In this section we will answer the following question:
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
- 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
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.
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
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.
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.)
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
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?
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?
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?
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?
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