Hydrometers are graduated floating devices that measure the densities of liquids. Hydrometers are commonly used in the making of wine, beer, and spirits to monitor the fermentation process and are also used by environmental scientists to monitor the concentration of salt and impurities in water. Commercial hydrometers are tubes of glass, weighted with metal shot, but you can make a simple hydrometer with materials you can find in a grocery, hardware, or sporting goods store and use it to test the density of water with various salt concentrations. Follow the steps below.
Building and Calibrating the Hydrometer - Seal off an end of a soda straw. You can plug the end with modeling clay, plasticine (Play-Doh or Cranium Clay), or a wad of chewed gum. Be sure to use something that won't let water seep through it, and use a piece large enough not to let water seep past it.
Fill a narrow jar or graduated cylinder partially with water. If using a graduated cylinder, fill it to the 100 milliliter line. If using a narrow jar like an olive jar, fill it with 100 ml (0.42 cup) of water and mark a reference line on the outside of the jar with a permanent marker. (If you don't have a measuring cup with metric measurements, use 1/2 cup of water.) You can use either distilled or tap water, but whichever you use, use the same type of water from the same source throughout this project. The water will form a curved arc called a meniscus. Where the bottom of the arc falls is considered the correct point to read the water level on a graduated cylinder or to mark the water line on an unmarked jar.
Put the straw in the water, sealed side down. The straw will serve as your hydrometer. Note how far up the straw pokes from the top of the jar or graduated cylinder. You want the open end of the straw to poke no more than 1 inch (2.5 cm) from the top of the container of water. If it does poke up more than this, add weight to the straw to keep it from floating up too high and flipping onto its side. You can add weight either by adding more clay, plasticine, or gum to the outside of the straw at the sealed end, or you can drop small weights such as BBs, split shot sinkers, small nails, or pebbles down the inside of the straw until it floats at the right level.
Mark the place on the straw where it touches the surface of the water with a permanent marker. This point is your zero line.
Remove the hydrometer from the water. Your hydrometer is now calibrated for "pure" water (either distilled or regular tap water). From here, if you wish, you can mark lines above and below this line. Ideally, each line should be 1 mm (about 1/32 inch) apart, or at least as close as the width of the point of your marker will allow. You may want to use multiple colors for your lines, such as red for the zero and 10-mm lines, and black for the lines between them.
Making Tests With the Hydrometer - Add 1 g (0.004 oz.) of salt to the water. Stir the water to ensure the salt dissolves completely. If you're doing this experiment in a home school setting and don't have a scale to measure out 1 g of salt, you can instead measure the salt by volume. One gram of salt takes up 0.89 ml (0.18 tsp.) volume, so you can get by using 1/4 tsp. of salt in place of 1 g.
Put the hydrometer in the water. Note the level to which the hydrometer floats. If you marked your hydrometer with measurement lines above and below the zero line, write down the measurement of the line which now floats at the water's surface. If you didn't mark your hydrometer with measurement lines, mark a line at the point on your hydrometer that now floats on the water's surface. (You can also mark this point if you did mark your hydrometer with measurement lines by using a different color marker than those used to make your measurement lines.)
Remove the hydrometer and add more salt. Add the same amount of salt each time, repeating the above steps and recording the level to which the hydrometer now floats.