Density, a physical property of matter, is the mass of an object divided by its volume. For a given substance, the density is the same regardless of how much you have; scientists call this an intensive property. Basic chemistry density experiments include measuring the density of objects, comparing objects of different densities through buoyancy, observing density changes and identifying unknown substances based on their densities.
Measuring the densities of several small items such as cubes, blocks and balls made of wood, metal and plastic makes a good introduction to the study of density. To determine the density of an object, students first measure the object's mass in grams. Next, they measure the object’s size and compute the volume based on the item’s shape. For example, the volume of a rectangular solid is its height multiplied by its width and depth. Using a calculator or pencil and paper, the students divide the measured mass by the volume to find the density of the object. In a similar manner, they can find the density of liquids in a beaker by first measuring the weight of an empty beaker, measuring it with liquid inside, then subtracting the first weight from the second to determine the mass of the liquid by itself. Then they can find the volume of the liquid by reading the markings on the beaker. Then, as before, divide the mass by the volume to determine the density of the liquid.
Students can easily compare the densities of a liquid and solid by placing the solid object in a beaker containing the liquid. If the solid object floats, its density is less than the liquid’s. If the object sinks to the bottom, it is more dense than the liquid. The same experiment works with oil and water. A less dense oil forms a layer on top of the water, while a more dense oil sinks to the bottom.
Observing Density Changes
Many things expand and contract due to temperature changes; because their sizes change but masses stay constant, their densities also change. Measure out 100mL of water, find its density and write down the results. In an open container, put the water in a freezer and let it turn to ice. Carefully remove the ice from the container, find its density, and compare it to the value for water. Note that water is one of the few substances that become less dense when turning from liquid to solid.
Identifying Unknown Substances
Sometimes, you can use the density of an unknown substance to help identify the substance. One typical lab experiment to illustrate this process uses small blocks of aluminum, lead, iron and copper, painted to hide their metallic appearance. Have students find the density of each unknown object. Then, using a reference table that lists the densities of various metals the students should be able to identify the substance based on the densities listed in the table.
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