Chapter 8: Density describes the amount of mass in a given ...



Chapter 8: Density describes the amount of mass in a given volume of a substance.

Section 8.1 Defining Density

To understand density we must first revisit the terms mass and volume:

Mass: the amount of matter in a substance.

Volume: the amount of space occupied by the substance.

So density is:

*A measure of the mass contained in a given volume.

*A substance with a lower density will float on substances with higher densities.

DENSITY AND THE PTM

(Particle Theory of Matter)

The PTM states that different substances have different sized particles.

The PTM also states that there are spaces between the particles.

The greater the spaces between the particles, the less particles therefore the lower the density.

Ex. Water vapour has a lower density than liquid water.

In general, gases are less dense than liquids and liquids are less dense than solids.

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So how are the state of matter and the density related in real life situations?

- Solid objects can move easily through liquids and gases. Take for example a diver. According to the particle theory the fluid properties of water and air allow the particles in these substances to move out of the way of the solid body of the diver.

- The particles in solids are very strongly attracted to one another and cannot be easily pushed apart. The attraction is stronger then the attraction between the particles in a liquid.

- Particles in a solid cannot slide past one another but this can happen in a liquid. So when a rock gets thrown in water the water particle slide apart to let the solid through as they are not strong enough to be kept together.

Section 8.2: Calculating Density

So as we know density is the mass of a given volume.

To calculate density we need to know the mass

and the volume.

To measure a regular shaped solid you can use a volume formula (ex. V=l x w x h) to calculate the volume. Then use a scale to get the mass ( grams/kilograms) . From these you can calculate density.

To measure the volume of an irregular shaped

solid , you will need to determine the amount

of water it displaces.

Displacement is the amount of space that an object takes up when placed in a fluid.

The amount of fluids displaced is the volume of your object.

SAMPLE PROBLEMS

1. Find the density of a 10g mass of a substance that has a volume of 2.0 cm³.

D=m D=10g D=5.0g/cm³

V 2.0cm³

2. You want to put 10g of salt into a container.

What is the volume of the container if the salt completely fills it?Use your pyramid to figure out the formula for volume!

Step 1: V = m

D

Step 2: So m = 10g

Step 3: To get density use the table on page 312.

D= 2.16 g/ cm3

Step 4: So V = m = 10 g = 4.63 cm3

D 2.16 g/cm3

3. What is the mass of 1500mL of helium?

m= D x V

m= 0.0002g/mL x 1500mL

m= 0.3g

Section 8.3: TEMPERATURE& DENSITY

Increasing temperature will cause the particles in a substance in gain energy, spread out and take up less space.

This results in decreased density.

EXAMPLES/ APPLICATIONS in everyday life:

*Hot air balloons



* Warm vs. cool tire pressure

*Water in each of its three states (water as a solid is less dense as the particles move apart as they freeze)

*Drying wood

*Swim bladder (fish)

*Submarines

* Lava Lamp

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