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Chemistry Review: Density

Defining Density

A large wooden block and a small stone in a large fishtank filled with water.  The wooden block floats, while the rock sinks.
  • A heavy wooden block floats in water but a much lighter stone sinks.
  • Why does this happen?

Density is about mass and volume

A small heavy rock and a large wooden block on a primary balance.  The wooden block weighs more than the rock.
  • A rock is kind of heavy for its size. It has a pretty high mass compared to its volume. So it has a fairly high density.
  • A block of wood is lighter for its size. It has a lower mass compared to its volume. So it has a lower density than the rock.
  • Water also has a certain mass compared to its volume and a certain density.
  • Comparing the density of an object to the density of water can tell you whether the object will sink or float.
  • The rock is more dense than water so the rock sinks. The wood is less dense than water so the wood floats.

Density is about sinking and floating

A small cup of water weighed against an equal volume of wax on a primary balance.  The water weighs more and is more dense.  The wax is also shown to float in a small cup of water.

When equal volumes of wax and water are weighed against each other, the wax is lighter. The wax is therefore less dense and will float in water.

  • You can compare the density of other substances to the density of water by using a balance.
  • Compare the weight of a wax candle to the weight of an equal volume of water.
  • Since the wax weighs less than an equal volume of water it must be less dense than water. Therefore the wax floats.
  • Since the clay weighs more than an equal volume of water it must be more dense than water. Therefore the clay sinks.

Comparing the density of liquids

A small plastic cup filled with vegetable oil and a small plastic cup filled with corn syrup on a primary balance.  The cup containing corn syrup weighs more than the vegetable oil.

When equal volumes of corn syrup and vegetable oil are weighed against each other, the corn syrup is heavier. It is therefore more dense.

  • You can compare the densities of water, corn syrup, and vegetable oil by weighing equal volumes on a balance.
  • Pour a volume of vegetable oil into one cup. Add paper clips to the other cup to get the approximate weight of the oil.
  • Weigh the same volume of water and corn syrup using paper clips in the same way.
  • Since equal volumes of each liquid was weighed, the one that weighed the most is the most dense and the one that weighed the least is the least dense.

A density tower

A plastic cup with three layers of liquids.  Corn syrup is most dense and on the bottom.  Vegetable oil is the least dense and is on top.  Water is somewhere in between and the layer of water is between the layer of corn syruup and vegetable oil.
  • From weighing the liquids, the relative densities of the liquids are known: Corn syrup is the most dense; Vegetable oil is the least dense; Water is in between.
  • Carefully pour the liquids in a clear plastic cup from most dense to least dense. The liquids will float or sink on one another based on their densities.
  • Place objects such as uncooked pasta, a paper clip, crayon piece, and piece of popsicle stick into the liquids.
  • Where the objects end up in the liquids can help you compare the density of the objects to one another and to the liquids.

Changing the density of a liquid

A carrot is placed in a cup of water and it sinks. When two spoons of salt are added, the water becomes more dense, and the carrot floats. Then, more freshwater is added to the cup such that rather than sinking or floating, the carrot “hovers” in the center of the solution, because its density is equal to that of the surrounding solution.

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  • You can see that the density of water can be changed by dissolving salt in it.
  • Place a carrot slice in water. The carrot should sink. This means that the water is less dense than the carrot.
  • Add salt and stir until the carrot floats. Since the carrot floats, the salt water must now be more dense than the carrot.
  • Dissolving the salt in the water increases the mass of the salt water more than its volume. So the salt water becomes more dense than the carrot.
  • Adding the right amount of fresh water can then make the salt water about the same density as the carrot so the carrot “hovers” in the salt water.

Density: hot and cold water

This video shows that hot water is more dense than cold water. Two jars are filled with water, one with hot water and the other with cold water. The hot water is colored yellow with food coloring and the cold water is colored blue. The jar containing the hot water is carefully inverted so that the opern mouth of the jar meets up with the mouth of the cold water jar. Even with gravity pulling it down, the hot water does not mix with the cold water, which we can observe by noticing that the two water samples colored yellow and blue do not mix to form green. When the process is repeated with the cold water on top of the jar of hot water, the samples mix quickly, and both water samples turn green.

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  • The hot water is less dense than the cold water and should float on top of the cold water without much mixing.
  • You can try this demonstration again but this time starting with the cold water on top. The cold water will sink in the hot water causing immediate mixing.

Changing the density of an object

An aquarium filled with water.  A ball of clay sinks and sits at the bottom of the tank, an ball of clay of equal size that is formed into a boat floats.
  • The density of an object can be changed by changing its shape.
  • Start with two balls of clay each about the size of a ping pong ball.
  • Flatten one into a small thick pancake and the other into a larger thinner pancake.
  • Fold the sides up on each pancake to make a small thick-sided box and a larger thin-sided box.
  • Place each in water. The larger box floats and the smaller box sinks.
  • Both boxes should weigh the same but the larger box has a greater volume. Since density = mass/volume, the larger volume lowers the density and the box floats. The density of the clay itself has not changed but the density of the object has.

Summary

  • Matter exists in three states or phases: solid, liquid, and gas.
  • A combination of the motion of molecules and their attraction for one another determines whether a substance is a solid, liquid, or gas.
  • Adding heat energy increases the motion of molecules.
  • Removing heat energy, or cooling, decreases the motion of molecules.
  • If heat is added to a solid, the molecules can move fast enough to change from a solid to a liquid. (Melting)
  • If heat is added to a liquid, the molecules can move fast enough to change from a liquid to a gas. (Evaporation)
  • If heat is removed from a gas, the molecules can slow down enough to change from a gas to a liquid. (Condensation)
  • If heat is removed from a liquid, the molecules can slow down enough to change from a liquid to a solid. (Freezing)
  • Heat always moves from an object at a higher temperature to an object at a lower temperature.

Molecular Animation Downloads

The animations featured in the slideshow above are available for download by clicking on the "Download this animation" link below each file.

Each video is offered for download in both Quicktime Movie (.mov) and Windows Media Video (.wmv) format.

To play these videos, you will need either Quicktime or Windows Media Player.