Why do objects that are the same size sometimes have different weights?
The answer has to do with their density.
An object's density is determined by comparing its mass to its volume.
If you compare a rock and a cork that are the same size (they have equal volume), which is heavier? The rock is, because it has more mass.
The rock is denser than the cork, then, because it has more mass in the same volume - this is due to the atomic structure of the elements, molecules, and compounds that make it up.
Liquids have density, too. You can perform several experiments with different types of liquids to determine which is more dense. These experiments can make a good science fair project; use them as a foundation and then come up with your own ideas of what to test.
Experiment 1: Sink or Swim
Question & hypothesis:
Will a raisin, paperclip, penny, small cork, ball of paper, and other small objects sink or float if they are placed in water, corn syrup and vegetable oil?
Write down what you think will happen when you place each object into the three different liquids.
What You Do:
- Pour 150 ml of water into beaker #1, 150 ml of corn syrup into beaker #2, and 150 ml of vegetable oil into beaker #3. (If you are using glass jars, use 2/3 cup of liquid, which is approximately 150 ml.)
- Gently set a raisin in each beaker. Does it sink or float? Write down what happens to the raisin in each beaker.
- Take the raisins out of the beakers and try a different object, such as a paperclip or cork. Record what happens in each beaker.
Conclusions: Were your predictions right? Did the raisins and other objects sink and float when you expected them to? Did they float in one liquid and sink in another? Why do you think they acted the way they did?
The denser a liquid is, the easier it is for an object to float on it. If one of your objects floated in the corn syrup but sank in the water, what does that tell you about the densities of water and corn syrup? Take the experiment a step further to find out more.
Experiment 2: Mix it up
Question & hypothesis: Which is the most dense: water, corn syrup, or vegetable oil? Which is the least dense? Based on your results from experiment #1, predict which liquid you think is the most dense and which you think is the least dense.
What You Do:
- Place a few drops of food coloring into the beaker of water so you will be able to tell it apart from the other liquids. (This is not necessary if you are using dark corn syrup.)
- Carefully pour each of the liquids into a 600 ml beaker or a large jar. Let them settle.
- What happened? Did the three liquids mix together or separate into layers? Which liquid is at the bottom of the jar? Which is at the top?
Conclusions: Was your prediction right? If so, the liquid you thought was densest should be at the bottom of the jar. The next dense will float on top of that, and the least dense will float at the very top.
Now you know how the densities of the three liquids compare to each other. If you want to find out the approximate density of each, you can calculate it using this formula: Density = Mass/Volume.
On Earth we measure mass (how much of a substance there is) by calculating weight (how heavy it is). Weigh each liquid in grams (make sure you subtract the weight of the beaker!) and then divide that number by the volume (number of milliliters) of the liquid.
The answer is density in grams per milliliter.
(Your answer will be more exact if you use a graduated cylinder instead of a beaker to measure the volume and weigh the liquid.)
Materials for Experiments 3 & 4
Experiment 3: Hot and Cold
You've found out how the density of water compares to the density of oil and corn syrup; now see if you can change the density of water itself!
Question & hypothesis: Does temperature change the density of water? Write down what you think will happen when you mix cold water and hot water.
What You Do:
- Fill two beakers with 150 ml (2/3 cup) of water. Put several drops of blue food coloring in one beaker, and several drops of red in the second.
- Add a handful of ice to the blue water and put it in the refrigerator for a few minutes. Put the red beaker in the microwave for a minute.
- Take the blue beaker out of the fridge and the red beaker out of the microwave. Pour some of the blue water into the 10 ml graduated cylinder or narrow glass. Using a pipet, slowly add red water a drop at a time and watch what happens. (This part may take a little practice—if you add the red water too fast you will force the colors to mix. Hold the pipet near the surface of the water and keep trying until you get it!)
Conclusions: Was your prediction right? What happened to the colored water? Did it stay in layers? Which layer was on the bottom? On the top?
What does this tell you about the density of hot water compared to cold water? What would happen if you left the cylinder out until the cold water warmed up and the hot water cooled off? Do more experimentation to find out!
Experiment 4: Salty or Sweet
Now you know that temperature can affect the density of water. In this part of the experiment, test to see if adding salt or sugar will make water more dense.
Question & hypothesis: Will adding salt make water more dense? Will adding sugar make water more dense? Which is denser, sugar water or salt water? Write down what you think will happen to the density of water if you add salt or sugar.
What You Do:
- Fill three beakers with 150 ml (2/3 cup) of water. Add food coloring to make blue, red, and green water.
- Add 2 teaspoons of salt to the red beaker and stir until the salt is dissolved. Add 2 teaspoons of sugar to the blue water and stir until it is dissolved.
- Try putting a raisin in each of the beakers. Does it float? Remove the raisins with a spoon.
- Pour some of the red (salty) water into the graduated cylinder. Using the pipet, slowly add the blue (sugar) water one or two drops at a time. Record which sinks to the bottom and which floats on top.
- Add the green (pure) water drop-by-drop to the other two and record what happens.
Conclusions: Were your predictions correct? Did adding salt and sugar to the water make the water more dense or less dense? Which was more dense, the salt water or the sugar water?
Liquid Density Science Lesson
Dive! Dive! Dive!
The study of density is very important if you sail the high seas! A ship is built to float because its average density is less than the density of water.
(Even though it is made out of very dense materials, like steel, most of its volume is actually filled with air, because ships are hollow on the inside. This lowers the average density.)
But what about submarines? They can float or sink, and they can remain steady at any depth they want! Have you ever wondered how they do that?
Submarines are built with special tanks called ballast tanks. When the submarine is on the surface, these tanks are filled with air, which makes the average density of the sub less than the density of water.
If the submarine needs to dive, the sailors flood the tanks with water from the ocean. Since water is much denser than air, this increases the average density of the submarine. As soon as the sub's density is more than the water's, the sub will sink.
So what prevents the submarine from diving all the way down to the ocean floor? When the sub reaches the correct depth, its density is adjusted by changing the amount of air or water in secondary tanks called trim tanks. When these tanks have the right balance of air and water, the submarine's average density equals that of the surrounding water, so it will neither float nor sink.
Try it out with a plastic drink bottle in the sink or bathtub. If the bottle is empty (filled with air), it will float on the surface. If you fill the bottle with water, its density will increase and it will sink. Can you figure out how much water should be in the bottle to make your "submarine" stay at a steady depth in the water?
The Navy has to keep track of the salinity of seawater (how much salt is in it) because salt makes water more dense. Changing water density could cause a submarine to rise or fall unexpectedly.
Try putting your bottle submarine in a sink full of salt water. Do you need to add more fresh water to the bottle to make it stay at a steady depth than you did when the sink had fresh water in it? This test helps you see the higher density of seawater, but a real sub doesn't have to add fresh water to the ballast tanks - it just fills up with the higher density salt water.