M Density and the Particle Theory

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Figure 5.1 There is very little space between the people on a crowded elevator — they are densely packed together.

Have you ever been on a crowded elevator? As Figure 5.1 shows, i t is definitely uncomfortable when too many people are jammed together tight­ly, or densely, on an elevator.

Using everyday words, density can be described as the "crowdedness" of the particles that make up matter. In scientific terms, density is the amount of a substance that occupies a particular space. When you describe a substance as being "heavy" or "light," you are referring to the property of density.

According to the particle theory, different sub­stances have different-sized particles. The size of the particles determines how many particles can "f i t into" a given space. Therefore, each substance has its own unique density, based on particle size. Furthermore, the particle theory suggests that there is empty space between the particles of mat­ter. Could as many people fit onto an elevator i f each person were surrounded by a large "spacing box"? Would larger spaces among the people increase or reduce the density (crowdedness) of the people travelling on the elevator? The answer is shown in Figure 5.2.

Density of Solids, Liquids, and Gases How is the density of a substance related to the substance's physical state? Imagine filling a film container with liquid water and another f i lm container with water vapour. Both liquid water and water vapour are the same substance and therefore have parti­cles of the same size. According to the particle theory, gas particles have more space between them than do liquid particles. Therefore, the water vapour in the container would have fewer particles than the liquid water. I t would be reasonable to conclude that the density of the water vapour is less than the density of liquid water.

For tips on how to make and use a

Science Log, turn to page 534.

PauseSc Reflect

Imagine you were to fill three balloons to the same size: one with plaster, which would harden solid; the second with water, a liquid; and the third with air, a mixture of gases. In your Science Log, draw these three balloons. Draw O's or X's to represent the particles and to show how the particles are spread out inside each balloon. Use the particle theory to try to explain how and why the density of the substances inside each balloon varies.

132 Fantastic Fluids

DidYouKnow?

O O 0

Figure 5.2 Increasing the spaces between people on an elevator will reduce the density.

Could the solid state be less dense than the liquid state of the same sub­stance? The answer is yes. You have probably seen ice cubes floating in a glass of water. As solid ice forms from liquid water, the ice expands. This means that there is more empty space trapped in a chunk ot ice than in the same amount of water Thus, solid ice is less dense than liquid water.

How are density and state of matter related to the physical properties of a sub­stance? Solid objects can move easily through liquids and gases. For example, dol­phins can leap through the air and then dive back underwater so smoothly that the activity appears almost effortless, as shown in Figure 5.3. According to the particle theory, the fluid properties of water and of air allow water particles and air parti­cles to move out ot the way of the firmer, non-fluid bodies of marine animals. V\Tiy do solid particles tend to hold together while fluid particles tend to move apart?

Figure 5.3 The particles of a fluid move apart easily when a solid object, such as a dolphin, travels through the fluid.

Density and Buoyancy 133

When an object moves through a fluid, i t pushes

particles apart and moves between them. Particles in

a solid cannot be pushed apart. To understand why,

imagine that you and a few friends are together. You

want to prevent anyone else from pushing your group

apart and moving between you. What would you do?

First, you would have to stand quite close together.

Then you would probably hold on to each other very

tightiy. I f you do not let go of one another, no one can

move between you. That is what particles in a solid do.

Attractive forces among the particles of a solid are

stronger than those between fluid particles and thus th(

particles in a solid cannot be pushed apart.

Figure 5.4 The cartoonized weight-lifters represent the tightly packed particles in the solid plate. The plate can hold its shape and support materials such as the large rock, because the weight-lifters are holding on to each other very tightly.

Differing Densities Using your knowledge of the particle theory, what

inferences can you make about the densities of

different substances?

What You Need S I

3-5 class sets of numbered film containers (pre­

pared by your teacher), completely filled with var­

ious "mystery substances"

^ ^ ^ ^ • ' ^ ^ ^ ^ ^ ^ ^ ^ ^

What to Do

1. Make a table with the following headings:

Container Rank order (heaviest Substance number to lightest) (your guess) (actual)

Find Our

2. With a partner, pick up each film container

and observe how heavy or how light each

one feels. Decide on the order of the film

containers from heaviest to lightest. In your

table, record the numbers of the film contain­

ers, from heaviest to lightest.

3. Predict what the substance inside each con-

.'^.^tainer might be. Record your predictions.

4. Find out what the substances are by check-

ing your teacher's master list. Record the

actual substances in the table.

What Did You Find Out?

1. Which substances did you predict correctly

(or closely)? Which substances, if any, sur­

prised you?

2. The volumes of the substances were identical

because each container was filled to the top.

Why is it important to keep the volumes

equal in this activity?

3. Make a general statement about the sub­

stances, in terms of the density of the

particles.

134 Fantastic Fluids

Figure 5.5 Although liquid particles are sometimes quite closely packed together, they cannot support objects in the same way that solids can, because the particles do not have a strong enough attraction for each other Thus, liquid particles move apart easily.

I f you were to step onto the surface of a lake, the water would not support your foot. Instead, your foot would go right through the water. In fact, you would continue to fall through the water, pushing the water particles out of the way, as shown in Figure 5.5. Liquids cannot support objects in the same way that sohds can, because the particles of a liquid move apart easily, allowing a dense, solid object, such as your foot, to pass through the liquid. The attractive forces between liquid particles are not strong enough to pre­vent your foot from pushing them apart.

Similarly, you cannot walk on air, because gases are even less dense than solids or liquids. When you move through air, you are moving through mostly empty space. You do not have to move as many particles of air out of the way as you do in*water (see Figure 5.6). This explains why running through air is much easier and faster than running through water. In general, gases are

less dense than liquids.

DidYnuKnow? "Empty space" does not mean "air" Empty space means a separation between two objects with nothing between those objects, not even air parti­cles. Simply because empty space and air /oo/(the same does not mean they are the same! For example, outer space is mostly empty space, but it has no air. Astronauts would suffocate if they were to venture out­side their spacecraft with­out masks and air tanks, because there is no oxygen in outer space.

Figure 5.6 When you move through air, you do not have to move as many particles of air out of the way as you do water particles in water

Density and Buoyancy 135

As temperature increases, a substance wil l change from solid, to liquid, to gas.

The particle theory states that the particles of a substance spread out as they gain

energy when heated. Thus, the particles take up more space, which means that the

density of the substance decreases. I t is almost always true that, for each pure sub­

stance (for example, gold), the density of its solid state is greater than the density

of its liquid state. The substance's sohd state and Hquid state are, in tum, denser

than its gaseous state.

In some cases, the densities of two substances can be so different that the liquid

state of one is denser than the soUd state of the other! One example of this is

shown in Figure 5.7A Many solid metals, such as copper, nickel, and silver, can

float on mercury, one of the densest substances known. A more familiar example

of differing densities is shown iti Figure 5.7B.

Figure 5.7A Liquid mercury is so dense tliat it can support a solid iron bolt. A layer of oil has been placed on top of the mercury to prevent vapour from escaping into the surrounding air.

Figure 5.7B A solid block of wood floats easily on the surface of liquid water.

Check Your Understanding 1. In your own words, explain what density means.

2. Explain why solids can support objects more easily than fluids can.

3. What is the only way in which the density of a pure substance can change?

4. Apply Find some small items in the classroom (for example, pencils or paper

clips) and determine whether these items are denser than water by dropping

them into a container fiiU of water. Organize your observations in a table.

5. Apply Butter is made by intentionally changing the density of liquid milk.

Find out how this is done and write a brief description. Explain the change in

terms of the particle theory.

6. T h i n k i n g Critiically The density of molten lava increases as i t cools and

hardens. List other examples of natural changes in density.

136 Fantastic Fluids