Hydrometers

INTRODUCTION

A hydrometer is one of the simplest ways to compare the density of different liquids. A hydrometer measures the density of liquids relative to that of water. When this is done the density is actually reported as the specific gravity, which is the density of the liquid measured divided by the density of water.

MATERIALS

• Straight drinking straws
• Modeling clay
• Ruler
• Permanent markers
• Transparent cup
• Vegetable oil
• Water
• Salt

WHAT TO DO

1. To make a simple hydrometer, take a drinking straw and cover one end of the straw with modeling clay, which is to seal the straw and to hold it upright in a solution. With a permanent marker, mark lines on the side of the straw at every centimeter.
2. Try to measure the relative density or specific gravity of water, salt water and cooking oil. Fill three cups to exactly the same volume with the three liquids. First place the hydrometer into the water. Count the number of marked lines that are submerged under the water. Record this information. Do the same measurement in the salt water and the vegetable oil.
3. The hydrometer should sink lower in the vegetable oil than in the water and should be higher in the salt-water solution. The hydrometer sinks deep in the lower density liquid and raises in more dense liquids. Therefore the vegetable oil is lower in density than water and water is lower in density than the salt-water solution.

QUESTION

1. What is another way that the density of a liquid could be measured and compared to that of water?

EXTENSION

1. Determine the density of two of the solutions using the triple beam balances and the graduated cylinders.

SOURCE

"Making a Hydrometer: How Science Works." Judith Hann, 1991, Dorling Kindersley Limited, London. ISBN 0-7621-0249-7.
"Pencil Hydrometers: Teaching Chemistry with Toys, Activities for Grades K-9." Jerry L. Sarquis, Mickey Sarquis, John P. Williams, McGraw-Hill, 1995, ISBN: 0-07-064722-4.

© S. Olesik, WOW Project, Ohio State University, 2000.