Beacon Lesson Plan Library

Density and Solubility of Liquids

Beverly Grim

Description

Students predict and test the densities of common liquids. Solubility is also being observed.

Objectives

The student knows that the vast diversity of properties of materials is primarily due to variations in the forces that hold molecules together.

The student knows that connections (bonds) form between substances when outer-shell electrons are either transferred or shared between their atoms, changing the properties of substances.

The student knows that investigations are conducted to explore new phenomena, to check on previous results, to test how well a theory predicts, and to compare different theories.

The student knows that scientists assume that the universe is a vast system in which basic rules exist that may range from very simple to extremely complex but scientists operate on the belief that the rules can be discovered by careful, systemic study.

Materials

Per group:
-1 test tube
-1 test tube stopper
-1 test tube holder
-1 (10 ml) graduated cylinder,
-4 different colors of food coloring
-laboratory liquids (2 ml of each)
Per class:
-denatured alcohol (ethanol)
- vegetable oil
- water
- glycerin

Preparations

1. A discussion of physical properties and density may be necessary prior to this activity.
2. Gather group materials.
3. Class liquids should be placed in an easy access location. In classes where food coloring is not permitted, consider adding color to each liquid prior to the activity.

Procedures

Knowledge/Skills:
-Students will use the scientific method to predict and test liquid densities.
-Students will understand the concept of density.
-Students will relate density to the forces that hold the materials together.
-Students will understand the property of solubility.
-Students will use writing skills to help analyze the concept material.

Procedure:
Part one: Prediction

1. In front of the class, the teacher should prepare 4 beakers showing liquids of different densities :
a) 25 ml of clear glycerine (density near 0.7854 g/ml)
b) 25 ml of water to which red food coloring has been added (density at 1.0 g/ml)
c) 25 ml of vegetable oil with its natural yellow cast (density near 0.8957 g/ml)
d) 25 ml of denatured alcohol to which blue food coloring has been added (density at 1.2 g/cc)

Swirl each beaker so that the students are able to observe the behavior of the liquid. Pour the liquid back and forth into a second beaker, or stir the liquids with a stirring rod. Have students predict the order of liquid densities based on their observations. Predictions should be made in the students' laboratory journals. Students should use observations and class content material to support their predictions.

Part two: Preparing the test

2. Separate students into groups of two or three. Students may work alone if there is an ample supply of materials.
3. Have the students create a mixture in the test tube using the following instructions:
a. Measure 2 ml of one of the liquids in the graduated cylinder.
b. Add a drop of food coloring to the liquid.
c. Transfer the liquid to the test tube.
d. Clean the graduated cylinder before adding the next liquid to avoid blending colors.
e. Repeat steps a - d for the remaining liquids using a different color of food coloring for each liquid.
4. Instruct students to place a stopper on the test tube and mix the liquids by inverting the tube several times. Vigorous shaking should be avoided as it will mix the colors of the various liquids.
5. Ask students to make observations as soon as the mixing stops. Test tubes should be placed in a labeled holder and placed in a safe location until the following day. Students should enter their observations in their journal and make predictions as to the order of liquid densities. The students should predict what the contents of the tube will look like the next day and describe what will occur and why.

Part three: Testing the prediction

6. Explain that solubility may affect their results. Have students answer the following questions:
a. What is solubility?
b. What is the solvent?
c. What is the solute?
d. How would solubilty affect the density results?
e. What causes solubiltiy?
7. Have the students check the test tubes the next day and compare the results to their predictions within their scientific journals.
8. Assess student understanding.

Assessments

Journals may be used for assessment. The teacher should read the journal entries to ascertain that the studentís entry reflects an understanding of what has occurred in the activity. All answers should be acceptable as long as the reason behind the choice is logical. (Most students will believe that the four liquids will go back to the original order due to the differences in densities. Some will know that oil is not soluble in water; that alcohol is soluble, but they will not be sure about glycerin. The glycerin will not be soluble.)

The following questions may also be used to assess student understanding:

1. Liquids that are defined as being very dense would be liquids that:
a. have large molecular chains.
b. have tight molecular packing.
c. have irregular molecular structure.
d. have many small molecules.
(Answer b: Density is defined as a property of molecular packing.)

2. Alcohol was soluble in water. Which statement explains this observation?
a. Alcohol and water hydrogen bond.
b. Alcohol and water have equal densities.
c. Alcohol and water have different densities.
d. Alcohol and water covalently bond.
(Answer a: The hydroxyl group of alcohol is attracted to the oxygen and hydrogen of water. This type of bonding is called hydrogen bonding.)

3. The density of oil is 0.87g/ml, and the density of water is 1.0g/ml. Based on this data and your observations, which of the following is true?
a. Water will float on the less dense oil.
b. Oil will dissolve in the more dense water.
c. Oil will float on the more dense water.
d. Water will dissolve in the less dense oil.
(Answer c: Oil floats on water.)

Extensions

Enhancement:

-Explore various types of molecular bonding.
-Explore hydrogen bonding.
Activity: Mix 10 ml of water with 5 ml of alcohol. Predict the results. Students may predict that a solution of 15 ml will result. In reality, the volume is less than 15 ml due to the forces that pull the solution together.
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