Sun Dried: How Can the Sun be Used to Separate Salt from Salt Water?

Problem

How can the sun be used to separate salt from salt water?

Materials

• Cookie sheet
• 2 sheets of black construction paper
• 2 tablespoons (30 ml) table salt
• 1 cup (250 ml) tap water
• Spoon 

Procedure

1. Cover the bottom of the cookie sheet with the black paper.
2. Add the salt to the water in the cup and stir. Most, but usually not all, of the salt will dissolve.
3. Pour the salt water over the paper. Try not to pour any undissolved salt onto the paper. Allow the undissolved salt to remain in the cup.
4. Place the cookie sheet in a sunny place where it will not be disturbed for several days. This can be by a window or outdoors if the weather is warm and dry.
5. Observe the paper daily until it is dry.

Results

At first, a thin layer of white crystals forms on the paper. Later, a few small, separate, white, cubic crystals form.

Why?

As the sun heats the salt water, the water evaporates and dry salt is left on the paper. While this experiment is similar to a method used by some salt companies to produce salt by the evaporation of seawater, the amount of salt in seawater is much less. This method of salt production is known as the solar process, and the product is called solar salt. Solar salt is still produced in large amounts in many countries, including the United States.

Let's Explore

1. How much solar salt would be produced if the water from a cupful of ocean water evaporated? Repeat the experiment, preparing mock ocean water using 1 teaspoon (5 ml) of salt in the cup of water.
2. A saltern is a place where salt is produced by the solar process. A simple saltern can be made by digging a shallow pool near the sea and allowing seawater to flow in. The flow from the sea is then shut off to allow the sun to evaporate the water, leaving a deposit of salt crystals. How does evaporating water from a shallow pool of salt water affect the salt crystals produced? Repeat the original experiment, replacing the cookie sheet with a small bowl. Science Fair Hint: Take photographs of the salt left in this experiment and the original experiment to represent the results. Use the photos as part of a display.

Show Time!

Example

Think!

1.  
   1. Salinity is the measure of the amount of salt dissolved in water. The average salinity of seawater is 35 parts per thousand. This is written as 35 ppt. This means that 35 units of salt are in every 1,000 units of seawater. The gram is the unit most commonly used in measuring salinity, but any unit of mass (the amount of material in an object) or weight (the force with which an object is pulled toward the center of the earth due to gravity and mass), such as pounds, can be used. While most samples of seawater have a salinity of 35 ppt, it does vary from place to place. The range of salinity of seawater is usually between 32 ppt and 38 ppt. Find out where these salinities are most common, and display a map indicating their location.
   2. Along with the map, prepare and display photos of samples of the three salinities: 32 ppt, 35 ppt, and 38 ppt. Label the samples and indicate the amount of salt and water in each. Following the example given for the preparation of a sample of seawater with 36 ppt, prepare your three samples.

36 ppt = 36 g salt + 1,000 g water

Fact: 1 g of water has a volume of 1 ml, thus 1,000 g of water has a volume of 1,000 ml (1 liter).

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