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Jan 20, 2013

Water Cycle: Oceans

Dissolving Salt in Water
  • Dissolving salt in water.
  • Observing how salt crystals form.
  • evaporation
  • salt
  • salt 
  • Epsom salt
  • trays
  • warm water
  • pan
  • rope-like string (mop string) 
  • measuring beakers
  • two jars or beakers. 
Students experiment with salt crystals.

BACKGROUND: Salts originate from the erosion of the land. Evaporation prevents salts from going into the clouds, so the waters become full of various salts.  The salt then precipitates if there is more salt than the water can “hold” within the molecules of water.  You can explain this as referring to a large structure made of paper cups, if there are too many cups, the weight of the cups will make all the cups fall down.  Precipitation is when the salt is “supersaturated” and the remaining salt falls to the bottom or precipitates.  
Salt water is salt mixed with fresh water.  However, salt in cold water does not dissolve as well as if the water is warm.  Warm water has more room between the water molecules, allowing more salt to fit.   Cool water molecules are tighter together and will not allow much salt to dissolve.  
  1. Students will make salt from water.  In this lab ordinary table salt will be used, but in reality the "salt" in salt water consists of various other compounds as will be discussed in the post lab.
  2. Water can just take so much salt.  Have the students measure about 10 ml of water into a jar.  Have the students stir in 1 ml of salt.  Discuss what happens.  Provide the students with warm water.  Have them repeat the procedure with the warm water.  Discuss that the salt will dissolve more readily in the warm water.
  3. Ask students if it is easy to tell if a liquid has salt in it without tasting it.  You may want to pour some water from one source into a dish but do not label it.  See if the children can guess, from its appearance whether the water is salty or fresh.  Have them tell you why they guessed the way they did.  Put the water in a place where it can evaporate and have the students see for themselves that the salt will be left behind.
  4. Students should measure about 100 ml of warm water (with beaker) and mix with 5 ml of Epsom salt (with spoon)  into the water.   They will pour half of the solution in another jar and place one end of the cotton string (mop string) in each of the two jars with Epsom salt solution, as shown in the diagram on the right.  Let the string dip between the jars and let the jars sit for several days.  The solution should flow from each jar toward the middle, where it will drip, forming “salt” pillars.  This will take a few days, so put the beakers  in a place where they will not be disturbed.
Check back in a week to see the results of our lab!
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