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Lesson 6.3

Forming a Precipitate

Key Concepts

  • The ions or molecules in two solutions can react to form a solid.
  • A solid formed from two solutions is called a precipitate.

Summary

Students will combine two clear colorless solutions (baking soda solution and calcium chloride solution) and see the formation of a solid and a gas. Students will analyze the chemical equation for the reaction and see that all atoms in the reactants end up in the products. They will make the connection between the chemical equation and the real substances and see that the solid and gas produced in the actual reaction are also in the products of the equation.

Objective

Students will be able to explain that for a chemical reaction to take place, the reactants interact, bonds between certain atoms in the reactants are broken, the atoms rearrange, and new bonds between the atoms are formed to make the products. Students will also be able to explain that this definition applies to the production of a solid called a precipitate.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Copper II sulfate is available from various chemical suppliers, including Sargent Welch, Product #WLC94770-06 or Flinn Scientific, Product #C0110. Follow all safety precautions regarding use, storage, and disposal of copper II sulfate and sodium carbonate. Sodium carbonate is Product #WLC94291-06 or #S0052.

Materials for the Demonstrations

  • Sodium carbonate
  • Epsom salt (magnesium sulfate)
  • 2 clear plastic cups
  • Test tube
  • Water
  • Copper II sulfate
  • Household ammonia
  • Hydrogen peroxide (3%)
  • Graduated cylinder
  • 2 droppers

Materials for Each Group

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

About the Materials

Copper II sulfate is available from various chemical suppliers, including Sargent Welch, Product #WLC94770-06 or Flinn Scientific, Product #C0110. Follow all safety precautions regarding use, storage, and disposal of copper II sulfate.

  1. Engage

    Do a demonstration by combining two clear colorless solutions that produce a white solid and introduce the term 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.

    Procedure

    1. Hold up the two clear colorless solutions and slowly pour the smaller amount into the larger.

      A teacher combines a sodium carbonate solution with a solution of magnesium sulfate

    Expected Results

    Particles of a white solid will form.

    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.

    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.

  2. Explore

    Have students combine two liquids to observe another precipitate.

    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.

    Procedure

    1. Use masking tape and a pen to label 2 plastic cups baking soda solution and calcium chloride solution.
    2. Use a graduated cylinder to add 20 mL of water to each cup.
    3. Add 2 g (about ½ teaspoon) of calcium chloride to the water in its labeled cup. Swirl until as much of the calcium chloride dissolves as possible.
    4. Add 2 g (about ½ teaspoon) of baking soda to the water in its labeled cup. Swirl until as much of the baking soda dissolves as possible. There may be some undissolved baking soda remaining in the bottom of the cup.
    5. Carefully pour the baking soda solution into the calcium chloride solution. Try not to pour in any undissolved baking soda. Observe.

      A student combines a solution of baking soda with a calcium chloride solution

    Expected Results

    Bubbling and a white precipitate appear.

  3. Discuss student observations.

    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.
  4. Explain

    Discuss the products produced in this chemical reaction.

    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 Chemical Equation for 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? Yes. Why?
    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.

  5. Explore

    Separate the products to show that the precipitate is a solid.

    Ask students:

    How do you think we could separate the precipitate from the other products?

    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

    Procedure

    1. Use a large enough coffee filter (or paper towel) so that you can push it about ⅓ of the way into the cup and still have enough left to hold it around the outside of the cup.
    2. While holding the coffee filter in place, pour the products into the center of the coffee filter.

      A student using a coffee filter to seperate a precipitate from the rest of the solution
    3. Allow the liquid to drip through the filter. This may take a while.
    4. Set the precipitate aside and allow the water to evaporate.

    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.
    How do you know that a chemical reaction occurred when you combined baking soda solution and calcium chloride solution?
    Because different products were formed.
  6. Extend

    Do a demonstration to show students another example of a precipitate and a color change.

    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 fairly 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.