Experiment 3-5

Density of Liquids

Background Information

In Experiment 3-4 you hopefully were able to determine that different kinds of solid materials have easily distinguishable densities.  But objects made from the same material have the same density within the uncertainty of your measurements and calculations.  Thus the property of density is a valuable characteristic property.

Solids maintain their shape and size, but liquids take the shape of their container.  These are called operational definitions because they specify what operation to do to determine if a material is included in the category.  For example, you place a material in a container and observe whether it flows to the shape of the container.  Based on the observed outcome, the substance can be determined to be a solid or liquid.  (Conceptual definitions will be contrasted later.)

The ancient Greeks presumed based on their four element theory that all liquids are elementally water but differ in the amount of other contained elements.  Today most people realize that while many of our beverages do contain water, water is not required for a material to be a liquid.  Instead we believe liquids have their defining behavior due to weak bonds inside the substance allowing their constituent building blocks to flow past each other.  Weaker internal forces are overpowered by the force of gravity which reduces stored gravitational energy by minimizing the average distance from the substance to the center of the earth.  That is to say that liquid flows to the lowest position available to it, pulled by gravity.  In a solid the internal forces are stronger, preventing gravity from reshaping the material.

As a result of Archimedes insights, the ratio of the mass of a substance and the volume it displaces is defined by density:

Density = mass / volume

Scientists develop the procedures based on their previous training and experiences mixed with small amounts of needed creativity.  Occasionally they get suggestions from their peers.  Since you are familiar with liquids and previously found the density for solids, you should be able to develop your own procedure for determining the density of liquids.

Experiment

In this experiment compare the densities of several distinct liquids.

Water is the most common liquid on earth.  The density of water was originally used to help establish the standard unit of mass.  So be sure to measure and calculate the density of pure liquid water.

Cooking oil, denatured alcohol, rubbing alcohol, and gasoline are other distinct substances worth measuring.  (Caution: These are flammable substances.  Vapors can form and be ignited by flames or sparks some distance away.  Their vapors can also impair your nervous system.  Work with adequate ventilation or outdoors where vapors will be dispersed.  Recycle or dispose of these materials appropriately.  Consult labels!)

Tip:  You will reduce the introduction of experimental error into your calculations by using the same graduated container for measuring the liquid volume as the container used to hold the liquid on the balance.  Just don't forget to subtract the container's mass before calculating the liquid's density.

As always, record your investigation into your journal and, if credit is needed, construct a written report.

to experiment 3-6