Mixtures & Solutions
Investigation 2
Part 2
Citric Acid Saturation
The students will be repeating the same procedures followed in Investigation 2, Part 1. Students make a saturated citric acid solution. They discover the amount of citric acid needed to saturate the water using the property of mass. They compare this amount of citric acid to the amount of salt needed to saturate the salt solution.
3. Lesson set up and Management
Properties of Substances (PR01 1.1.1)
- Use properties to sort natural and manufactured materials and objects, for example, size, weight, shape, color, texture, and hardness. Grades K, 1, 2, 3, 4, & 5.
- Use physical and chemical properties to identify and describe substances; for example: density, boiling point, and solubility. Grades 6, 7, & 8.
Structure of Systems ( ST01, STI02, STI03, & STI04 )
- Know that matter is made of small particles called atoms and molecules. Grades K, 1, 2, 3, 4, & 5. (ST01 1.2.3)
- Understand that all matter is made up of atoms, which may be combined in various kinds, ways, and numbers to make molecules of different substances. Grades 6, 7, & 8. (ST01 1.2.3)
Changes in Systems (CH01 1.3.3)
- Know that matter can undergo changes of state such as evaporation, condensation, or freezing and thawing. Grades K, 1, 2, 3, 4, & 5.
- Understand physical and chemical changes at the particle level and know that matter is conserved. Grades 6, 7, & 8.
- A solution is saturated when as much solid material as possible has dissolved in a liquid.
- Different solids have different saturation points.
- A simple solution is one solid dissolved in one liquid. Solubility is another property used to identify materials.
3. Lesson set up and Management
Materials:
Have students use Handout # 8 for instructions for this investigation.
Set up a water station and a citric acid station with 25ml spoons.
Check the student tubs to be sure they have all the materials listed. The mass set can be the small cubes from a Base Ten Kit as they are 1 gram cubes. If your school has the Measurement Kit, there are gram cubes in this kit.
Student Management:
The lesson suggests having the students work in pairs. It is just as easy to keep the same groups of four students, and have them take turns filling and shaking the vial. Also, the post-it notes seem to stick fine without tape. Here is a text copy of the handout with compete instructions.
Citric acid has such a high saturation point that the dissolving and shaking portion of the lab may take a bit of time. It takes between 45-60 g to saturate the 50 ml. This lesson may take more than the 50 minutes listed in the manual. It may take an hour to an hour and fifteen minutes.
When students' solutions have reached saturation, and they have filtered and weighed their solution, ask students to talk in their groups about how to separate the citric acid from the water. Discuss the groups' plans and then proceed with using the evaporation trays. It is important to do these discussions rather than telling the students what to do, as this discussion allows them to develop procedural thinking.
Reaching saturation
Materials:
In Basin:
Safety goggles
2 bottles with lids
5 plastic cups
1 funnel
2 filter papers
Sticky notes
1.5 ml spoon
Balance
Gram weights
Worksheets and journals
Pencils
At Chemistry Station:
Citric acid
25 ml spoons
Water containers
Syringes
Paper Towels
Tape
Procedure:
1. Get the basin of group supplies.
2. Get 50 ml of salt into a plastic cup.
3. Label the cup salt with a sticky note.
4. Put 50 ml of water in both plastic bottles.
5. Put a sticky note so the top of the note is at the top of the water level in the bottles.
6. Using the funnel, without a filter, put citric acid into one of the bottles using 5 ml spoonful increments.
7. Cap and shake the bottle between each spoonful.
8. Keep track of the number of spoonfuls added, and make observations after each spoonful, making sure to note the water level each time.
9. Note when you have a saturated solution in your data.
Part 2
1. Place a short cup labeled salt solution under the funnel.
2. Get and dampen a filter (not dripping), and place it in the funnel.
3. Shake and then pour the salt solution through the damp filter.
4. Place the salt solution onto the balance.
5. Take an empty short cup and place 50 ml of water into it.
6. Place it on the other side of the balance.
7. Place gram cubes one at a time into the short cup with 50 ml water until the balance is level.
8. Note how many gram cubes you used in your data.
Part 3
1. Take 20-25 ml of the solution into a syringe.
2. Put the solution into an evaporation tray.
3. Take it to the rack and label your tray.
Remind students to continue to add the chemical and shake completely to be sure their solution has reached saturation. Citric acid has a high saturation rate, and students quit way too early because they think they are doing something wrong. Students will also notice that the water changes color. They may ask you about this. This might be a time to introduce them to the fact that chemicals can come in different colors, and that is another piece of data used to identify chemicals.
Record the class results on the board or overhead. At this time, show the students how to present data in a chart for comparison. You may want to print this chart for the students (the unknown column is for the next part). Have students paste these into their journals, or on the back of the instruction sheet.
Be sure to save two of the evaporation trays with crystals for Lesson 4.
Group # |
NaCl (g) |
Citric Acid (g) |
Unknown (g) |
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When the students have evaporated their solution and the crystals have formed again, ask them how they know it is citric acid? Ask them what are the properties they already know (solubility)? Have them do a web search to see if they can find the crystal structure of citric acid. Have them compare the solubility and the shape of the sodium chloride and the citric acid.
Writing:
Have students complete Handout # 9 and add solubility to their definitions list.
Reading Support:
Read “Sour Power” in the Mixtures and Solutions: Science Stories book. A great extension for homework or as an in-class project is the crystal growing experiment. Have the students read and follow the procedure, then have them describe what happens in their journals. Have them describe the solubility of the borax in room temperature and hot water. Have them compare the size, shape and solubility of the crystals from their crystal growing experiment to the size, shape and solubility of the crystals from their previous investigations. It is important to have the students see the value of connecting past experiments with their current investigation as a way to compare results, and to begin the process of applying procedures that work in one case to another.