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Author: Janice VanCleave

Let's Explore


To construct a hydrometer.


  • 10-ounce (300-mL) plastic cup
  • tap water
  • scissors
  • drinking straw
  • grape-size piece of modeling clay
  • 5 BBs
  • fine-point permanent black marking pen
  • metric ruler


  1. Fill the cup about three-fourths full with water.
  2. Cut a 4-inch (10-cm) section from the straw.
  3. Plug one end of the piece of straw with clay.
  4. Drop three of the BBs into the straw. Then try to stand the straw, clay plug down, in the cup of water. You want the straw to stand upright in the water with about half of the straw above the water line. If the straw does not stand upright and/or is not halfway above the water line, add one BB at a time until it is.
  5. With the marking pen, draw a line on the straw even with the water line.
  6. Remove the BB-filled straw from the water. Label the line 1.0.
  7. Using the ruler and marker, start at the line on the straw and mark as many lines as possible 1 cm apart toward the clay end of the straw. Number these 1.1, 1.2, and so on.
  8. Repeat step 7, making lines toward the open end of the straw. Number these 0.9, 0.8, and so on.
  9. Riser


You have made a hydrometer.


A hydrometer is an instrument used to compare the density (mass per unit volume) of a liquid substance to the density of pure water. This comparison of densities is called specific gravity. The scale on the hydrometer in this investigation is a model of how a hydrometer works and cannot be used to measure actual specific gravity. Starting at 1.0 at the water line, this is the height of the hydrometer in any liquid having the same density as that of the tap water used. The other marks on the scale indicate a relative increase or decrease in specific gravity. The lower the hydrometer sinks in a liquid, the lower its specific gravity. Thus, scale readings less than 1.0 mean the liquid's density is less than the density of water. The higher the hydrometer rises in the liquid, the greater the liquid's specific gravity, indicated by scale readings greater than 1.0.

For Further Investigation

Ocean water is salty. Is salt water denser than freshwater? A project question might be, How does the salinity (salt concentration) of water affect its specific gravity?

Clues for Your Investigation

  1. Using the hydrometer made in the original experiment, determine the density of different salt solutions. Make the salt solutions with equal amounts of water but different amounts of salt, such as the solutions shown here in the Specific Gravity Data table. NOTE: Since the hydrometer was made using tap water, tap water must also be used in the test solutions. You may wish to repeat the original experiment using distilled water so that it indicates the density of pure water and not tap water that has some dissolved materials in it. Measure and keep a record of how much salt you put into each cup.
  2. For each cup of liquid, take four or more readings with the hydrometer and average them. For each reading, place the hydrometer in the liquid and allow it and the water to become still. Then read and record where the water line touches the mark on the hydrometer in a Specific Gravity Data table like the one shown. Estimate measurements between marks. For example, 1.25 would be halfway between marks 1.2 and 1.3. Average the test results of each liquid.


References and Project Books

Churchill, E. Richard. 365 Simple Science Experiments with Everyday Materials. New York: Black Dog & Leventhal,1997.

Edom, Helen. Science with Water. London: Usborne, 1992.

Gibson, Gary. Science for Fun Experiments. Brookfield, Conn.: Copper Beech Books, 1996.

VanCleave, Janice. ]anice VanCleave's A+ Projects in Chemistry. New York: Wiley, 1993.

Janice VanCleave's Chemistry for Every Kid. New York: Wiley, 1989.

Walpole, Brenda. 175 Science Experiments to Amuse and Amaze Your Friends. New York: Random House, 1988.

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