NASA: National Aeronautics and Space Administration NASA Goddard Space Flight Center Home Page NASA Jet Propulsion Laboratory Home Page
NASA Logo - Goddard Space Flight Center NASA | GSFC | JPL | Site Map
SEARCH NASA
Aquarius satellite image Aquarius title
Education: Student Outcomes

Filtered by outcome: 0-4q2
Click here to begin a new search
Outcome: Compare the basic properties of fresh and salt water (e.g., density, ability to dissolve salt, freezing point).
Grade level: K-4
Theme: water cycle
Activity: http://aquarius.nasa.gov/sea_water_freeze.html

Big Idea
Salt causes water to freeze at a lower temperature. The oceans do not freeze (except in extreme polar areas) because of the salinity (i.e., concentration of salts).

Key Concepts
Dissolving any substance in pure water raises or lowers the freezing and boiling point.
When water freezes - goes from the liquid state to the solid state - its molecules go from a disorganized state to an organized state.
When water freezes to a solid, molecular motion slows down enough that the molecules become permanently fixed in an orderly arrangement called a crystal.
The individual particles that make up salt (known as ions) arrange themselves around the water molecules. In doing so, they shield the water molecules from interactions among themselves, making it less likely that they will find each other and form ice.
The water molecules have to be slowed down even more in the presence of salt in order to form a solid. So you have to go to a lower temperature in order to freeze water that contains salt.Salt is excluded in the formation of ice; therefore ice made from salt water is essentially salt-free.
Salt water is denser than freshwater because of its salt content, and, as a result, an object is more buoyant in the ocean than in a freshwater lake. The density of salt water depends on its salinity and varies around the world.

Background
Under normal conditions, ordinary fresh water freezes and melts at 0°C, or 32°F. If water and ice are at 0°C, which is the melting point of ice and the freezing point of water then molecules of ice are escaping into the water (melting), and molecules of water are being captured on the surface of the ice (freezing). (more)
Grade level: K-4
Theme: water cycle
Activity: http://aquarius.nasa.gov/liquid_rainbow.html

Big Idea
When solutions of two different densities meet, the lower density (less dense) solution will move on top of the higher density (more dense) solution, resulting in a layering or stratification of the solutions. Density is an important feature of seawater since many physical and biological processes are affected by it, such as moving heat around the globe influencing climate and feeding and reproduction by marine organisms.

Key Concepts
Different densities of water, or solutions, will stratify to form layers.
Density of ocean water is influenced by temperature and salinity.
Cold water with dissolved salts (higher salinity) is denser than warm water without dissolved salts (low or no salinity).
In the oceans, the deep, bottom layer is colder and saltier than the surface layer.

Background
Density is a property of matter that can be introduced at the elementary level by thinking of it in terms of the relationship between weight and volume. How can two objects that are the same size 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. Thus the rock is denser than the cork because it has more mass in the same volume.
Grade level: K-4
Theme: water cycle
Activity: http://aquarius.nasa.gov/prop_fresh_sea.html

Big Idea
Water has unique properties. About 97 percent of all water is in the oceans. Salt water or seawater has characteristics similar to fresh water with some noticeable differences because of the salts that are dissolved in water.

Key Concepts
Life on Earth is possible because of water's unique properties.
Water is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth.
Water can absorb a tremendous amount of heat acting like a heat buffer for the Earth.

Background
Liquid water (H2O) is often perceived to be pretty ordinary as it is transparent, odorless, tasteless and ubiquitous. Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) - at the temperatures normally found on Earth. Earth's water is constantly interacting, changing, and in movement. Water freezes at 32° Fahrenheit (F) and boils at 212° F (at sea level). In fact, water's freezing and boiling points are the baseline with which temperature is measured: 0° on the Celsius scale is water's freezing point, and 100° is water's boiling point. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats.