Simple Specific Gravity Experiments
Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water. Liquids and solids can be measured using displacement because liquids can float or sink just as a solid object will. A variety of experiments can demonstrate and explore the specific gravity of solids and liquids.
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Water Density
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The specific gravity of a liquid compares its density in relation to fresh water. Add salt to a glass of water until it no longer dissolves. Add a couple of drops of food coloring and pour the salt water slowly into a glass of fresh water. A colored layer will form on the bottom and a clear layer on top because fresh water at 4 degrees Celsius has a specific gravity of 1 and salt water has a specific gravity of around 1.022. Fresh water is less dense than salt water and has a lower specific gravity. Explore by pouring different liquids into fresh water. If they float, they have a lower specific gravity than 1. If they sink, they have a higher specific gravity.
Make a Hydrometer
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Hydrometers are used to measure the specific gravity of liquids. To make your own, mark a plastic straw at regular intervals and number them. Add clay to one end of the straw and place it into a tall glass of water. Adjust the size of the piece of clay so that the straw floats vertically. Place the hydrometer in different liquids. The higher it floats, the denser the liquid and the higher the specific gravity.
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Specific Gravity of a Rock
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Cut off the top of a soda bottle. Punch a small hole in the rim and insert the flexible end of a drinking straw. Bend the straw so that the long end points down. Place a measuring cup beneath it. Fill the bottle with water so that it rises just above the straw. When the water stops draining, empty the cup and place it under the straw again. Tie a long string around a rock and record its weight in grams. Lower the rock into the bottle until it reaches the bottom. Record the amount of water (in milliliters) that drains into the cup, and determine the specific gravity of the rock by dividing the mass of the rock by the mass of the water displaced. For example, if the rock is 150 g and it displaced 60 mm of water, you would divide 150 by 60 (1 mm of water weighs about 1 g, so the mass of 60 oz. of water is 60 g). The specific gravity of the rock in this case would be 2.5.
Specific Gravity and Product Packaging
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Products that appear not quite full are typically weighed based on their specific gravity. Water has a specific gravity of 1, meaning one fluid ounce of water weighs 1 oz. Glycerin has a specific gravity of 1.21, which means one fluid ounce of glycerin weighs 1.21 oz. A container of glycerin will appear less full than the same weight of water because the glycerin has a higher specific gravity. Do an experiment with household products to determine whether they are sold by weight or volume. Fill a jar of water three-quarters full. Add a small amount of the item you want to test. If the object floats to the top of the water, the specific gravity of that product is less than 1. If it sinks, it's greater than 1.
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References
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