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Properties of Compounds

Investigation 4 – Lab

Back to Investigation 4

BE PREPARED

Supplies and Equipment:

Class Materials:

3 400 ml beakers
3 100 ml gradated cylinders
20 15 ml centrifuge tubes
250 ml vinegar
250 ml rubbing alcohol
250 ml hydrogen peroxide
46 g baking soda
50 g salt
34 g sugar
4 g cream of tartar
4 lab scoops
1 roll of masking tape
1 marker

Group Materials:

4 400 ml beakers
3 100 ml beakers
1 liter pitcher
400 ml water
9 pH indicator strips
1 pH chart
1 20 cm piece of masking tape
1 glass stir rod
Paper towels

Individual Materials:

1 Student Data Record

Teacher Preparation:

1. Prepare the materials needed for Trials 1 and 2.

a. Fill one 400 ml beaker with approximately 250 ml of vinegar and label “vinegar.”

b. Fill one 400 ml beaker with approximately 250 ml of rubbing alcohol and label “rubbing alcohol.”

c. Fill one 400 ml beaker with approximately 250 ml of hydrogen peroxide and label “hydrogen peroxide.”

d. Fill five falcon tubes with approximately 7 ml of baking soda and label “B.”

e. Fill five falcon tubes with approximately 7 ml of salt and label “Sa.”

f. Fill five falcon tubes with approximately 7 ml of sugar and label “Su.”

g. Fill five falcon tubes with approximately 1 ml of cream of tartar and label “T.”

Note: The cone portion of the falcon tube has additional markings from 0.5 ml to 1.0 ml
(see illustration). This scale should be used when preparing the falcon tubes with 1 ml of cream of tartar.

Instruction:

1. Direct each student group to obtain the following necessary materials from the distribution point: one (1) falcon tube of baking soda, one (1) falcon tube of salt, one (1) falcon tube of sugar, one (1) falcon tube of cream of tartar, four (4) 400 ml beakers, three (3) 100 ml beakers, one (1) liter pitcher, nine (9) pH indicator strips, one (1) pH chart, one (1) 20 cm piece of masking tape, one (1) glass stir rod, and paper towels. Instruct one student from each group to fill one (1) 100 ml beaker with 50 ml of vinegar, one (1) 100 ml beaker with 50 ml of hydrogen peroxide, one (1) 100 ml beaker with 50 ml of rubbing alcohol, and one (1) liter pitcher with 400 ml of water. The beakers should be labeled using the piece of masking tape.

GET FOCUSED

Investigation Four gives students an opportunity to study chemical formulas as a way to understand that the properties of a compound result from the combination of elements within that compound and allow students to investigate the pH of various solid and liquid compounds.

INVESTIGATE

Trial 1

1. During this Trial, students will determine the pH of four liquid compounds: hydrogen peroxide, rubbing alcohol, vinegar, and water. Students should realize that pH is a quantitative property of compounds.

To measure pH, students will use a tool from the Procedural Toolbox, pH indicator strips. By dipping the strip in each liquid compound, students will discover whether it is an acid, a base, or a neutral compound.

Through this Trial, students should discover that liquid compounds that are composed of similar elements may or may not have a similar pH and that compounds composed of different elements may or may not have a different pH. Therefore, the pH of a compound depends upon the unique combination of elements in the compound.

As they complete this Trial, students should consider the following questions:

Do compounds that are composed of similar elements have the same pH?

Do compounds that are composed of different elements have a different pH?

Do all liquid compounds have the same pH?

Trial 2

2. In this Trial, students will use the pH indicator strips to determine the pH of four solid compounds: sugar, baking soda, salt, and cream of tartar. Students will discover that in order to test the pH of a solid, the solid must be made into a solution. Before creating the solution, students must first test the pH of the water in order to determine the contribution of the pH of water to the solution. Students should discover that in general, water is considered a neutral substance.

Through this Trial, students should discover that solid compounds that are composed of similar elements may or may not have a similar pH and that compounds composed of different elements may or may not have a different pH. Therefore, the pH of a compound depends upon the unique combination of elements in the compound.

As they complete this trial, students should consider the following questions:

Do compounds that are composed of similar elements have the same pH?

Do compounds that are composed of different elements have a different pH?

Do all solid compounds have the same pH?

Note: Water is generally considered a neutral substance and for that reason when solutions are made with other solid solutes, the pH of the solution reflects the pH of the solute rather than the pH of the water. However, there are occasions in which the pH of a particular water source may not be exactly 7 as indicated by pH strips. These variations are due to the presence of additional substances in the water supply. In these cases, the pH of water may be measured as 6.5. However, even in these cases, the pH of a solution is still considered to be a result of the solute rather than the water as the pH of water is considered close to neutral.