The density of a material can be found by measuring its mass (g) and volume (mL for liquids, cm3 for solids), then dividing the mass by its volume.  But can the density of a material change?  Below we will address this question by determining of shape, size/amount, or temperature can change the density of a material.

As we have observed, a material that is more dense than the material around it will sink.  Material that is less dense than the material around it will float. This explains why the Earth layers in the way that it does!  The most dense layer (inner core) "sinks", while the least dense layer (crust) "floats"!

Since the mass and volume both do not change when re-shaping the clay, the density does not change.  Even though it may seem as if the clay is more compressed (tightly packed) in the oval than in the hollow square, the spacing between those tiny particles that make up the clay is constant (does not change).  Therefore, the shape of a material/substance does not affect its density.

2.  Does size (or amount) affect density?
Dr. OPHERC holds two different amounts of molasses that came from the same jar.  Does the amount of molasses affect its density?

Again, in order to answer this question, we simply need to measure the mass and volume of the molasses at each amount.

The mass and volume both change when changing the amount of molasses.  However, the density does not change.  This is because the mass and volume increase at the same rate/proportion!  Even though there is more molasses (mass) in test tube A, the molasses also takes up more space (volume).  Therefore, the spacing between those tiny particles that make up the molasses is constant (does not change).  In other words, the size or amount of a material/substance does not affect its density.

3.  Does temperature affect density?
If we heat or chill the same amount of water, then does the density of the water change?  Fairly measuring the mass and volume of the water (in order to address this question) is a bit more challenging, because water evaporates when heated, thus affecting both the mass and the volume of water being measured.

Check out the video below on "lake stratification" in order to learn more about how temperature affects the density of water (and other materials).

Assignment
Complete the assignment on Google Classroom.

Continental Drift
About 100 years ago, German scientist Alfred Wegener noticed that the coasts of western Africa and eastern South America matched up like pieces of a jigsaw puzzle.  Although he was not the first to notice this, Wegener was the first to formally present evidence suggesting that the two continents had once been connected.  Wegener's hypothesis was called "continental drift".  

Wegener was convinced that the two continents were once part of an enormous super-continent called Pangaea, meaning “all earth,” that split apart over hundreds of millions of years.  He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America.  Mesosaurus, which lived in  freshwater lakes and rivers, was only about one meter long and would not have been able to swim across the salty Atlantic Ocean.   

Wegener also used climate clues as evidence to support his hypothesis (claim) of continental drift. Fossils of warm-weather plants had been found on the island of Spitsbergen in the Arctic Ocean. Wegener explained that Spitsbergen likely "drifted" from warmer, tropical regions.  In the same way, glacial deposits and rock surfaces polished and marked by glaciers are found in warm regions where glaciers do not exist, such as South America, Africa, India, and Australia.  Wegener argued that these continents were likely connected and partly covered with ice near Earth's south pole a long time ago.

Plate Tectonics
For a very long time, other scientists did not accept Wegener’s theory of continental drift.  But today, scientists know that the Earth's lithosphere is broken into massive tectonic plates that float on the asthenosphere, and are always moving and interacting.  This is called the theory of plate tectonics.  

So then what is causing the tectonic plates to move?  Convection currents in the Earth's mantle.  As a result of the uneven heating of the Earth's plastic mantle (asthenosphere), hotter, less dense mantle is continually rising, and cooler, more dense mantle is continually sinking.  This transfer (and cycling) of heated material is called convection.   Convection can occur in any gas or liquid where there is uneven heating. 

The Earth is Forever Changing
The constant movement changes the Earth's surface, rearranging and reshaping its landmasses, creating mountains, new sea floor, volcanoes, Earthquakes, and land rifts.  In fact, the continents are still moving.  North America and Europe are moving away from each other at the rate of about 2.5 centimeters per year.  In another 360 million years, it is even possible that another super-continent may form someday!