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°Specific Gravity

  Printable Version of Lesson 20
















































































Lesson 20:


Specific Gravity

In this section we will answer the following question:

  • What is specific gravity?


Specific Gravity

Specific gravity is a way of relating the density of an object to the density of water to determine whether or not the object will float.  The formula for specific gravity is given below:

If an object's specific gravity is less than one, then the object will float.  If the object has a specific gravity of greater than one, it sinks.  So dense objects sink in water and less dense objects float.



Importance of Specific Gravity

Specific gravity is a very important concept in the water/wastewater field.  The specific gravity of a substance will determine where a compound can be found in water in case of a spill.  

Let's consider gasoline.  The density of gasoline is 0.6 g/mL and the density of water is 1.0 g/mL.  So the specific gravity of gasoline is:

Since its specific gravity is 0.6 (less than 1), gasoline floats in water.  So when a ship leaks gasoline into the water, the gasoline stays at the top of the water.

In contrast, the specific gravity of palmalive is 1.1, so it sinks in water.  



In this section we will answer the following questions:

  • What is the difference between dissolved and suspended substances?
  • What is solubility and what factors affect it?


Suspended and Dissolved

When a substance is mixed with water, one of three results will occur.  The substance may float or settle out of the water, the substance may become suspended, or the substance may become dissolved.

You can see these three results using foods out of your kitchen.  Oil is an example of a substance which will not mix with water.  If you pour oil into a cup of water and stir the water vigorously, the oil will break up into beads.  But as soon as you allow the water to become still, the oil will float up to the surface of the water and form a film of oil with no water intermixed.  So oil in water neither becomes dissolved nor suspended.  

In contrast, try adding a spoonful of flour to a cup of water. When you mix the flour and water vigorously, the flour will become suspended, meaning that the flour is well mixed with the water, but can be removed by physical means.  If you allow the flour and water mixture to sit still for long enough, then the flour will settle out of the water. Alternatively, you could strain the flour out of the water using a filter.  

Finally, add a spoonful of salt to a cup of water and mix well.  Like the flour, the salt mixes into the water, but unlike the flour the salt will not settle back out of the water.  The salt has become dissolved, meaning that it has become a part of the water solution.  Salt has broken down into its constituent parts (Na and Cl), so the salt can only be removed from the water if the water evaporates away.  Substances which are dissolved in water cannot be filtered out.



Solubility is the amount of a substance that will dissolve in water at a certain temperature.  You can determine the solubility of salt in water at room temperature by adding spoonfuls of salt to water, stirring after each spoonful. Eventually, the salt will stop dissolving in the water, and some will settle to the bottom of the cup.  Now the solution is saturated, meaning that it contains as much salt as it can hold.

Solubility is influenced by temperature.  Try heating your solution to a warmer temperature.  Now add more salt.  You will be able to dissolve several more spoonfuls of salt in the warm water before this solution, too, becomes saturated.  

If you allow the saturated, warm solution to cool, then the solution becomes supersaturated.  This means that the salt will remain suspended in the cool water even though there is more salt present than you were initially able to suspend in the room temperature water.  You can grow a rock crystal by laying a string in this supersaturated solution and leaving it in place for a few days.



The density, percent moisture, and percent organics of soil influence the uses to which the soil can be put.  Density, the closeness of materials, can be calculated using the following formula:

Percent moisture is calculated using the following formula:

Percent organics is calculated using the following formula:


Specific gravity relates the density of an object to the density of water.  If the object's specific gravity is less than 1, then the object will float in water.  If the object's specific gravity is greater then 1, then it will sink in water.  A substance's specific gravity can be calculated using the following formula:

Some substances become suspended when mixed with water and can be filtered back out.  Others become dissolved in water, and can only be removed if the water is evaporated away.  Solubility is a measure of the amount of a substance that will dissolve in water at a certain temperature.



  1. A trailer truck hauling a load of vegetable oil wrecked beside the river feeding your water treatment plant.  All of the oil bottles broke and the oil oozed out into the river upstream of the plant's influent pipe.  The intake for your water treatment plant is located three feet below the surface of the river.  You have to decide whether to close down the treatment plant or to allow it to operate.  Will the oil enter your treatment plant?  Why or why not?  (Hint: The density of the vegetable oil is 0.8 g/mL.)

  2. Find a bottle of liquid in your kitchen or in a grocery store which lists both the weight and the volume of the liquid. (Weight in grams is often given under "Serving Size" in the nutrition facts.  To find the weight of the whole bottle of liquid, multiply the weight of one serving by the number of servings per container.)  What is the density of the liquid? Do you think it will float or sink in water?  Pour a bit of the liquid into a cup of water to test your math.  

  3. You want to test a sample of soil in an area where you hope to install a septic field.  You dig a hole with a volume of 3 liters.  The soil's original weight is 4,000 grams.  After drying the soil, it weighs 2,759 grams.  Then, after baking the soil, it weighs 2,011 grams. What are the density, percent moisture, and percent organics in the soil?  
  4. Tiny particles of dirt can be mixed water.  The dirt particles are so light that they do not settle out of the water, but they can be removed from water using a filter.  Is the dirt suspended or dissolved in the water?  

  5. Fill three separate cups with water.  Add oil to the first cup, flour to the second cup, and salt to the third cup.  Mix each cup.  What do you observe?  Allow each cup to sit for a few hours.  What do you observe now?

  6. Fill another cup with water.  Add cornstarch to the water and mix.  Then allow the cup to sit for a few hours.  Does the cornstarch act most like the oil, the flour, or the salt?  Do you think mixing the cornstarch and water caused the cornstarch to dissolve, suspend, or neither?

  7. How many spoonfuls of salt can you add to a cup of water at room temperature before it becomes saturated?  Now heat the water until it is almost boiling.  How much more salt were you able to add to the hot water