The Density Lab -Answer Sheet- Part A:

(1)

LAB FOLDER #: _______

Name: ________________________________

Partner: _________________________

Due Date: ______________

The Density Lab

-Answer Sheet- Part A:

1. Volume =_____________________

Show work for #1:

a. Equation b. Plug-In Values c. Final Answer

2. Graph #1 Graph #2

a. Explanation for Graph #1:

______________________________________________________________________________

b. Explanation for Graph #2:

______________________________________________________________________________

3.

a. ____________________

b. ____________________

c. ____________________

HEAT

DENSITY

PRESSURE

DENSITY

MASS (g)

VOLUME (ml)

(2)

4.

_______________________________________________________________

5.

Henry’s Work:

Suzette’s Work:

Who is closer to the Correct Answer? _________________________

6.

PART B – Regents Questions:

1. _______

2. _______

3. _______

4. _______

5. _______

6. _______

7. _______

8. _______

9. _______

10. _______

(3)

Original KMP, Modified by C. Carroll, CHSN E.S. Dept 2011-12 3

Density Concepts Lab

DO NOT WRITE ON LAB – CLASS COPY!!!

BACKGROUND:

Density is a measure of the CONCENTRATION of matter (mass) in a given space.

You have seen that some things are “heavier” than others, and that the difference is more than just a difference in size or volume. If you had samples of many different materials, all the same volume, you could classify them by weight from heaviest to lightest. Since all the samples have the same volume, the heavier ones have more mass packed into the same volume than the lighter ones do. This is what is meant by density. It is a characteristic of a material that DOES NOT depend on the size of the sample.

DENSITY is defined as the MASS PER UNIT VOLUME of a sample of matter. Density is the number of grams in 1 cubic centimeter. Mass being measure in grams and volume measured in cubic centimeters.

Finding Density: Density = Mass Volume

The density of a material is its mass per unit of volume. Density depends partly on the mass of the particles in a substance and partly on how closely they are packed.

EFFECT OF MOLECULAR STRUCTURE ON DENSITY:

In Figure B, the density is greater than in Figure A, because there are more particles (with the same mass) in the same volume. In Figure C, the density is greater than in Figure A because the particles in Figure C have more mass.

EFFECT OF MASS & VOLUME ON DENSITY:

To understand the usefulness of the idea of density look at the following diagram. In A, we see a bar of aluminum that has a mass of 270 grams and a volume of 100 cubic centimeters (100 ³ cm). Applying the density formula, we find that the density of the bar is 2.7 grams per cubic centimeter.

(D= 2.7 g/cm³)

In B, the bar has been cut in half. Each piece now has only half of the mass of the original

piece – 135 g. But the volume of each is also only half as large – 50 cm³. When we calculate the density of each piece, it comes out to the same value – 2.7 g/cm³.

Looking at the diagram you can see the density of aluminum is the same regardless of the size, shape, or mass of the sample. This is true of any pure substance.

Bar A:

Mass = 270 g Volume = 100 cm³ Density = 2.7 g/cm³

Bars B (A cut in half) Mass of each = 135 g Volume of each = 50 cm³ Density = 2.7 g/cm³ 1cm

20cm 5cm 5cm

10cm 10cm

1cm

(4)

EFFECT OF DIFFERENT SUBSTANCES ON DENSITY:

The ratio of the mass of a pure substance to its volume should be the same no matter how large or small a piece we have. In short the density of a piece of aluminum is always the same, regardless of the size shape or mass.

On the other hand, the density of some other material would be different. In the next diagram we have a bar of iron. It has exactly the same dimensions as the bar of aluminum. Its volume is therefore the same 100 cm³, but its mass is much greater - 790 grams. The density of iron is 7.9 g/cm³. Its density is almost three times as great as pure aluminum.

Every pure substance has its own density. Density is therefore a characteristic that helps to identify a material.

We have said that each substance has a definite density that is characteristic of the material. This is strictly true only when the temperature and pressure remain the same. This is important in the case of gases. It is also important to remember that only pure substances have a definite density.

CHANGING THE DENSITY OF A SUBSTANCE

You have probably heard it said that cold air is heavier than warm air, and that is why cold air sinks and warm air rises. The statement is made more scientifically correct by changing the word “heavier” to

“denser”.

Heating a gas adds energy to it. A gases molecules begin to move more rapidly, they collide with more force, and they therefore tend to spread apart. Thus a heated gas tends to become less dense.

Removing energy by cooling the gas tends to make it more dense. These changes in density occur only if the gas is able to expand or contract. In a sealed container, heating a gas raises the pressure, rather than lowering the density, since the agitated molecules have no way to get out.

Solids and liquids also expand and contract with changes in temperature, but to a much lesser extent than gases. Therefore their densities are also affected to a much lesser extent.

Pressure is another factor that affects the density of gases. (Solids and liquids are also affected by pressure changes, but to a lesser extent.) Increasing the pressure packs the molecules closer together.

The gas contracts. This means that more molecules can fit in a given volume, and so the density is increased. If the pressure is reduced, the gas molecules move farther apart. The gas expands. So there are fewer molecules in a given volume, and the density is decreased.

DENSITY AND THE STATES OF MATTER:

Heating or cooling a substance can cause other changes besides a change in volume or density. Heating may cause the material to melt, evaporate, or boil; cooling may cause it to condense or freeze. In other words, it may change its physical state or phase.

The three state of matter are solid, liquid, or gas. Elements and many chemical compounds change state at definite temperatures, called their boiling and melting points. As a substance changes state, its volume usually changes sharply also, especially between the gas and liquid states. Its density therefore changes in the opposite direction.

1cm

20cm 5cm 1cm

20cm 5cm

Aluminum:

Iron:

(5)

Water is the only substance that we find naturally in all three states on the earth. In fact, wherever the temperature is below the freezing point ( 0° C), we are likely to find all three state of water present at the same time.

Ice floats in water because it is less dense than water in its liquid phase. An object floats in a liquid only if the object is less dense than the liquid. Most substances are more dense in the solid state than they are in the liquid state. Therefore, a solid normally sinks in its own liquid. But ice floats in water, the solid form floats in its liquid form. Thus ice must be less dense than liquid water.

As water changes from a liquid to a gas, it is doing what other substances do, which is to become much less dense in the gaseous state. But in the process of warming, up from its melting point at

0°C, water behaves strangely. Normally, a liquid expands as it is heated. But here we see that as the temperature of the water increases from 0°C, its volume decreases at first. Its volume does not begin to increase until its temperature passes 4°C.

What this means is that the maximum density of water occurs in the liquid state, at a temperature of 4°C.

At 0°C, the density of water is slightly less than at 4°C, and the density of ice is even less than that.

FACTS:

Below 4°C water expands slightly and becomes less dense. Ice is less dense than water at any temperature.

If you measure the temperatures in a frozen pond, the water at the bottom which has to be the densest must be at 4°C. The ice on the top must be at 0°C or lower. So the water in between must gradually increase in temperature from 0° to 4° as you go deeper.

Because water near 0°C is less dense than water at 4°C, and because ice, when it forms, floats on water, ponds and lakes freeze from the top down. The ice acts as an insulator for the water below it, most lakes do not freeze all the way through in the winter. This means that water organisms can survive the winter quite nicely in the protected waters near the bottom of the ponds.

IMPORTANT: Unlike most other substances, water decreases in volume when it changes from a solid (ice) to a liquid. Like most other substances, it increases in volume when it changes from a liquid to a gas.

The volume of a given quantity of water is smallest at 4°C.

When an object is put under pressure,

the density increases. (Same mass but in a smaller volume)

When you heat an object, the volume expands, mass remains the same and therefore the density decreases.

If volume is not changed by heat and pressure then different sizes of an object have the same density.

Temperature

Density

Pressure

Density

Volume

Density

(6)

Questions: Answer the following questions in the spaces provided on your answer sheet.

PART A:

1. Calculate the volume with the given dimensions. *SHOW WORK*

The density of the object is 42.8 g/cm³ and a mass of 128g.

Make sure you round your answer to the nearest tenth and use appropriate units.

2. Using graph #1, show how heating an object will affect its density. Using graph #2, show how applying pressure to an object will affect its density.

3. Using the following data to draw a line graph. Answer the questions below.

Four samples of the Mineral Magnetite:

MASS (g)

VOLUME (cm³)

15 g 3 cm³

40 g 8 cm³

50 g 10 cm³

25 g 5 cm³

a) What is the density of magnetite?

b) What would be the mass it the volume is 6 cm³? c) What would be the mass if the volume is 12 cm³?

4. Pat learned that heavy objects sink and light objects float. But, when she did the experiment show below, she found that the heavier object was the one that floated. Explain why the heavier object was the one that floated.

5. Henry and Suzette were arguing over the correct height of tree. Henry determined the tree to be 180.0 ft and Suzette had calculated it to be 182.3 ft. The teacher explained that they were both off by a little bit and should determine which answer they were going to hand in, but they should pick the answer that is closest to the correct answer. Using the formula below calculate percent error if the correct (accepted) height of the tree is 181.2 ft. Whose answer was the closest to the correct answer? * Remember to show your work*

Percent Error

6. Using the data below, what is the density of the unknown object?

a. Mass of the 80 mL filled graduated cylinder is 82.0 g.

b. Mass of the unknown object and 80 mL liquid filled graduated cylinder is 224.8 g.

c. Starting Volume of graduated cylinder is 80 mL of liquid; ending Volume with unknown object is 120 mL.

(7)

Part B

(8)