Exploring Our Fluid Earth

Separate the substances in seawater by evaporation

• Seawater
• Fresh water
• Graduated cylinder
• 2 Evaporating dishes (e.g. 5 cm aluminum pan)
• Oven (alternative: sun, heat lamp, or hot plate)
• Scale
• Dissecting microscope

1. Find the mass of a 10 mL sample of fresh water and a 10 mL sample of seawater (note: if tasting samples at end, use a food-safe graduated cylinder or 2 teaspoons of each liquid) 
   
   1. Weigh an evaporating dish. Record the weight in Table 2-1.  
   2. Using a graduated cylinder, measure out a 10 mL sample of fresh water and pour it into the evaporating dish.
   3. Weigh the evaporating dish and the water. Record in Table 2-1.
   4. Subtract the mass of the evaporating dish from the weight of the evaporating dish and water together to find the weight of water. Record in Table 2-1.
   5. Repeat a-d for seawater.
   6. Label each evaporating dish
2. Evaporate the water from the samples.
   1. Bake the samples at in an oven until all the liquid has evaporated. Keep your oven temperature under 200˚F.
   2. Alternately, you can evaporate the liquid using the sun or heat lamp as a heat source. You can also use a hot plate. To use a hot plate, heat the evaporating dish to no more than 60˚C. Do not let the water boil. When the water is almost gone and the crystals look slightly wet, you can turn up the heat a little to remove the remaining water until the crystals are dry. If the crystals begin sputtering, turn down the heat. You will get better crystals using the oven or sun than using a hot plate.
3. Find the mass of the crystals in the samples.
   1. a.Weigh the evaporating dish and the dry crystals for seawater. Record in Table 2-1.
   2. b. Subtract the mass of the evaporating dish from the weight of the evaporating dish plus the salt crystals. Record in Table 2-1.
   3. c. Repeat a and b for your fresh water dish.
4. Describe the crystals in each evaporating dish under a dissecting microscope. How many kinds do there appear to be in each dish? How are they distributed? What is the evidence for your answers? Draw the shapes of all of the crystals you see in each of your samples. Record any other observations in your notebook.