Specific Gravity

Introduction

Four different metal cylinders are used to study measurement and perform density experiments.

Concepts

• Measurement
• Specific gravity
• Density

Materials

Safety Precautions

If glass graduated cylinders are used, be very careful adding the metal cylinders in order to avoid breaking the glass cylinder. Follow all laboratory safety procedures.

Procedure

Density by Measurement

1. Find the mass of each metal cylinder using an analytical balance. Record mass of each cylinder to the nearest 0.1 g.
2. Carefully measure the dimensions (diameter and length) of the metal cylinder to the nearest millimeter (0.1 cm). If possible, estimate to the nearest tenth of a millimeter (0.01 cm).
3. Calculate the volume of the cylinder using the equation: V = πr²L, where r is the radius and L is the length of the cylinder. Remember, the radius is one-half the diameter.
4. Calculate the density of each cylinder using the equation: Density = Mass/Volume (g/cm³).

Density by Displacement

1. Find the mass of each metal cylinder using an analytical balance. Record mass of each cylinder to the nearest 0.1 g.
2. Fill a 50- or 100-mL plastic graduated cylinder about half-way with water. Record the volume of water in milliliters.
3. Carefully place the metal cylinder into the graduated cylinder. It works best to tip the graduated cylinder and slide the metal cylinder along the side.
4. Record the new volume of water in the cylinder.
5. Calculate the volume of the metal sample using the equation: Volume_metal = Volume_{metal & water} – Volume_water
6. Calculate the density of each cylinder using the equation: Density = Mass/Volume (g/cm³). Remember, 1 mL = 1 cm³.

Discussion

Density is a characteristic property of a material and pure elements or compounds may be identified by their density. Density is defined as the mass of a substance per unit volume.

Density is commonly expressed as g/cm³ or g/mL.

Specific gravity is a similar term to density. Specific gravity is a comparison (or ratio) of the mass of a substance to the mass of an equal volume of water. Since the density of pure water is 1.00 g/cm³ at 20 °C, the specific gravity is equivalent to density. Specific gravity, however, is unitless.

The four metal cylinders and their approximate densities are: The metal cylinders are about 12.5 – 12.7 mm in diameter and about 50 ±1 mm in length. Due to limitations in measuring the volume of the metal cylinders, the density calculations may be off by 10–15%. Estimating the next decimal place greatly increases the accuracy of the measurement (12.5 is more accurate than 12).