University of Virginia
Physics Department

Chemical Change with Mixed Solutions

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

 

Background Information

 The reaction taking place here is a double replacement reaction or an ion-exchange reaction. In this type of reaction, two ionic compounds are made into solutions and mixed together. The positive ion of one of the compounds will react with the negative ion of the other and form a precipitate. When the precipitate forms, the results are dramatic - a colorful insoluble blue solid appears. The students will observe two liquids with a particular mass resulting in a liquid and solid with the same mass.

In the Copper nitrate solution the positive ion is copper (Cu) and the nitrate radical (NO 3 ) is the negative ion. In the sodium hydroxide solution the sodium (Na) is the positive ion and the radical (OH) is the negative ion. These two solutions form the reactants in the chemical reaction. When mixed, an ion exchange occurs and the copper and sodium switch places in the compounds. One of the products formed after this exchange is copper hydroxide Cu(OH) 2 which is an insoluble, blue gelatin-like solid. The other product, sodium nitrate, NaNO 3 is also a solid but is soluble in water and will remain in solution undetected.

 

Reactants

Products

chemical eqn

Student Activity

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Materials

 

Teacher Preparation

A 0.5 molar solution of sodium hydroxide is required to perform this experiment. Measure out 20.0 grams of sodium hydroxide pellets and dissolve them in 500 mL of water.

This will be enough for a class of 20 students working in pairs. You can provide a small container for each lab station and the students can mass out 25 mL and return the remainder to a central location.

Copper nitrate can be dispensed from a central location or distributed in small containers to each lab station. In any event the students will have to mass out 4.8 grams.

50 grams should be enough for a class of 20 students working in pairs.

Caution: Avoid skin contact with the sodium hydroxide pellets. Use rubber gloves while preparing the solution.

 

Procedure

Safety glasses and aprons should be used, avoid touching or tasting chemicals.

  1. With your balance mass out 4.8 grams of copper nitrate on a piece of paper.
    (When massing chemicals on most balances, paper or some type of container must be used to keep the chemical from coming in contact with the metal pan because the chemical may be corrosive. The paper or container must be massed before placing the chemical on it, and then again with the chemical on it. The two masses must be subtracted to obtain the mass of the chemical alone if using a triple arm balance. When using an electronic balance the "tare" function will remove the mass of the paper or container.)
  2. Measure out 25 mL of water in a graduated cylinder and pour it into a 100 mL beaker.
  3. Empty the copper nitrate into the beaker of water and stir until the mixture is completely dissolved. Let the beaker and solution remain on the balance.
  4. Place the second beaker on the balance, measure out 25 mL of the sodium hydroxide solution.
  5. While both solutions are on the balance in separate containers, record the mass. Enter the value in the data section below under "mass of beakers and contents before reaction."
  6. Pour one of the beakers into the other, observe the reaction and record the mass of both the empty and full beakers again. Enter the value in the data section below under "mass of beakers and contents after reaction."

 

Data Sheet

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Mass of Beakers and Contents Before Reaction

Mass of Beakers and Contents After Reaction

___________________

___________________
 

  1. Describe the solution that was made when you mixed copper nitrate in water.

     

  2. Describe the appearance of the sodium hydroxide solution.

     

  3. What did you observe when you mixed the two solutions together?

     

  4. Describe the state of matter, color and solubility of the new substance formed.

     

  5. A double replacement reaction is also called a ion exchange reaction. Describe the exchange of ions that is taking place.

     

  6. In a double replacement reaction one product must always be an insoluble solid. Which of the products formed in this reaction is the insoluble solid?

     

  7. In a double replacement reaction one of the products must always be a soluble solid. Which of these products is the soluble solid?

     

  8. Describe what would happen if the products you obtained in this reaction were filtered through filter paper.

 

Extensions

Students can perform the experiment, filter the product to separate the precipitate from the soluble solid. Then evaporating the remaining solution on a hot plate will yield the white powder sodium nitrate.

Students with special needs

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Assessment

1. Which of the following equations is a double replacement reaction?

a. 2CO ==> 2C + O2

b. 2Na + 2H2O ==> 2NaOH + H2

c. 2Al + 3Cl2 ==> 2AlCl3

d. Na2CO3 + Ca(OH)2 ==> CaCO3 + 2NaOH