Introduction

When filling out medical forms, we are often asked for our height, weight, and date of birth. Although our height and weight may change from year to tear, our date of birth remains constant.

We can distinguish elements and compounds by the properties they possess. Properties that depend on the quantity of the sample are called extensive properties. Like our height and weight, the size and mass of a compound vary from sample to sample. In contrast, properties that do not depend on quantity are called intensive properties.

For example, as long as the temperature remains constant, the density of a specific compound, like a birth date, is a constant, which makes it an intensive property. We define density as the ratio of the mass of a substance to its volume, as shown here.

The units we use most often to express the density of as liquid are grams per milliliter, or g/mL. For solids, we usually express density in units of grams per cubic centimeter, or g/cm³. Because 1 mL = 1 cm³, we can use the density units g/mL and g/cm³ interchangeably.

We can measure a solid object's mass on a balance, but how can we measure the object's volume? One approach is to find the volume of liquid that the object displaces. To do so, we partially fill a graduated cylinder with a liquid in which the object does not dissolve, which is usually water. We record the exact volume of water in the cylinder. Then we carefully slide the object into the cylinder until it is completely covered with water. The new volume reading represents the combined volume of thee water and the object. The difference between these two volumes is the volume of the object.

We follow a similar procedure in order to measure the mass of a liquid. First, we determine the mass of an empty container. Then we add our liquid sample to the container, and reweigh the container plus sample. The difference between these two masses is the sample mass.

We can use density to help identify a substance, but density alone is not enough for a positive identification of its composition. Just as many individuals share the same birth date, many different substances have the same density. In the case of a person, we would need additional information, such as the exact location of birth, to make a positive identification. Similarly, if we know a substance's density and some additional information (such as its melting point, boiling point, or availability in the laboratory), we can usually identify the composition of the substance.

Example

Problem
When a metal sample is placed into a 50 mL graduated cylinder containing 25.0 mL of water, the water level rises to 34.4 mL. The mass of the sample is 67.7 g. Identify the metal sample as either nickel (d = 8.90 g/mL), iron (d = 7.87 g/mL), or zinc (d = 7.13 g/mL).

Solution
(1) Determine the volume of the sample.
metal vol, mL = (water & metal, mL) - vol of water, mL
                    = 34.4 mL - 25.0 mL = 9.4 mL

(2) Calculate the density of the metal sample.


(3) Identify the metal.
The metal must be zinc.

Objectives

1. Use a balance and graduated cylinders to measure and mass and volume of a liquid and a solid.
2. Using observed mass and volume data, determine the density of ethanol, a rubber stopper, and an unknown object.
3. Using its calculated density, identify the composition of the unknown object from a list of possibilities.

I. Determining the Density of Ethanol

1. Grab the scale.

2. Weigh a dry, 10mL graduated cylinder. Record the mass of the cylinder.

3. Add 5-6 mL of ethanol. Read and record this volume as Sample 1. Be sure to read the bottom of the meniscus, or curved liquid surface, as shown here.

4. Use the scale to determine the mass of the graduated cylinder plus Sample 1. Record this mass.

5. Remove the cylinder and add 2-3 mL of ethanol. Read and record this volume as Sample 2.

6. Use the scale to determine the mass of the graduated cylinder plus Sample 2. Record this mass.

7. Reset the lab.

II. Determining the Density of a Rubber Stopper

1. Grab the scale.

2. Add the rubber stopper. Record the mass.

3. Reset the lab.

4. Get a 100 mL graduated cylinder.

5. Add 50-60 mL of water. Read and record the volume of water in the cylinder.

6. With the cylinder slightly tilted, carefully slide the rubber stopper into the water. Read and record the volume.

7. Reset the lab.

III. Determining the Density of an Unknown

1. Grab the scale.

2. Add the unknown. Record the mass.

3. Reset the lab.

4. Get a 100 mL graduated cylinder.

5. Add 50-60 mL of water. Read and record the volume of water in the cylinder.

6. With the cylinder slightly tilted, carefully slide the unknown into the water. Read and record the volume.

7. Reset the lab.