Forming a Precipitate

Materials for the Demonstration

• Magnesium sulfate (Epsom salt)
• Sodium carbonate
• Water
• 2 clear plastic cups
• 1 tablespoon
• 1 teaspoon

Teacher Preparation

• Pour 100 mL of water in one clear plastic cup and add 10 g (about 1 tablespoon) of magnesium sulfate. Stir until the solution is clear.
• Pour 50 mL of water in another clear plastic cup and add 5 g (about 1 teaspoon) of sodium carbonate. Stir until the solution is clear.

Ask students:

• Would you consider adding a sodium carbonate solution to a magnesium sulfate solution a chemical reaction?
  Yes.
  
  Why or why not?
  Combining the two clear colorless liquids is a chemical change because a different solid substance is formed.
  Tell students that a precipitate is an insoluble solid that forms when two solutions are combined and react chemically. Insoluble means that the solid will not dissolve.

Give each student an activity sheet.

• Lesson 6.3 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 6.3 Activity Sheet Answers PDF | DOCX | Google Doc

Download the student activity sheet, and distribute one per student.

All Downloads

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.

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.

Question to Investigate

How do you know when a precipitate is formed in a chemical reaction?

Materials for Each Group

• Baking soda
• Calcium chloride
• Water
• Graduated cylinder
• Measuring spoon (½ teaspoon) or balance
• 2 clear plastic cups
• Masking tape
• Pen

Note: If you would like students to practice using a balance to weigh grams, have them weigh two grams each of baking soda and calcium chloride.

Expected Results

Bubbling and a white precipitate appear.

Ask students:

• What did you observe when you mixed the baking soda solution and the calcium chloride solution?
  The solutions bubbled and little white particles of solid formed.
• Did you observe a precipitate?
  Yes. The white particles appeared after the two solutions were combined.
• Do you think this was a chemical reaction? Yes. Why?
  The two substances that were combined were liquids and the substances that were produced were a solid and a gas. These products seem to be different from the reactants.

Remind students that in the chemical reactions they have seen so far, certain atoms in the reactant molecules unbond from one another and then rearrange and rebond in different ways to form the products. They saw that the same type and number of atoms were in the reactants as were in the products.

Project the image Calcium Chloride and Sodium Bicarbonate.

Ask students:

• What products of the reaction do you recognize?
  Students should recognize sodium chloride (NaCl), water (H₂O), and carbon dioxide (CO₂).
• Look at the product side of the chemical equation. What gas is produced in the chemical reaction?
  Carbon dioxide gas.
• What do you think is the precipitate?
  The salt and water are clear and colorless as a solution, so the precipitate must be CaCO₃, which is calcium carbonate. Tell students that calcium carbonate is ordinary chalk.
• How many of each type of atom is on the reactant side of the equation?
  1 calcium atom, 2 chlorine atoms, 2 sodium atoms, 2 hydrogen atoms, 2 carbon atoms, and 6 oxygen atoms.
• How many of each type of atom is on the product side of the chemical equation?
  1 calcium atom, 2 chlorine atoms, 2 sodium atoms, 2 hydrogen atoms, 2 carbon atoms, and 6 oxygen atoms.
• Is this a balanced chemical equation? If so, how do you know?
  Yes. The same type and number of atoms are in the reactants and products.

Make sure students see that every type of atom on the left side of the equation is also on the right. Also be sure that they see that there is an equal number of each type on both sides of the equation.

Ask students:

• How do you think we could separate the precipitate from the other products?
  Put everything in a filter and let the liquid run through.

Question to Investigate

Can you separate the calcium carbonate from the rest of the products?

Materials for Each Group

• Coffee filter or paper towel
• Tall clear plastic cup

Expected results

A white solid will remain in the coffee filter. After the water evaporates, the calcium carbonate will be a white powder.

Note: If you’d like to separate the sodium chloride from the water that flowed through the filter, pour the liquid into a clean empty cup and allow the water to evaporate for a few days. As the water evaporates, students will begin to see cubic-shaped salt crystals forming in the solution. Eventually, only salt crystals will remain in the cup.

Ask students:

• What is the solid white substance on the paper?
  Calcium carbonate (chalk).
• Is filtering out the calcium carbonate and allowing the water to evaporate a chemical change or a physical change?
  Physical change.
• Why?
  These substances were already present in the water, so no new chemicals are made.
• What evidence was there that a chemical reaction occurred when you combined baking soda solution and calcium chloride solution?
  A gas and a white solid were formed.

Ask students:

• How could we compare the precipitate to the reactants to be sure that the precipitate is actually different from both of them?
  Do a solubility test on all three substances.

Question to Investigate

Is the solubility of the precipitate different than the solubility of baking soda and calcium chloride?

Ask students:

How should we set up the solubility test?

• Should we use the same amount of each substance?
  Yes
• Should we use the same amount of water?
  Yes

Materials for Each Group

• Dry precipitate on paper towel
• Balance
• 3 small plastic cups
• Graduated cylinder
• ¼ teaspoon
• Popsicle stick (optional)
• Calcium chloride
• Baking soda
• Water

Tell students that you will show them another reaction that forms a precipitate and a little something extra.

Materials for the Demonstration

• Copper II sulfate
• Household ammonia
• Hydrogen peroxide (3%)
• Water
• Graduated cylinder
• Test tube
• 2 droppers
• 1 clear plastic cup (empty)

Note: The copper compound is called “copper II” because copper can make different types of ions. It can lose one electron and be just Cu⁺ or it can lose two electrons and be Cu 2⁺. This type of copper ion is called copper II. The “sulfate” in copper II sulfate is also an ion. This ion is made up of more than one atom. It is one of the polyatomic ions discussed in Chapter 4, Lesson 3. The sulfate ion is made up of a sulfur atom bonded to four oxygen atoms and is treated as one ion (SO₄²⁻).

Teacher Preparation

Make a copper II sulfate solution by adding 5 g of copper II sulfate to 50 mL of water.

Procedure

1. Pour 15–20 mL of copper II sulfate solution into a test tube so it is about ½ full.
2. Add about 10–20 drops of ammonia.
3. Add about 10–20 drops of hydrogen peroxide.

Expected Results

After adding the ammonia, a whitish precipitate will form at the top of the copper II sulfate solution. As more ammonia is added, the color on top of the liquid will change to a deeper darker blue. As the hydrogen peroxide is added, the dark blue area at the top of the solution will turn dark green and a dark precipitate will form.

Note: The details of the chemical reactions that produce the different precipitates and different color changes are complicated. The main idea for students is that atoms or groups of atoms in the reactants rearranged and bonded in different ways to form different substances in the products.

Let students know that when they see the production of a gas, a precipitate, or a color change, that this is evidence that a chemical reaction has taken place.

Ask students:

• How can you tell that something new was made when the copper II sulfate and ammonia reacted?
  A precipitate was produced.
• How can you tell that something new was made when these substances reacted with hydrogen peroxide?
  The color change and other precipitate are evidence of another chemical reaction.

In-Class or At-Home Project

Have students use objects such as gum drops, beads, M&Ms, Legos, or other small objects to represent the atoms in two of the three reactions they have explored so far. Students can tape or glue the objects to poster board and write down the chemical formula for the reactants and products.