||This activity may be thought of as a simple application of the central concept for separation, namely that separation of a mixture depends on differences in the properties of the components of the mixture. Students understand an idea more deeply if they can study it in many contexts.
||Students will apply what they have learned in Activities 7A.1, 7A.2, and/or 7A.3 in order to separate a mixture of salt and sand. They should identify the key differing property (solubility) that will enable the separation.
- basic concept of separation by differing properties, as stated above
- filtration, filtrate, filter paper
Per lab group:
- 10 mL table salt (NaCl)
- 10 mL fine sand
- Clear plastic cups or beakers (2)
- Filter paper
- Ring stand
- Metal ring
- Large beaker for stock mixture
- Aluminum pie pan
Ability to fold filter paper for funnel and set up filtration apparatus.
One 45-minute period plus overnight to evaporate water.
No special safety measures.
|Student Ability Level and Grouping
This activity may be done by middle school students working individually or in groups of two or three.
- Teacher preparation:
- Before class, thoroughly mix approximately equal volumes of table salt and fine sand in a large beaker. Prepare enough so that each lab group doing this activity can have about 20 mL of the mixture. (Alternatively, you could mix the salt and sand at the beginning of the class so that students can see that they are getting a mixture of salt and sand, since it can be difficult to see the salt grains once the salt and sand are well mixed.)
- Distribute 20 mL portions of mixture in clear plastic cups or beakers for student groups (one 20 mL portion per group).
- Have filter paper, funnels, ring stand, and ring available. If your students have never done a filtration, delay showing them the set-up and procedure until the point at which they need to know the method (see below).
- Ask students to devise a procedure for separating the salt and sand. Remind them to apply the basic principle they learned about separating mixtures from Activities 7A.1, 7A.2, and/or 7A.3 to this task. Have them write down the steps in their procedure on the Investigation Record Sheet for teacher approval.
Note: This is a good time to introduce to students the idea of reserving a portion of the sample when testing an unknown procedure so that, if the proposed procedure fails, they will not have lost the entire sample.
- As long as the student procedures are safe, allow the students to proceed.
- If students propose filtering the mixture, this is the appropriate time to show them the filtration set-up. Note: If no one proposes the filtering the mixture, you will have to demonstrate the filtering method yourself in class (see procedure below), after the students have tried out their procedures.
- Add water to the mixture and stir until the salt dissolves.
- Place a funnel in the mouth of a second clear plastic cup.
- Fold a circle of filter paper into quarters and open it into a funnel shape.
- Place the filter paper into the funnel.
- Pour the mixture slowly into the paper funnel to filter out the sand.
- After filtering the entire mixture, pour the contents of the clear plastic cup into an aluminum pie pan.
- Evaporate the water to re-crystallize the salt. You can place the pie pans on hot plates or in an oven, or leave them out overnight to dry out.
- Have the students write down their observations on their Investigation Record Sheet as they try out their procedures.
- Once students have tried their procedures, ask for volunteers (or select students) to discuss whether their procedures worked, and why or why not. Probe students to see whether they applied the “difference in properties” concept to this separation. Reinforce the concept in your discussion. Summarize the property(ies) they used to make the separation. Summarize the points you wish students to remember.
- Go over vocabulary terms for this lab: filtration, filtrate, residue, solubility, evaporation, properties, separation.
- Have them record the outcomes of the discussion on their Investigation Record Sheet.
||See Activity 7A.7, Assessment: A Challenging Separation.