Density is a fundamental property of physical objects. Chemists sometimes use it to help in the identification of materials. The effects of density are often demonstrated by considering whether an object will float in water. Water has a density very close to exactly 1 g/cm^{3}. When an object has a density greater than 1 g/cm^{3} then it will not float in water. When its density is less than 1 g/cm^{3} then it floats. For example, common vegetable oil will not dissolve in water and has a density of around 0.92 g/cm^{3}, which is why Italian salad dressing looks the way it does.

Density is defined as the amount of mass per unit volume and mathematically by the equation D = m/V. The letter D stands for density, m for mass, and V for volume. The SI unit of density is kg/m^{3} but it will suffice in the lab for us to use g/cm^{3} (remember, 1 cm^{3} = 1 mL so 1 g/cm^{3} = 1 g/mL). The measurement of density requires two pieces of data and therefore requires great care in the making of both measurements so as not to lose precision.

In this lab you will measure the density of two substances.Successful completion of this investigation will include:

- The density of two of the following, as measured in multiple trials:

- Tap Water or
- Prepared Salt Water

- Aluminum Shot or
- Mossy zinc (irregularly shaped pieces of zinc metal)

- 10 mL, 50 mL, and 100 mL graduated cylinders
- lab balance
- water

- lab spatula
- pen
- weighing boats
- lab notebook or paper

- mossy zinc
- aluminum shot
- salt water

Density is what chemists call an *intensive* property. That means that *the density of a material does not depend on how much of it you have*. Other examples of intensive properties include temperature, concentration, color, odor, and molar mass. You cannot add intensive properties together. If you have 2.70 g of aluminum it has a volume of 1 cm^{3} (at ordinary room temperature and pressure). If you have 2,700 g of aluminum it has a volume of 1,000 cm^{3}. If you have 1 m^{3} (1 × 10^{6} cm^{3}) of aluminum it has a mass of 2,700 kg. Work all of these out using D = m/V and you will always get a result of 2.70 g/cm^{3}.
No matter how much aluminum you add, the density, an intensive property, does not change. An intensive property can be understood in this way: there is a ratio of two measurements, which together have a constant value. Just like the slope of a line in an algebraic equation: the slope is the same no matter what the values of *x* and *y*. With density, this can be shown in a simple graph, as seen at right. The slope of the line in the graph *is* the density of aluminum, according to the data.

Contrast intensive properties with *extensive properties*: measurements like mass, volume, height, width, and depth all depend on the size of a particular object. These measurements are extensive properties. You can always add more volume or mass.

Density is often confused with weight. We often say that lead is heavy (ask your teacher if there is a piece of lead around to use as an example). But which is heavier, one pound of lead or one pound of feathers? … So weight is not the difference between them. Try another question: which is heavier, 1 liter of lead or 1 liter of feathers? Clearly, even if you compressed the feathers to the maximum degree practical, they would never weigh as much as the lead, assuming you have the same volume of each. So the substance lead is not heavy (an extensive property), it is dense (an intensive property). Still not convinced? Answer this: is 0.0001 g of lead heavy?

An individual formal lab report is due. Turn it in through the Google Classroom page. You will use a computer to enter your data in Excel and graph it as a way to find the density of a substance. Averaging multiple measurements is one way, finding the slope of a line for volume (x-axis) vs. mass (y-axis) is another. You will compare these two analysis methods in the Analysis section of your formal report.

### Procedure

#### Measuring Density for a Liquid

#### Measuring Density for a Solid

Items to include in your Introduction:

- Definition and formula for density
- Explanation of density as an intensive property
- Discussion of the application of density to flotation
- Brief overview of what you did in the lab

Items to include in your Analysis

- A data table showing mass and volume measurements for both materials you measured.
- A graph for each data set (mass on the y-axis and volume on the x-axis)
- The slope of the lines (with units) for the graph for each substance
- The meaning of the slope of the lines
- Look up the density of each substance you measured online. Compare your result to this ‘correct’ value.
- If your values are inaccurate discuss
**specific physical reasons**that could have led to the results you have - In general, discuss events and situations that may have affected the accuracy of your data

We are measuring density by measuring the mass and volume of five samples of the same material. Each sample will be different from the others in size. By making a graph of volume (x-axis) vs. mass (y-axis) and finding the slope of the line of best fit you will determine the density. The slope of the line in such a graph is the mass divided by the volume, which is the density. This procedure produces a more accurate result than simply finding an avereage.

**Your task is to find the density of one liquid material and one solid material.** Precise measurement is absolutely critical to making a successful and accurate measurement. Measure between the lines on all measurement tools to estimate one tenth of the smallest division on the tool. On three-beam balances this means you must measure to the nearest 0.01 g. On a 10-mL graduated cylinder measure to the nearest 0.01 mL. On a 50-mL or 100-mL graduated cylinder measure to the nearest 0.1 mL.

Measuring the volume of a liquid is easy. Place a random sample of tap water or salt water (or some other liquid provided by your teacher) into a graduated cylinder and then read the volume. Measuring the mass requires a bit of care but is not too hard.

- Using a three-beam balance measure the mass of a
*dry*graduated cylinder to the nearest 0.01 g. Write this number down! - Put a small sample of liquid into the cylinder. The amount does not matter very much because density is the same regardless of the size of the sample. Still, fill it only about one fifth full.
- Read the volume precisely, estimating to one tenth of the smallest division marked on the cylinder. Write this volume down in your data table (see the next page).
- Look for ways to avoid measurement error. For example, make sure that the outside of the cylinder is not wet and that the balance pan is also not wet. Make sure that no liquid is clinging to the top part of the cylinder where it will add to the mass you measure but not the volume.
- Measure the mass of the cylinder now that it contains your sample. Subtract the mass of the dry, empty cylinder from the mass you find. Write down this number in your data table.
- Do not dump out any of the liquid in your cylinder. Instead, just add a little more. Read the new volume and write it down in your data table.
- Measure the mass again and subtract the mass of the dry, empty cylinder. Write down this mass in your data table.
- Repeat these steps until you have five measurements of volume and mass.
- Calculate the density of each of your samples. All five results should be nearly the same if you have made careful measurements.
- Find the average density and write it in your data table. Take a look at your data and evaluate it. Does it appear that you have a consistent set of data? If there are densities that you’ve calculated that are very different from the others then you must make further measurements. Careful measurement will result in consistent results and you can recognize a poor data point and replace it with a good one. Do so.

Measuring the mass of a solid is easy. Place a random sample of zinc or aluminum (or some other solid provided by your teacher) onto a balance and measure the mass. Measuring the volume requires a bit of care but is not too hard. You will use the method of Archimedes and submerge your solid samples in water to find the volume by difference.

- Obtain a sample of your chosen solid. For aluminum the minimum first sample size should be between 8 and 12 grams. For zinc the minimum first sample size should be between 20 and 25 grams.
*Do not let your sample get wet before you weigh it.* - Measure the mass of your first sample to the nearest 0.01 g and record it in your data table.
- Obtain a graduated cylinder and fill it partway with tap water. Make sure no water is clinging to the inside of the cylinder where it could later drop into the liquid below. Measure the volume of the water as precisely as possible, estimating to one tenth of the smallest division marked on the cylinder. Write this down.
- Add your solid to the cylinder containing your measured amount of tap water. Be careful not to splash any of the water out. Be careful about bubbles. If there are bubbles hidden in among the pieces of zinc or aluminum then they will make your measured volume bigger than the volume of the actual material. Gently tap the cylinder to dislodge bubbles.
- Measure the volume in your cylinder now that the solid is submerged in the water. Subtract the original volume of water. Write the result in your data table as the volume of the material.
- Do not dump out your cylinder and start over. Instead, measure the mass of an additional amount of dry solid that is about as large as your first sample. Add this mass to the mass of your first sample and record this total mass as the mass of your second sample.
- Add this additional amount of solid to your graduated cylinder.
- Being careful about bubbles and splashed water read the volume on your cylinder after you add the new amount to the original amount. Subtract the
*original*volume of water from this new measured volume. This is the total volume of the new sample plus your original sample. Record this in your data table. - Repeat these steps until you have a total of five samples with different sizes. Calculate the density of each sample and find an average. Evaluate your data for consistency. If there are one or more data points that seem inconsistent (or which are very far off from the true density of the material) then do new trials to replace your poor-quality data.

You will find the density of * two* substances, including

Follow the procedures on the previous page.

Write your data in the tables below. **All of your data points must have very different masses from one another.**

Identity of Material: | Mass of Empty Cylinder: | ||

Volume (mL) (x-axis, independent variable) |
Mass (g) (y-axis, dependent variable) |
Density (g/mL) | |

1 | |||

2 | |||

3 | |||

4 | |||

5 | |||

Average |

Identity of Material: | Original Vol. in Cylinder: | ||

Volume (mL) (x-axis, independent variable) |
Mass (g) (y-axis, dependent variable) |
Density (g/mL) | |

1 | |||

2 | |||

3 | |||

4 | |||

5 | |||

Average |