clay and water, you can compare the weight of the same “size” or volume, of each substance.

As you show the animation, explain that since a piece of clay weighs more than the same amount, or volume, of water, clay is more dense than water. Since clay is more dense than water, a ball of clay sinks in water, no matter how big or small the ball of clay is. 

Show the Animation – Density: Wood and Water

If you compared the weight of wood and an equal amount, or volume, of water the sample of wood would weigh less than the sample of water. This means that wood is less dense than water. Since wood is less dense than water, wood floats in water, no matter how big or small the piece of wood is.  

Extend

5. Show photos to explain how the density of an object can be changed.  

The key to floating is being light for your size. So if you can add size to an object without adding much weight, the object will be lighter relative to its size. This means that the density of the overall object will decrease and be more likely to float.

Ask students to describe how this principle can be used to explain how a lifejacket can help someone float in water.

The key to sinking is being heavy for your size. If you can add weight to an object without adding much size, the object will be heavier relative to its size. This means that the density of the overall object will increase and be more likely to sink. 

Ask students to explain how this principle can be used to explain how a weight belt can help a SCUBA diver sink in water when they might otherwise float. 

Note: A student might want to know why a boat made out of steel can float when steel is more dense than water. This is not an easy question and requires a different approach than what students have seen so far. We do not necessarily recommend the following explanation for 5th graders but here is the idea:

An object floats when it displaces a volume of water that has a mass equal to the mass of the object. So if a material like steel is shaped into a boat and made larger and larger, it will displace more and more water.  When it is large enough to displace a volume of water that has a mass equal to the mass of the boat, the boat will float.