The density of a substance is the amount of mass there is per unit volume of the substance. As you discovered in the first homework problem, densities are normally measured in units like kilograms per cubic meter or grams per cubic centimeter. Since the density of water is very close to one gram per cubic centimeter, it is quite common to specify densities as multiples of the density of water, in which case the number is dimensionless and is called the specific gravity of the substance.
Your goal today is to measure the densities of a number of substances commonly found in the interiors and at the surfaces of planets. The list of substances to be measured is the following:
- Basalt
- Pyroxene
- Iron or Steel
- Olivine
- Feldspar
You should measure the density of three samples of each substance. It is necessary to do this to get some feeling for the uncertainty of the measurement. Very often in planetology one is attempting to use density measurements to determine which substances are actually present (without going for a sample!), and so it is important to know something of the range of densities which a given substance presents. Perfect crystals exhibit very little variation, as you can imagine, but natural materials are not always perfect specimens.
To determine the density of a sample, you need only measure the mass of the sample with the three-beam balance (Treat it gently- if you are not sure how to use the balance please ask for assistance!), then measure the volume by seeing how much volume of water is displaced when you immerse the sample in water in a graduated cylinder. DO NOT DROP the samples into the glass graduated cylinders, as they are likely to break the glass. Then, paying careful attention to the units, divide the mass by the volume for each specimen. Dry the specimens off gently before you set them aside for the next investigator.
To give yourself a sense of the relative variations in density, you should prepare a one-dimensional plot of your data. Draw a straight line across the page and label it with specific gravity values that cover your range of measurements (a linear scale). Then mark the positions of all of your determinations along the line, labeling each one. Are all of the substances distinct, or do some of the ranges overlap?