When you use a graduated cylinder to measure the volume of a solid such as a granulated substance like salt, air pockets form between the granules, which affects the accuracy of the measurement. Air bubbles trapped in the solid take up space, lowering the density of the solid and inflating the volume measurement slightly. To reduce the effects of air bubbles in solids, compact the solid with the end of a small pestle, rubber “policeman” or stirring rod.
TL;DR (Too Long; Didn't Read)
Minimize the affect of trapped air by tamping down the solid material you are working with.
Density is the mass of a substance divided by its volume, and is typically stated in units such as grams per cubic centimeter, kilograms per cubic meter and so on. Because the density of a substance is the same regardless of the quantity, scientists call it an “intrinsic” property. As the densities of thousands of substances have been accurately measured and published, looking up a density figure is one way of identifying an unknown material.
To measure the density of a granulated solid, first weigh it on a balance, then find its volume in a graduated cylinder, beaker or other container. Divide the mass by the volume. When working in a chemistry lab setting, it is usually preferable to determine the density of a substance yourself; however, if you are absolutely sure of the nature of the compound and its purity, you can find the density in a reference book or online.
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Density of Solids and Air
The density of normal solids varies from light elements such as boron at 2.37 grams per cubic centimeter to heavy ones like osmium at 22.6 grams per cubic centimeter. By comparison, air’s density is nearly negligible -- 0.001205 grams per cubic centimeter, or less than a thousandth of the value of a solid.
Density of Mixtures
The density of a pure substance is relatively straightforward, but measuring density becomes complicated when two or more substances are mixed together. In that case the density is determined by the ratio of the substances involved, by volume. For example, if 80 percent of the volume of a substance is sulfur and 20 percent is air pockets, the overall density will be lower than for pure sulfur -- about 20 percent less, as the density of air is negligible compared to that of the sulfur.