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There is more to dissolving table salt into water than meets the eye.

A new substance is not created during a physical change.. The substance is changed but can always be changed back. Think of water as ice which is melted can be refrozen. Water as a gas can be condensed to produce liquid water. A subtle form of this is when you dissolve a substance into water. Technically, you can allow the water to evaporate and the substance will still be there.

We all know you can dissolve more sugar into hot water than cold water. However, most people do not know that this is not true for table salt (NaCl). Temperature does not affect how much some salts can be dissolved in water. The reason lies with the fact that salts are ionic, inorganic compounds and sugar is a covalent, organic compound thus vastly different substances.  

The following experiments illustrate this point.

Take one cup of chilled water. You might use ice in the water but remove the ice before pouring.
Test how many teaspoons or 1/2 teaspoons of sugar you can dissolve completely in the water.

Take another cup of chilled water.
Test how many teaspoons or 1/2 teaspoons of table salt you can dissolve completely in water.

3. Take a cup of hot or warmed water.
Test how many teaspoons or 1/2 teaspoons of sugar you can dissolve completely in the water.

4. Take another cup of hot or warmed water.
Test how many teaspoons or 1/2 teaspoons of table salt you can dissolve completely in water.

5. Have students put their data on a table and make bar graphs of their results.

:You can add skills by have students mass in grams the solute and measure in volume (mL) the solvent.

Vocabulary: Physical Change - A change in a substance which does not change the substance and is reversible.
Solute - That which gets dissolved.
Solvent - That which does the dissolving.

pre-K to 2nd grade - dissolving salt or sugar in water is an appropriate activity. In 2nd grade, you can introduce dissolving salt or sugar in different temperatures of water.,
Measuring should be introduced with Grade 3. The comparison of different amounts to affect the reaction should be grade 5 and above.
You can spend a lot time getting children to do activities that are grades ahead or you can lay the foundation so they can learn more readily later.
For high school, have students get the mass and volume of water and solute. Then get the volume and mass after the solute has been dissolved in water. Allow students to combine their combinations in a 1000 mL graduated cylinder or beaker. This allows students to see that the volume of the solute and water does not account for the volume of the solute and solvent before mixing. Ask students why this happened. It is because the solute molecules fit between the solvent molecules and the substance is more dense.

5 comments:

  1. What are the risks of dissolving salts in water?

    ReplyDelete
  2. This is the value of input. Thanks for this question because it illustrates an important point. Each salt has its own characteristics and can affect human and environmental health differently.
    Table salt should be used. In the United States, KCl is used as a salt substitute for those on a low sodium diet. This could be used if you want to use a secondary salt.
    A successful science lab is based on an experiment in which student safety is number one. The nature of an accident is that it is not expected nor planned. With so many experiments to choose from, why choose one that can adversely affect the environment.

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  3. I forgot to suggest dangers with table salt. Mildly salted water should be no problem on skin or ingested. In the eye, it could cause irritation.
    Table salt is rough on skin. About 50 years ago, a friend of mine poured a cannister of salt on my head during a Girl Scout picnic. I was immensely proud of the skin that peeled from my scalp the next week at school. It was sort of like having sunburn. Anyway, control the horseplay. We don't want to find out what unique dangers lurk in salt and water for kids who just want to have fun.
    Submit your input.

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  4. "Temperature does not affect how much most salts can be dissolved in water."

    That is not a true statement. You can dissolve more salt in boiling water.

    ReplyDelete
    Replies
    1. Here are two sources showing how much NaCl can be dissolved in water at different temperatures.
      http://galileo.phys.virginia.edu/education/outreach/8thgradesol/TempSolubility.htm
      http://www.middleschoolchemistry.com/multimedia/chapter5/lesson6
      Table salt is relatively unaffected by temperature. Gases decrease their solubility with temperature. Solubility depends on the substances physical properties.
      I appreciate your response. Good scientists always question what they read.

      Delete

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