University of Virginia
Physics Department

## Properties of Mixtures and Solutions

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2
• PS.5

Objectives

Students will

• identify and separate the components in mixtures;
• determine the iron content in cereal;
• determine the concentrations of components of a mixture;
• use the following skills: making and applying observations, organizing and analyzing information, and measuring and recording data.

Motivation for Learning

Discrepant Event

Materials

• 4 g salt
• Two beakers (at least 500 mL)
• 4 g calcium carbonate or flour or talcum powder
• stirring rod
• flashlight
• water

Procedure

1. To illustrate the difference between a solution and a suspension, add about 4 grams of salt to 500 mL of water. Also, prepare a solution of calcium carbonate (4.0 grams) and 500 mL of water.
2. Make certain that each mixture is stirred well. Ask the students to list any differences or similarities between the two mixtures.
3. Darken the room and shine a flashlight through each beaker. The beaker with calcium carbonate will appear cloudy. Point out that this method is one way to differentiate between a solution and a suspension.

The particles in a suspension are larger and more unevenly distributed than those of a solution. When light passes through the suspension, the larger suspended particles scatter the light in all directions producing a cloudy appearance. This phenomenon is known as the Tyndall Effect. Most students have experienced the same effect in a movie theater, when light from the projector becomes visible through the dust in the air.

Background Information

Mixtures are combinations of substances that are not combined chemically. The amounts of substances in a mixture can vary. Students should understand that mixtures can change in appearance but this does not mean that any substances have changed in their chemical composition. Mixtures can be combinations of elements or compounds. Most substances found in nature are mixtures. A pure element or a pure compound is rarely found. Mixtures can be in any of the four phases of matter- or they can be in combinations of different phases. Air is a mixture of gases, milk is a mixture of solids and liquids, alloys are mixtures of solids. Mixtures that do not appear to be distributed the same throughout are said to be heterogeneous, and those that are the same throughout are called homogeneous.

Solutions are common types of homogeneous mixtures. Sugar and water form a solution when mixed. The sugar becomes evenly distributed throughout the solution, so that one portion is not sweeter than another. The dissolved portion of the solution is called the solute (sugar) and the dissolving portion is the solvent (water). If more sugar is added to the solution, the entire solution becomes sweeter and we say that it is more concentrated. In solution, sugar and water have not lost their properties, only combined them. Pouring a solution, like sugar and water, through filter paper will not separate the mixture, the sugar particles are too small. The best method is distillation; the water evaporates and the sugar is left behind.

All materials are not soluble. A suspension is a mixture in which the solute particles are larger than molecules or ions. Particles this size cannot adhere to the molecules of the solvent and will eventually settle out. The particles in a suspension are in the range of 10-2000 angstroms in diameter (1A = .00000001 cm). The particles in a solution are usually much less than 10 A. As a result, in addition to settling, the particles in a suspension scatter light when it is passed through, giving it a cloudy appearance (Tyndall Effect).

### Student Activity

Materials

 50 mL sand Filter paper 50 mL iron filings Triple beam or electronic balance 50 mL salt Funnel Water 100 mL beaker Cooking oil (100 mL) Stirring rod Evaporating dish Safety goggles Bunsen burner Plastic cups (8 oz) Box of Total breakfast Cereal Mortar and pestle Neodymium Magnet

Teacher Preparation

All materials should be available for each group of students (workstation). The sand, salt, iron filings and cooking oil can be in cups or small beakers. Go over the background information with the students. Show them examples of mixtures, solutions, and suspensions, and go over any new vocabulary words.

Explain that in the following activity, each group has to make two mixtures using three ingredients each from the list of ingredients in the procedure. One of the mixtures must contain water; the other does not. After making the mixtures, each group must come up with a procedure for separating the ingredients.

In part #2, the students have to grind Total Cereal into a powder and determine the percentage of iron in the mixture. Have a cup of cereal, magnet, scrap paper, empty plastic cup and balance available at each station.

Procedure - Part 1: Making and Separating Solutions

1. Use three ingredients from the following list to include in your first mixture: sand, salt, water, iron filings and cooking oil. Enter the name and physical description of the ingredients that you chose in the data sheet.
2. Mass out 2.0 grams of each solid and 15 mL of water if needed.
3. Mix the ingredients in the beaker and stir with the stirring rod. Record the description of the mixture in the table.
4. Write out a procedure for separating the mixture in the data sheet. Follow your procedure and separate the mixture. Save all the parts of the mixture and show them to your teacher.
5. Repeat the procedure in steps 1-4 for your second mixture.

Data Sheet

To print out the Data Sheet only (Parts 1 and 2), click here.

Mixture 1
 Ingredients Description

Description of Mixture

Procedure for Separating Mixture #1

1._____________________________________________________________________
______________________________________________________________________

2. ____________________________________________________________________
______________________________________________________________________

3. ____________________________________________________________________
______________________________________________________________________

4. ____________________________________________________________________
______________________________________________________________________

Mixture 2

 Ingredients Description

Description of Mixture

Procedure for Separating Mixture #2

1._____________________________________________________________________
______________________________________________________________________

2. ____________________________________________________________________
______________________________________________________________________

3. ____________________________________________________________________
______________________________________________________________________

4. ____________________________________________________________________
______________________________________________________________________

Procedure - Part 2: Determining the Percentage of Parts of a Mixture

1. Place an empty cup on the electronic balance, press "tare" to remove the mass of the cup.
2. Fill a 100 mL beaker with the breakfast cereal. Transfer the cereal, a little at a time, to a mortar and grind it to a powder. Transfer the ground cereal to the cup. When you are finished grinding all the cereal from the beaker and transferring it to the cup, find the mass. Enter the results in the table.
3. Place the magnet into the ground cereal and stir it around several times to collect the reduced iron. Remove the magnet and rub off the particles that stuck to it onto a piece of scrap paper. Repeat this procedure several times to insure you have collected as much iron as you can. Remove any items that are clearly not magnetic.
4. Mass the cup and contents again and record the mass in the table.
5. Subtract the mass of the cup and contents after the iron was removed from the original mass of the cup and contents to find the mass of the iron that was removed. Enter the value in the table.
6. Find the percentage of iron in the cereal by dividing the mass of the iron (#5) by the original mass of the cereal (#2) and then multiply by 100.

Data Table

To print out the Data Sheet only (Parts 1 and 2), click here.

 Original Mass of Cereal (g) Mass of Cereal After Iron is Removed (g) Mass of Iron (g) Percentage of Iron Contained in Cereal

Extensions

1. A good extension or even an assessment would be to give each group an unknown mixture or solution and have them determine the ingredients. You could use either the substances given here or others.

2. Students can investigate how salt depresses the freezing point of water. They can design their own experiment or follow the procedure below.

• Fill a 250 mL beaker 2/3 full of crushed ice. Add water to fill up the spaces between the ice. Stir well. Measure and record the temperature.
• Add 5 grams of salt (NaCl) to the ice water mixture and stir well to dissolve the salt. Measure and record the temperature.
• Predict what will happen when a second 5 grams of salt is added to the solution. Add the salt (total 10 grams). Stir to dissolve and record the temperature.
• Add another 5 grams (total 15). Stir and dissolve. Measure and record the temperature again.
• Keep adding salt, 5 grams at a time, dissolving and recording the temperature each time.
• Make a data chart and graph Grams of Salt vs. Temperature.

Students with special needs

Assessment

1. How do the properties of a mixture differ from the properties of the components of the mixture?

2. In terms of physical properties, what is there about mixtures that makes it possible to separate out the components?

3. Describe in detail the steps you would take to find the percentage of salt in a sample of salt water.