Lesson 1: The Hydrosphere

Getting Started

In this unit you will explore one of the unique features of the Earth, the hydrosphere. The primary component of the hydrosphere is water, and because of its abundance, Earth is often called the "water planet." The hydrosphere is made up of oceans, lakes, and rivers, as well as the water beneath the ground. It is important to understand the characteristics of water that make it a unique molecule and one very necessary for life on Earth.

One important characteristic of water is density. Density is a measurement found by dividing an object's mass by its volume. Mass measures how much matter is in an object, and volume is the amount of space that an object takes up.

Note that mass is different from weight. Weight is a measurement of how heavy something feels — it is the amount of force an object will exert on something when you also include the effect of gravity pushing down on the object. An object's weight is greater on Earth than it is on Mercury, for example, but the object's mass is the same on both planets.

In this lesson, you will conduct investigations to help you understand density and how density influences the behavior of water.

Stuff You Need

  • Painless Earth Science by Edward J. Denecke, Jr. (green edition)
  • blue food coloring (kit)
  • eye droppers (kit)
  • graduated cylinder (kit)
  • green food coloring (kit)
  • ice cube tray (kit)
  • measuring cup
  • measuring cups and spoons
  • pot
  • red food coloring (kit)
  • salt
  • scale (kit)
  • small clear cups (kit)
  • yellow food coloring (kit)

Things to Know

  • The density of a material is its mass per unit volume. Density is mass divided by volume (D = M/V).
  • All substances do not have the same density.
  • The density of a solution is influenced by what the solution contains. For example, tap water is less dense than salt water.
  • Density plays a significant role in the movement of the oceans' waters.
  • Salinity and temperature influence density.

Ideas to Think About

  • What are some unique characteristics of water?
  • How are these characteristics important in the behavior of water?
  • How are these characteristics important for life?


  • Experiment with variables. (S)
  • Observe, collect, organize, and analyze data. (S)
  • Analyze evidence to explain observations, make inferences and predictions, and develop the relationship between evidence and explanation. (S)
  • Use oral and written language to communicate findings. (LA)
  • Analyze and evaluate information from a scientifically literate viewpoint by reading, hearing, and/or viewing scientific texts, articles, and events in the popular press. (S)
  • Analyze the unique properties of water, including density. (S)

Introducing the Lesson

In this lesson, your child will begin learning about concepts associated with the hydrosphere, specifically its primary component, water. He will conduct investigations to understand more about water's characteristics; this lesson focuses on understanding how density impacts the behavior of water and how this characteristic makes water a unique molecule that is important for life on Earth.

Your child will also be challenged to comprehend two very important components of scientific investigation — claims and evidence and cause and effect relationships. As the unit progresses, the goal is for your child to make connections between evidence that he gathers and what this evidence reveals to him about the subject he is investigating. In Lesson 1 and other lessons throughout the unit, your child will be focused on the unique properties of water and why these properties, which are taken for granted every day, can mean the difference between life and death.
Reading and Questions
Read the web article "Water Density" from the United States Geological Society (at the following web link). Answer the following questions, and keep in mind the important question, why is density important for life?
Web Link

  1. If two substances have exactly the same mass, but one takes up less space (i.e., has less volume), which substance will be more dense? Why?
    The substance that takes up less space will be more dense. This can be thought about in two ways: 1) since the density of a substance is defined as mass/volume, if the mass stays the same and the volume decreases, then the density will increase (i.e., if the denominator of a fraction decreases, the value of the fraction increases); or 2) a substance that is dense can be thought of as having its molecules "packed tightly together." If a substance is squashed into less space (i.e., less volume), yet still has the same mass, then its molecules are packed more tightly together, so it's more dense.
  2. Why is ice less dense than water? For most substances, is the solid form less dense than the liquid form?
    Molecules in ice are more spread out than molecules in water, so the ice takes up more space (has more volume), and is less dense. Most substances take up less space and are more dense in solid form than they are in liquid form.
  3. What would happen in a lake if ice was more dense than liquid water? Would this be good or bad for life in the lake?
    Frozen ice would sink to the bottom of the lake, and the lake would freeze from the bottom to the top. It would also make it hard to melt ice at the bottom of the lake even in the heat of summer. This would be bad for life in the lake.
Reading and Questions
Materials: Painless Earth Science by Edward J. Denecke, Jr. (green edition)
Read pages 103-104 and "Circulation in the Oceans" and "Convection Currents" on pages 108-109 in the book Painless Earth Science. For this lesson you will focus on salinity and density. As you read, also keep in mind the question, why is density important for life?

Note that the "Brain Ticklers" in Painless Earth Science are always optional unless they are specifically mentioned in a question or reading assignment.
  1. Is warm saltwater more dense, less dense, or the same density as cold saltwater? Why?
    Warm saltwater is less dense than cold saltwater. Adding heat energy to water causes its molecules to move around more, which causes them to bump into each other more and thus bounce further away from each other. This causes the warmer water to take up more space, i.e., to increase its volume. If a given mass of water increases in volume, its density decreases. (Note that in general, warmer fluids are less dense than colder fluids, so this same effect holds true for freshwater, liquid solutions, gases, etc.)
  2. Where is the most dense ocean water found?
    This question could be answered in two ways: 1) The most dense ocean water is found at the bottom of the ocean (since at any particular ocean location on the planet, warmer, less dense, water rises to the surface, and colder, more dense, water sinks to the bottom; and 2) The most dense ocean water is found in the polar regions (since if we consider the ocean water for the entire planet, the water at the poles is colder, and therefore denser, than the water closer to the equator).
  3. What is upwelling and how does it support the growth of plants and animals in the ocean?
    Upwelling occurs when cold seawater, flowing along the ocean floor from the poles, displaces warmer water upward. These "convection currents" cause ocean water to circulate, and this circulation moves nutrients from the sea bottom to other parts of the ocean.