Specific Gravity Specimens

Introduction

Determine the specific gravity of each of the ten different materials provided in this set and then use the values to identify each material.

Concepts

• Specific gravity
• Mass/volume/density
• Buoyancy

Background

Density is an intensive physical property of matter—that is, it is a property unique to each substance (at a specified temperature) and can therefore be used to distinguish one substance from another. An unknown substance is often identified by determining its density and comparing this experimental value to the actual density recorded in chemical literature.

The density of a substance is defined as the amount of matter in a given unit of space, also thought of as the “concentration” of a substance. Density is calculated as the ratio of mass to volume, where mass is the amount of matter in an object (given in grams) and volume is the amount of space the object occupies (given in milliliters or cubic centimeters). The equation to calculate density is as follows:

Densities vary with temperature and pressure. The standard for comparing densities is water, which at 4 °C, has a density of almost exactly 1.00000 grams per milliliter (0.999973). (Note: Water attains its maximum density of 1.00 g/mL at 4 °C.) For liquids and gases, density is determined in a straightforward way by measuring, independently, the mass and volume of a sample. For solids, however, determination of density is not as straightforward of a process. The mass of a solid sample is measured in the usual way using a balance. The volume of a sample can be found indirectly using the method of water displacement. Water displacement is commonly used for irregularly shaped objects and involves submerging the object into a known volume of water and measuring the volume change. The volume of water displaced by the solid is equal to the object’s volume. If accuracy is crucial, the exact temperature of the water should be measured and the exact density of the water at that temperature should be used. These values can be found in a handbook such as the CRC or Merck Index.

Specific gravity is another term often used interchangeably with density. Specific gravity is the ratio of the density of a substance to the density of an equal volume of a reference substance (water) at 4 °C or other specified temperature. Specific gravity is an abstract number, which has no units. Since the density of water is 1.00 g/mL at 4 °C, then for solids and liquids, specific gravity is numerically equal to density (except without the units).

The densities for the ten materials provided in this set, as well as for water, are listed in order from highest to lowest in Table 1.

Materials

Safety Precautions

Carefully lower the specific gravity specimens into the graduated cylinder. Dropping them in may cause the glass graduated cylinders to break.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Specific gravity specimens may be dried and stored for use from year to year.

Procedure

1. Obtain a specific gravity specimen. Record observations about its characteristic appearance in the data table.
2. Weigh the specific gravity specimen on a balance to the nearest tenth of a gram. Record the mass in the data table.
3. Tie a string (or thin wire or thread) around the sample.
4. Fill a 50-mL graduated cylinder with tap water approximately to the 40-mL mark. Read the exact volume of the water in the cylinder to the nearest tenth of a milliliter and record this as initial volume of water in the data table.
5. Hold the sample by the string and carefully lower it into the water until it is fully submerged. Be careful not to drop the sample as this could break the graduated cylinder. Again read the exact volume of the water in the cylinder to the nearest tenth of a milliliter. Record this as final volume of water in the data table. (Note: For those samples that do not sink, use a toothpick or needle to push the material into the water until it is fully submerged.)
6. Repeat steps 1–5 for each of the ten specific gravity specimens.

Calculations

7. Calculate the volume of water displaced by each object. Record this as volume of sample in the calculations table.
8. Compute the specific gravity of each sample. Record each specific gravity in the calculations table.
9. Determine the identity of each sample by comparing the experimental values for specific gravity to the actual values provided in the background material. Record each sample’s identity in the calculations table.

Student Worksheet PDF

12279_Student1.pdf

Teacher Tips

• This handout is written for teacher reference and is not specifically intended for student copy purposes. Purchase of this specific gravity set, however, provides you with permission to use any part of this handout in the manner desired.
• Ideally, there could be one specific gravity set per group of students so that they may determine the densities of all ten materials at their lab stations. However, if you only have limited specific gravity sets, one suggestion is to set up this lab in stations and have the student groups move from station to station to determine the density of each material.

Answers to Questions

1. What is the purpose of measuring the mass of each sample prior to determining the volume?

   The mass should be measured first so that the samples are not wet; the water will add mass.

2. Make observations about which materials float or sink in water. Write a general statement regarding the densities of those materials that float versus those materials that sink.

   Materials with a density of less than 1.0 g/mL will float in water while materials with a density of greater than 1.0 g/mL will sink in water.