Fifth Grade NGSS
Water, Ecosystems, Human Impact

 

Learning how water dissolves substances,

 
FIFTH GRADE - SOLVENTS AND SOLUTES

OBJECTIVES:
·        Comparing fresh and salt water.
·        Distinguishing solvents and solutes.

·         clean
·        
dirty
·         dissolved
·        
fresh
·         polluted
·         salty
·         sediments
·         solution
·        
solvent

MATERIALS:
·         Beakers
·         Measuring spoons
·        
Stirrer
·         Salt, sand, sugar, baking soda, Epsom salt, mud, warm and cool water
·        
Worksheet
·         Solute and Solution (ppt)

BACKGROUND:

Water is a universal solvent which means it can dissolve many other substances within the molecular structure of water.  Water becomes salty because many different components that erode from the land will dissolve and become part of the water.  Over eons of time the water cycle evaporates only fresh water, leaving the “salts” behind. 

The term solution means a system in which one or more substances are uniformly mixed or dissolved in another substance.  A solution has two components, a solute and a solvent.  The solute is the substance that is dissolved.  The solvent is the substance doing the dissolving.  A solute plus a solvent equals a solution.

Water is considered a universal solvent, in other words, many other substances can be dissolved into water.  Seawater is an example, it contains many ions of dissolved elements like sodium, chlorine, bromine, calcium, carbon, and many more.  Seawater starts as fresh water but as water falls on the land causing the erosion of rocks, minerals become a part of the water, and then become part of seawater.   Salt water is neither dirty nor polluted, it is a solution that is clean, unless polluted by humans or nature.

The students should remember that the hydrogen and oxygen "bond" together, or "hold hands."  The bond is very strong and is called a covalent bond. Because the bond is so strong, water is considered a universal solvent, since many things dissolve in it. Water is a special type of covalent bond called a hydrogen bond.  Salts on the other hand hold hands very weakly and break up very easily in water.  This is called an ionic bond. 

The break up of salts in water causes the water to have the ions of that salt.  For instance,  table salt is sodium chloride (NaCl).  When it is dissolved in water it turns into a positive ion of sodium (Na+) and a negative ion of chlorine (Cl-).  Dissolving does not mean that the compound breaks into its elements.  If that was the case, sodium, the element is reactive with water and chlorine is a deadly gas.  It is important to use the correct terms early in a student’s education, so they don't get confused later on.

PROCEDURE:  

1.    Ask students if a body of salt water were to evaporate, would anything be left.  If the students debate over this you might want to set out a dish of salt water and a dish of fresh water and have them observe what happens when the water evaporates. 

Use the powerpoint presentation and go over Solute, Solution, Solvent, and Mixture.  When you introduce the materials make sure you go over each one.  For example:  water is the solvent + salt is the solute = results in a solution that is salty. 
  

2.    In this lab, the students will measure all the correct amounts and follow the lab sheet.  Have them describe what happens when they mix the materials.  Once the students record their findings on one item, have them clean out the dish and proceed to the next item.  Do not dispose of the sand and mud in the sink.  You may want to save the sand and dry it out, to be used again.   

3.    Students may ask you why the oceans aren't sweet.  The oceans are not sweet because sugar does not dissolve as fast as salt and does not stay dissolved.  Sugar has strong bonds (covalent) whereby salts have weak bonds(ionic).  Also, salts are much more abundant in the rocks. Have you ever tasted a sweet rock?

 

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