Determine the mass of the unknown liquid by placing it on a scale that measures in metric units such as grams. For example, assume the sample mass is 1.20 kg.
Determine the volume of the unknown liquid using a graduated cylinder. To get an accurate measurement, hold the graduated cylinder so the top of the fluid is at eye level. Note the fluid surface has a slight curve similar to a flattened U. This is called the meniscus. Find the volume by reading where the bottom, or center, of the meniscus aligns with a measurement line on the graduated cylinder. For this example, say the volume is one liter.
Convert the volume from liters to cubic meters by dividing your volume by 1,000. Therefore, one liter is equivalent to 0.001 cubic meters (m^3).
Calculate the density of the unknown liquid by dividing the mass of the sample by the volume of the sample: 1.20 kg/0.001 m^3 = 1,200 kg/m^3
Divide the density of the unknown liquid (1,200 kg/m^3) by the density of water (1,000 kg/m^3) to determine the unknown liquid's specific gravity: 1,200 / 1,000 = 1.20. The specific gravity of the unknown liquid is 1.2.
Place a floating glass hydrometer in a sample of the unknown liquid, making sure the container is large enough so the hydrometer does not touch the sides or bottom of the container.
Stand at eye level with the surface of the liquid to get an accurate reading of the hydrometer scale. Read the salinity value for the liquid on the hydrometer scale where the top surface of the liquid intersects the scale.
Compare the salinity reading of the sample to the known salinity of salt water, which ranges between 30 to 55 parts per thousand, to identify the liquid or to determine the starting point from which to adjust the sample's salinity.