4 Unknowns Lab
Introduction
In this lab you will uncover 5 properties of 4 unknown substances. The 4 unknown substances
will be referred to as unknowns A,B,C, and D. The 5 properties you will uncover are density,
reactivity with NaCO3, flammability, boiling point, and melting/ freezing point.
Safety
Students must wear safety goggles at all times.
Part A: Density
Pre-lab questions.
1. What 2 quantities do you need to measure to determine the density of a substance? What
units will you use to measure each quantity? What instrument do you have that can measure
each quantity?
quantity 1 ____________
units ____________ instrument ____________________
quantity 2 ____________
units ____________ instrument ____________________
2. What equation will you use to calculate density? _________________________
3. What units will you use for density? ______________________
Procedure
.
Write a procedure to find the density of an unknown liquid using the materials you are given.
Procedure must be written in steps, must be a valid procedure for finding density, and must
be clear with enough details that someone can follow your procedure.
Prepare a data table for every measurement you will need to take.
Carry out your procedure and complete your data table as you go.
Post-lab questions/ analysis
1. What is the difference between you highest and lowest density substance?
2. What properties of the substances might account for a difference in density?
3. What sources of error did you have in your measurements or your procedure? Name at least 3
specific errors.
4. Choose one error and explain how you could improve the experiment.
Prelab questions answers correct: 1point. each
Procedure
Written in steps 1 point
Valid: 2 points
Clear: 4 points
Detailed: 2 points
Data table
Prepared correctly/ completely 2 points
Values are correct: 1 point per measurement
Values have units: 1 point per measurement
Correct # of significant digits: 2 points
Calculations correct: 4 points per calculation (4 step)
Post-Lab
question 1: 1 point
question 2: 1 point
question 3& 4: 4 points each
Part B: Reactivity with NaHCO3
Pre- lab questions
1.
What does reactivity mean?
2.
What is NaHCO3? Look on pg. 157 of the textbook if you do not know.
3.
Is reactivity a chemical or physical property?
4.
When you test the unknowns for reactivity with NaHCO3, you will be looking for a
chemical change. What would be some observations that might give evidence of a chemical
change? Name at least 3.
5. Is it possible for a physical change to take place between 2 substances that are not reactive
with each other? Name 2 physical changes you might observe.
Procedure
Write out a procedure to find the reactivity of each unknown with NaHCO3. You can assume that
if the 2 substances are reactive together, they will react when mixed together. You do not need to
do anything to trigger a reaction. Materials you can use are test tubes, samples of each unknown,
and a beaker of NaHCO3 with a scoopula. Your procedure will be very simple, but you still need
to write it out describing the tools, and explaining how you will collect your data. Be sure to
include the observations you will expect if the 2 substances are reactive.
Prepare a data table to record your observations.
Post lab questions/ analysis
1. How many of your unknowns were reactive with NaHCO3? Which one(s)?
2. For each substance you think is reactive with NaHCO3 describe the specific evidence that the
2 substances reacted together.
3. You tested your unknowns for reactivity with NaHCO3. Is it possible that another substance
(besides NaHCO3) would react differently wit your unknown substances? Explain your
answer.
Part C: Flammability
Pre- lab questions
1. What is flammability?
2. Is flammability a chemical property or a physical property?
3. Is burning a chemical or physical change? Give 2 pieces of evidence to support your claim.
Procedure
1
Write a procedure to test whether each unknown is flammable. Be sure to include safety
precautions. Your procedure will be very simple, but you still need to write out the
procedure. Be sure to include the observations you expect to make.
2. Watch demonstration and record your observations in the data table.
Post- Lab questions
1. Propose a possible explanation for why one substance might be flammable and another might
not be flammable.
2. For each substance that is flammable, describe the color and behavior of the flame.
3. When you try to put out a burning liquid, it is very hard to blow it out. Why? What is a better
way to put out a liquid fire?
Part D:
Density
Unknown A
Unknown B
Unknown C
Unknown D
Reactivity
with NaHCO3
Boiling
Flammability point
Melting
point