The Science Notebook
Liquids - Part 1

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LIQUIDS

WATER
Water - What Is It Made Of?
Observing Water Seek It's Level

BUOYANCY
Exploring Archimedes' Principle
More On Archimedes' Principle
Buoyant Force
More on Buoyancy
When Will an Object Float?
Specific Gravity
Specific Gravity of Salt Water
Making a Model Submarine
Cartesian Diver

WATER PRESSURE
Making A Manometer
Using the Manometer
How Water Pressure Is Exerted
Water Pressure In Different Bodies of Water
Pressure Of Different Solutions
Water Pressure Engine

LIQUIDS

A liquid is matter that has a definite volume, but not a definite shape.  You can measure a liter of water, and no matter what kind of container you pour a liter of water into, it will always be a liter in volume, but it will always take the shape of the container into which it is poured.

Liquids are all around us, particularly the most common liquid of all, water. In these experiments, we will investigate some of the properties of liquids.  Because water is the most common liquid, many of these experiments will use water to explore those properties.


WATER

Water - What Is It Made Of?


CAUTION!  Use only batteries for this experiment!  Never use household current!

Materials Needed: Clear glass or plastic container; two AA, AAA, C or D cells; homemade battery holder; insulated wire; salt; water.

Procedure: Cut two pieces of wire about 30 cm (12 in) long, and remove about 1 cm (½ in) insulation from each end.  Assemble one end of each wire into a homemade battery holder with two cells.  Do not allow the free ends of the wires to touch each other.

Fill the container with water and dissolve about a teaspoon of salt into the water.  Place the two wires into the water close to each other, but do not allow them to touch.  What do you see?

What Happened: You should have seen bubbles begin to form at the end of each wire, particularly on the negative side. 

Water is a chemical compound made of two parts hydrogen and one part oxygen. Hydrogen and oxygen are both gases.  These two gases combine to make the liquid water. The electrical energy splits the molecules of water into the hydrogen and oxygen that make up the water.  Adding the salt to the water allows the electricity flow through the water more efficiently, which allows the water molecules to be split up more rapidly.  (Pure water does not conduct electricity very well.)

Going Further: You might want to try this experiment without salt, and using different amounts of salt in the water to determine how much difference it makes in how fast the bubbles are formed.


Observing Water Seek It’s Level


CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Small plastic soft drink bottle with cap; nail; clear plastic tubing or two clear flexible drinking straws and tape; modeling clay; water.

Procedure:   If you are using two straws, push one straw inside the other and tape them together as shown.  You may need to cut a small slit in one of the straws to join them together.  Wrap a couple of turns of tape around the straws to make them watertight.

Punch a hole in the bottle top with the nail.  Using the nail or a pair of scissors, make the hole large enough to allow the straw or tubing to fit through it snugly.   Push about 3 cm (1 in) of the straw or tubing through the top, and seal the top with modeling clay.  Punch a hole near the bottom of the bottle using the nail.

With your finger over the hole, fill the bottle about half full with water. Screw the top with tubing on the bottle and carefully turn it and the tubing over.  Now remove your finger from the hole.  Raise and lower the container and move the tubing around.  As you do, observe the level of the water in the bottle and in the tubing.

What To Look For: What happens if you lower the tubing below the level of the water in the bottle?

What Happened: As you shifted the bottle and straw tube, the water level in both was the same.  If the end of the tubing was allowed to drop below the water level in the bottle, water came out of the tube.  A liquid will always seek the lowest level possible.

Going Further: Why did you need to punch a hole in the bottle?  If you place a piece of tape or your finger over the hole and repeat the experiment, what difference, if any, does it make? 

(HINT: Check out the experiments on this Gases page.)


BUOYANCY

The next few experiments will explore how and why some objects float and other objects sink.


Exploring Archimedes’ Principle


Materials Needed: Graduated cylinder; water; small rock.

Procedure:   A homemade graduated cylinder will work just fine, or you can borrow one from your school lab.  Either way, the rock you use must be able to fit inside the cylinder.

Fill the cylinder about half full of water.  Measure the water level and write this number down.  (Always read the water level from the lowest point.)  Carefully lower the rock into the water and let it sink to the bottom.  Read and record the water level again.  Subtract the first reading from the second.

What Happened: An object that sinks will displace it’s own volume in water.  (Displace means “push out of the way.”) If your rock displaced 7 ml of water, then it’s volume is 7 ml.  You will remember that 1 ml equals 1 cubic centimeter, so you can also say that the volume of this rock is 7 cubic centimeters. (Cubic centimeters may be abbreviated “cc” or “cm3".)

This is the first part of Archimedes Principle: An object that sinks displaces it’s own volume in water.


More On Archimedes’ Principle

 

In the last experiment, you saw that an object that sinks will displace its own volume in water. What about an object that floats?

Materials Needed: Small block of wood such as a baby block; plastic soft drink bottle; straw; modeling clay; scissors; nail; graduated cylinder; centimeter ruler; homemade balance (or school scales); paper or light plastic cup.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Procedure:   Carefully cut the top of the soft drink bottle below the curved part.  Near the top of the cut, punch a small hole in the side of the bottle and enlarge it so that the straw will fit snugly through it.  Cut about 15 cm (6 in) length of the straw and fit it through the hole from the outside at a right angle as shown.  Seal the straw around the opening with modeling clay.  Carefully trim the straw on the inside of the bottle so that it is as close to the inside edge as possible.

Set up the balance with two cups and ensure the arm is level.  (If you are using a scale with a single pan, you will only need one cup.)

Measure the length, width and height of the block.  Calculate the volume in cubic centimeters using the formula from the chapter on measurement.  (V = L x W x H)

Fill the bottle until water begins to flow out of the straw.  Allow this water to flow out. Hold the graduated cylinder under the straw while lowering the block into the water.  Catch the water that is displaced and flows from the straw.  How much water is displaced?  Is it the same amount as the volume of the wooden block?

If you are using the balance, dry the wooden block and place it on one side of the balance.  Place the water in the cup on the other side.  What happens to the balance?

If you are using a school balance, weigh the block, and then weigh the water.   If you are using a scale with a single pan, first weigh the block.  Then, remove the block and weigh an empty cup.  Next, pour the water you collected into the cup and weigh it.  Finally, subtract the weight of the cup from the weight of the cup and water to get the weight of the water.

Next, refill the bottle so that water is again level with the straw.  Hold the graduated cylinder under the straw to catch the water, and again place the block in the water, but this time, use a small nail to push the block down until it is completely submerged.  Make a note of the volume of water and its weight.

What Happened: When the block was floated, the volume of water displaced was less than the volume of the block.  However, the weight of the water displaced should have been equal to the weight of the block.  When the block was completely pushed under the water, the volume of the water dispaced should have been equal to the volume of the block.

This is the second part of Archimedes Principle: For any object that floats, the object will sink into the water until it has displaced its own weight in water.

One other thing.  It is not absolutely necessary to weigh the water if you know its volume.  That's because 1 ml of water has a mass of one gram.  That is explained in a bit more detail on the Measuring Weight and Mass page.


Buoyant Force

Have you ever tried to swim down to the bottom of deep swimming pool?  Did it feel like something was pushing up on you?  The force you felt was the “buoyant force” of the water.  This may be described as the “floating force” of the water, and it is the upward force exerted by water against the downward pull of the force of gravity.  This force may actually be measured.

Materials Needed: Spring or rubber band scale; small rock or metal object; string; container filled with water.

Procedure:   The spring or rubber band scale on the Measuring Weight and Mass page may be used for this experiment.  If you prefer, you can borrow a spring scale from your school.

Tie a string to the rock and hang the rock on the scale.  Weigh the rock and write down this weight.  Leave the rock attached to the scale, but submerge the rock in a container of water.  The rock should be completely under water, but not touching the bottom of the container.  What is the weight on the scale now?

What To Look For: The reading on the scale should be less when the rock is under water.

What Happened: Water exerts a force pushing upward on the rock.  This is the buoyant force of the water.  If the rock weighs 23 grams when weighed in air, but only 11 grams when weighed in water, then the buoyant force is equal to the difference in that weight.  23 g - 11 g = 12 g.  Gravity is pulling down with a force of 23 grams, but the water is pushing up with a force of 12 grams.  This causes the weight to appear to be only 11 grams.  However, since the weight of the object is not completely canceled out by the buoyant force, the object still sinks.

Going Further: Can you figure out a way to do this experiment using the homemade balance?

Also, the next time you go for a swim, try picking up a heavy object such as a brick.  Notice how heavy it feels.  Then, try the same thing with the brick under water.  Does the buoyant force make it feel lighter?


More on Buoyancy

As you have seen, buoyancy is the upward push of water on an object.  Let’s see how this works with an object that floats.

Materials Needed: Balloon; sink or bathtub filled with water.

Procedure:   Blow up the balloon and pinch it shut, but do not tie it off.  Push the balloon completely under the water.  Can you feel the buoyant force of the water pushing up?  While keeping the balloon under water, slowly begin letting air out of the balloon.  Do you feel any change in the buoyant force?

What Happened: The force pushing the balloon upward was very strong in the beginning, but as you let the air out of the balloon, it grew weaker.


When Will an Object Float?

Materials Needed: Small glass container such as a small bowl or baby food jar; sink or bathtub filled with water. 
 
Procedure:   Place the empty container in the water.  Does it float?  Gradually add water to the container until it sinks.

What Happened: As long as the buoyant force is greater than the weight of the container and it’s contents, the container will float.  When the weight of the container is increased by adding water, it sinks lower. The buoyant force pushing upward doesn’t change, but the force of gravity pulling down increases.  When the weight, or force of gravity pulling downward, is greater than the buoyant force, the container sinks.

Going Further: Try several different plastic containers.  Can you sink them?  Why or why not?


Specific Gravity

Time for a little math.  Now don’t panic!  The math is easy, and what you are going to calculate is a very useful number called “specific gravity”.  But if you do need any help with the math, get your math or science teacher to help you out.

Materials Needed: Graduated cylinder; several small objects, including some that float and some that don’t; scale or balance.

Procedure:   Weigh each object in grams and record the weight.  (If you are using the homemade balance, weigh to the nearest gram.)

Use the graduated cylinder to determine the volume of each object.  Fill the graduated cylinder about half full of water and record the water level.  Then carefully lower the object into the cylinder.  If the object sinks, record the new water level.  If the object floats, use a pencil point to push the object completely under the water and then record the water level.  Subtract the first reading from the second to get the volume of the object in ml.  (Remember, the submerged object displaces it’s own volume of water.)

Next calculate the specific gravity.  To do this, you divide the weight of the object in grams by it’s volume in ml (or cubic centimeters).  

Suppose you have a rock that weighs 80 grams and has a volume of 40 cubic centimeters.  It’s specific gravity would be calculated like this:

                                        Weight of object (grams)
Specific gravity        =      ---------------------------------
                                            Volume of object (ml or cc)

     80
=    ----
     40

   =    2   


To help you do this experiment, make a table like the one below to record your results:

Object Does it float? Weight (grams) Volume (ml) Specific gravity
Rock* No 80 40 2

What To Look For: How does the specific gravity relate to whether the object floats?

What Happened: Objects that float in pure water have a specific gravity of less than 1.  Objects that sink in pure water have a specific gravity of more than 1.  The specific gravity of pure water is exactly 1.

Going Further: Can you calculate the specific gravity of an ice cube?  Is it the same as liquid water?


Specific Gravity of Salt Water



Materials Needed: Clear container such as a drinking glass; water; salt; raw egg.

Procedure:   Fill the container about 3/4 full of water.  Lower the egg into the water.  Does it float or sink?  Remove the egg.

Next, dissolve as much salt into the water as you can.  Try floating the egg again.  What happens?

What Happened: The egg sinks in pure water, but it floats in salt water.  The specific gravity of the egg is slightly greater than 1, so it sinks. A solution of salt water has a higher specific gravity than pure water, so if you add enough salt, the specific gravity of the salt water will become greater than that of the egg.  Since the egg has a lower specific gravity than the salt water, it floats.

Going Further:  Can you calculate the specific gravity of the egg?  Can you apply what you have learned to calculate the specific gravity of the salt water?


Making a Model Submarine


We can apply what we’ve learned so far to make a model that will show us how a submarine works.

Materials Needed: Plastic drink bottle with cap; modeling clay; plastic tubing or flexible drinking straws and tape; nail; small rocks or gravel; rubber bands or duct tape; bath tub or swimming pool.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Procedure:   Use the nail to punch a hole in the cap.  Using the scissors, enlarge this hole to allow the tubing or a straw to fit snugly.  If you are using flexible straws, join several together with tape.  Place one end of the tubing or straws into the bottle cap and seal with modeling clay.  Use the nail to punch small holes in both sides of the bottle and place tape over these holes for the time being. 

Place rocks or gravel in the bottom of the bottle and place the cap and tubing assembly on the bottle.   You should add or take away gravel until the top of the bottle just floats just at the surface.

Now remove the tape from the holes in the lower part of the bottle, and place your thumb over the end of the straw or tube.  Place the submarine in the water and release your thumb.  What happens?  Replace your thumb.  What happens now?  Slowly blow into the straw or tube.  What do you observe?

What Happened: When you removed your thumb, the air was able to escape from the bottle and water was able to come into the bottle.  This increased the weight of the submarine and it began to sink.  By forcing air into the bottle, you forced water out, and the weight of the submarine decreased. This allowed your submarine to surface.

Going Further:   A real submarine works in much the same way.  The top of the bottle is similar to the part of the submarine where people live and work.  The bottom portion of the bottle is similar to the ballast tanks on a real submarine.  These tanks have air forced in or out of them to raise and lower the sub.  

You can make a model that is a bit more realistic by taping two bottles together on their sides.  The top bottle should be sealed by leaving the cap on.  The bottom bottle should be constructed as above. except that you will probably need more rocks to get the two bottles to sink.  

The top bottle represents where the crew works, while the bottom bottle represents the ballast tanks.


Cartesian Diver

 


Materials Needed: Glass medicine dropper (or clear soda straw and modeling clay); drinking glass; 1 or 2 liter plastic soft drink bottle with cap; water.

Procedure: This experiment is a little easier to observe and understand if you can find a glass medicine dropper.  If you don’t have one around the house, you may be able to borrow one from your school lab or purchase one from your pharmacist. They are also available at many school supply stores and hobby shops.  However, if you can’t get a glass medicine dropper, you can make a substitute using modeling clay and a clear straw. 

If you are using a medicine dropper, fill the dropper with water until it just barely floats in a glass of water.  Otherwise, cut a 5 cm (2 in) piece from the soda straw and plug one end with modeling clay.  Place another lump of clay around the other end, but leave the straw open at that end.  You will need to put just enough clay on so that the straw barely floats in the glass.


Substitute for a medice dropper.

Fill the bottle completely with water.  Place the dropper or straw in the bottle and screw the cap on tightly.  Place the bottle on a table top or other flat surface and squeeze the sides.  What do you see?  What happens to the water level inside the dropper or straw?

What Happened: When you squeezed the sides of the bottle, you increased the pressure inside.  Since a liquid cannot be compressed, only the air inside the dropper or straw was compressed.  This forced more water into the cap or dropper and increased it’s weight, causing it to sink.  When pressure on the outside of the bottle was released, the cap or dropper rose again.  If you used a dropper, you could easily see the water level rise inside when pressure was increased and see it fall as pressure was decreased.


WATER PRESSURE

You have already learned quite a lot about buoyancy.  You already know that  when you swim to the bottom of a deep swimming pool, the water is trying to push you back up.  You also feel the pressure of the water that surrounds you.  The deeper you go, the greater that pressure becomes.  The next few experiments will explore water pressure. 


Making A Manometer


In order to study water pressure, you will need to make a device to help you measure that pressure.  Such a device is called a manometer.  The one you will make will not measure the exact water pressure, but it will show changes in pressure.  You will need it for several experiments.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Small plastic soft drink bottle with cap (see procedure); scissors; balloon; clear tape; rubber band; nail; modeling clay; about 1 meter (39 in) of plastic tubing (from the hardware store); ruler; small piece of wood.

Procedure:  Cut the top of the bottle to make a funnel. Trim  any rough or sharp edges.  Place a strip of tape all around the cut part.  Cut off the neck of the balloon to make a sheet of rubber and stretch it across the cut part of the bottle.  If you are using a round balloon, the rubber should seal itself around the funnel. Make sure that the rubber is tight across the opening.  Don’t worry if the funnel is a little bent. It won’t hurt anything.  Seal your lips over the mouth of the bottle and blow gently to insure that the no air is escaping from the balloon and funnel.

Use the nail to punch a hole in the bottle top.  Enlarge this hole with the scissors so that it is large enough for the plastic tubing to fit snugly.  Insert about 1 cm (1/4 in) of the tubing into the top of the cap and seal around the top with modeling clay.  The funnel you have just made is the sensor portion of your manometer.  A sensor in any device that detects or “senses” changes. Do not screw the cap onto the funnel just yet.

Press a lump of clay onto the small piece of wood and insert the ruler into the clay lump. Tape the tubing to the ruler as shown.  There should be about a 15 cm (6 in) loop at the bottom.  Use a dropper to add water to the open end of the tube at the top of the ruler.  This water may be colored if you like. You should add water until it is about 5 cm (2 in) from the bottom of the loop on each side.  You may need to tap the tube or puff gently into the tube to get rid of any air bubbles.


Place the cap on the bottle assembly.  Gently press the balloon.  The water level should rise on the long side of the tube as you place pressure on the outside of the balloon.   The manometer detects changes in pressure when water presses in on the sensor.  As it does, this pushes on the air inside, which in turn, forces air up the tube and to the loop.  The air pushed up the tube from the sensor pushes the water up the other side of the tube.  The higher the water level on this side of the tube on the other side of the loop, the greater the pressure.  When you use the manometer, you will use the numbers on the ruler only to let you know how much higher or lower the water level is raised.  You can use either inches or millimeters because the numbers do not tell you the actual pressure, only the change in pressure.

If your manometer does not work properly, check your work to make sure everything is tightly sealed and that you have assembled everything correctly.


Using the Manometer

Materials Needed: Manometer from the last experiment; bucket of water.

Procedure:   Lower the bottle part of your manometer into the water.  As you slowly move it to the bottom, notice what happens to the water level in the tube.

What Happened: Pressure increased with depth of the water.  This is a very important characteristic of water. The pressure exerted by water always increases as the depth increases.  This is very important to people who build submarines.  The outside of a submarine must be made extremely strong so it will not be crushed by the pressure of deep water.


How Water Pressure Is Exerted

Materials Needed: Same as the last experiment.

Procedure:   Submerge your bottle sensor about half way into the water and note the reading on the manometer.  Move the sensor around at that depth.  Do you see a change in pressure?  Move your sensor near to the bottom of the bucket.  Again, move the sensor around.

What Happened: You should not have seen any significant change in pressure as you moved the sensor around the in either the middle of the bucket or near the bottom.  (You might have noticed a little bit due to the size and shape of your bottle sensor.)  This shows that the pressure at a particular depth is the same from the bottom, sides and top.  When you moved the sensor near the bottom, the pressure increased, but it should not have changed all that much as you moved the sensor around at that depth either.


Water Pressure In Different Bodies of Water

Materials Needed: Manometer; meter stick; bucket of water; sink; bathtub; swimming pool.

Procedure: Pick a depth to measure pressure between 15 and 30 cm (6 and 12 in).  Use the meter stick to measure the depth and measure pressure at this depth in all of the containers.  Is there any difference?

What Happened: The pressure should have been the same in all of the containers at the depth you chose.  The pressure of water at a particular depth is not related to the size of the body of water. The pressure at 30 cm (12 in) is the same in a bucket, a bathtub, or a lake.


Pressure Of Different Solutions

Materials Needed: Large food tin or similar container; salt; dish detergent; water; manometer; ruler to measure depth.

Procedure:   Fill the container about 3/4 full of water.  Carefully note the water’s depth. Measure the pressure at a convenient depth, about 2/3 of the way down.  Record the depth  and the pressure.

Dissolve as much salt as you can into the water.  Again, measure the pressure at the same depth.  Is it the same? Record your results.

Empty the container and rinse.  Refill it with water to the same level as before.  Add some dish detergent to make a very soapy solution.  Measure the pressure at the same depth as you did with the plain water and the salt water.  How does it compare with the other two.

What Happened: Dissolving substances in water will affect the amount of pressure the water exerts.


Water Pressure Engine


Materials Needed: Three square or rectangular plastic containers with screw caps such as a 1 liter (1 quart) milk jug; nail; strong string; monofilament nylon fishing line; paper clip or button; water; sink or bathtub. (If you don’t have a rectangular container, with a few changes, you can probably use a cardboard milk carton instead.)

Procedure: Using a nail and hammer, punch a hole in the center of the plastic cap of one of the containers.  Run a 60 cm (2 ft)  piece of string through the hole in the cap and tie the end under the cap to the paper clip or button.

Next, punch two holes near the top of the container.  This will allow air to enter the container. Then, punch two holes, one hole near the left hand end of each of two opposite sides as shown. 

This next part should be done over a sink or bathtub!  While holding your fingers over the two holes at the bottom, fill the container with water and screw the cap back on.  Hold the container by the other end of the string.  (You can loop the string around your hand a couple of times if you need to.) Uncover the bottom holes.

Repeat this experiment using the same type of container, but punch the holes on the right hand ends instead of the left.  Does it make any difference in either the direction or speed of the spin?

To help you understand why there might be a difference, try suspending the third container full of water with no hole in it.  Does this container spin?

Now replace the string with nylon fishing line and repeat this experiment.  Do you see any difference?

What Happened: Both containers with holes began to spin as water pressure forced water out of the holes.  (NOTE: If they did not begin spinning on their own, you might need to repeat the experiment, giving the jug a slight push above one of the bottom holes in the direction away from the running water.)  As the water level in each container dropped, the pressure decreased, and the spinning eventually slowed down. 

A good scientist always has to be careful to ensure that there are no unexpected influences in an experiment.  When you suspended the third container, you probably saw that it spun around slowly for a little while.  If you examine the twine you are using carefully, you will notice that it is made of several threads twisted together.  The weight of the water in the third container pulled down on the string and began to untwist it. 

When the first container spun one way, it tended to untwist the threads, and the rate or speed at which the container spun was increased by this untwisting.  However, when the container spun in the other direction, it twisted the threads tighter, and the container did not spin as fast.  Also, when the container is nearly out of water, it stopped spinning, and you probably noticed that the container began spinning in the opposite direction as the twine untwisted.   

You should not have seen the twisting or untwisting when you used the nylon fishing line since it is usually made of a single strand that is not twisted.  "Monofilament" means single strand.

Going Further:  This is the same principle that is used in making lawn sprinklers that spin or move back and forth. Examine one to see whether you can figure out how it works. 


If you want to know more about liquids, don't stop here.  Take a look at our Liquids - Part 2 page.


"The Science Notebook"  Copyright 2008-2017 - Norman Young