Why Does Water Dissolve Salt?

Project the image Sodium Chloride Crystal. 

Remind students that the green balls represent negative chloride ions, and the gray balls represent positive sodium ions.

Ask students:

• What is it about water molecules and the ions in salt that might make water able to dissolve salt?
  The positive and negative polar ends of a water molecule are attracted to the negative chloride ions and positive sodium ions in the salt.

Give each student an activity sheet.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms & Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

• Lesson 5.3 Student Activity Sheet PDF | DOCX | Google Doc

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Show students a series of four pictures to help explain the process of water dissolving salt.

Project the image Sodium Chloride Dissolving in Water.

Point out that several water molecules can arrange themselves near an ion and help remove it from the crystal. Show students that the positive area of a water molecule will be attracted to the negative chloride ion and that the negative area of a water molecule will be attracted to the positive sodium ion.

Point out that the water molecules are attracted to the sodium and chloride ions of the salt crystal. Explain that the positive area of a water molecule is attracted to a negative chloride ion. The negative area water of a water molecule is attracted to a positive sodium ion. Dissolving happens when the attractions between the water molecules and the sodium and chloride ions overcome the attractions of the ions to each other. This causes the ions to separate from one another and become thoroughly mixed into the water.

Tell students that the amount of a substance that can dissolve in a liquid (at a particular temperature) is called solubility. Point out the similarity in the words dissolve and solubility. Also tell them that the substance that is dissolved is called the solute. The substance that does the dissolving is called the solvent.

The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.

• Lesson 5.3 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 5.3 Activity Sheet Answers PDF | DOCX | Google Doc

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Ask students to make a prediction:

• Think about the polarity of water molecules and alcohol molecules. Do you think alcohol would be just as good, better, or worse than water at dissolving salt?

Discuss how to set up a test to compare how water and alcohol dissolve salt. Be sure students identify variables such as:

• Amount of water and alcohol used
• Amount of salt added to each liquid
• Temperature of each liquid
• Amount of stirring

Question to Investigate

Is alcohol just as good, better, or worse than water at dissolving salt?

Materials for Each Group

• Water
• Isopropyl alcohol (70% or higher)
• Salt
• Balance
• 2 clear plastic cups
• 2 small plastic cups
• Graduated cylinder

Ask students:

• Is alcohol just as good, better, or worse than water at dissolving salt?
  Alcohol does not dissolve salt as well as water does.
• How do you know?
  There was more salt left behind in the cup with the alcohol.
• Think about the polarity of water and alcohol to explain why water dissolves more salt than alcohol.
  Have students look at the models of water and alcohol molecules on their activity sheet.

Remind students that isopropyl alcohol has an oxygen atom bonded to a hydrogen atom, so it does have some polarity but not as much as water. Since water is more polar than alcohol, it attracts the positive sodium and negative chloride ions better than alcohol. This is why water dissolves more salt than alcohol does. Another way of saying this is that the solubility of salt is greater in water than in alcohol.

Read more about polarity in Teacher Background. 

• Lesson 5.3 Teacher Background PDF

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Compare the solubility of the ionic substances calcium carbonate (CaCO₃) and sodium bicarbonate (Na₂HCO₃) in water.

Ask students:

• How could you compare the solubility of sodium bicarbonate and calcium carbonate?
  Students should suggest measuring equal amounts of each substance and adding equal amounts of water at the same temperature.

Question to Investigate

Do all ionic substances dissolve in water?

Materials for Each Group

• Sodium bicarbonate
• Calcium carbonate
• Water
• 2 clear plastic cups
• 2 small plastic cups
• Balance

Ask students:

• Do all ionic substances dissolve in water? How do you know?
  Because calcium carbonate does not dissolve in water, students should realize that not all ionic substances dissolve in water.

Explain that on the molecular level, the ions that make up calcium carbonate are attracted so strongly to each other that the attraction by water molecules cannot pull them apart.

That is a good thing because calcium carbonate is the material that seashells and bird eggs are made of. Calcium phosphate is another ionic solid that does not dissolve in water. This is also good because it is the material that bones and teeth are made of.

With enough water, sodium bicarbonate breaks apart completely into ions that are incorporated throughout the water, forming a solution. The sodium and bicarbonate ions will not settle to the bottom and cannot be filtered out of the water.

But calcium carbonate does not break up into its ions. Instead, it is just mixed in with the water. If given enough time, the calcium carbonate will settle to the bottom or can be filtered out of the water. Sodium bicarbonate dissolved in water is a good example of a solution, and undissolved calcium carbonate is a mixture, not a solution.

Note: The bicarbonate ion is different from the single atom ions such as sodium (Na⁺) and chloride (Cl^–) that students have seen so far. The bicarbonate ion (HCO^3-) is composed of more than one atom. These types of ions, called polyatomic ions, are made up of a group of covalently bonded atoms that act as a unit. They commonly gain or lose one or more electrons and act as an ion. Another common polyatomic ion is the sulfate ion (SO^2-). This ion is part of Epsom salt as magnesium sulfate (MgSO₄) and many fertilizers as potassium sulfate (K₂SO₄). You can decide if you would like to introduce students to these two common polyatomic ions.