Identifying Unknown Substances Using Density
Objectives:
 Students will be expected to collect data and make measurements with precision.
 Students will be able to calculate the density of an unknown in order to properly identify it.
Introduction
Mass is a measurement of the amount of matter in a sample while volume is a measurement of the
space occupied by the matter. In this experiment, you will measure the mass and volume of samples of
metals. You will then calculate the density of the different metals in order to properly identify them.
Measurements of mass are made on a triple beam balance or an electric balance. Volume
measurements are made in different ways depending on the physical state of the sample being measured.
The volume of a liquid is commonly measure in a graduated cylinder. Measurement of volume in a
graduated cylinder is always made by reading the mark at the bottom of the meniscus. The reading is
made with the eye positioned at the level of the liquid surface.
The volume of a solid may be calculated from its dimensions, if the solid is a geometric solid and
free of air space. If, on the other hand the solid is an irregular object, its volume must be determined
another way, such as water displacement. Water displacement measurements are made by recording a
certain amount of water in a graduated cylinder then placing the solid object carefully in the graduated
cylinder. There will be an increase in the amount of water in the graduated cylinder. Subtract the 2
water volumes and the difference is the volume of the solid. The solid must be completely submerged
under the water and must not dissolve.
Materials
Electronic balance
Graduated Cylinders
3 Unknown substances
Water
Ruler
Procedure
1. Obtain samples of your 3 unknown substances. Be sure the substances are clean and dry before
you begin.
2. Describe what your unknown looks like and record in your data table. (cube, cylinder, color, etc.)
3. Find and record the mass of your substances.
4. Find and record the volume of your samples. For at least one of your samples, you will use the
ruler to determine the volume (show your calculations!). For the other(s) you will use the water
displacement method. **BE EXTREMELY CAREFUL when you place the metals into the graduated
cylinder – tilt the graduated cylinder so you are careful not to splash water out!!**
5. Record all data in your data table, WITH UNITS!!
6. Calculate the Density of your unknowns. YOU MUST SHOW YOUR CALCULATIONS (what #s are
you divided, etc.)!!
7. Take your results and look at the “TABLE OF UNKNOWNS”. Compare your results and try to
match as close as you can to the correct density to identify your unknown substance.
8. Calculate % Error.
Name: ________________________________________________
Class Period: _______________
Identifying Unknown Substances Using Density
Pre-Lab Questions
1. What does mass measure in an object?
2. What does volume measure in an object?
3. What is the method of measuring the volume of an irregular shaped object called?
4. What is the formula for Density? What units can we measure density in?
Data Table – DON’T FORGET TO WRITE ALL YOUR UNITS!!!
Unknown
(describe its appearance)
Mass
Volume of
Unknown
Density of Unknown
MUST SHOW YOUR CALCULATIONS!!
Unknown #1
Unknown #2
Unknown #3
Results – use the TABLE OF UNKNOWNS to determine the identity of your unknowns
Unknown #1 = ___________________________________________ = ___________________
Name
Density from table
Unknown #2 = ___________________________________________ = ___________________
Name
Density from table
Unknown #3 = ___________________________________________ = ___________________
Name
Density from table
% Error – using the values of density from the TABLE OF UNKNOWNS, calculate your % error.
SHOW YOUR WORK!
% error = (accepted value – your value) x 100
accepted value
Unknown #1
Unknown #2
Unknown #3
Post Lab Questions
1. What would be considered a “good” % error? What would be considered a “bad” % error? Look at
your values for % error; were they “good” or “bad”? Explain.
2. Explain a way you could improve your % error.
3. Which of your unknowns would sink the fastest in water? Explain.
Density Practice – when necessary - SHOW YOUR WORK, include UNITS, and pay attention to SIG FIGS!!
1. A man has a 50.0 cm3 bottle completely filled with 163.0g of a slimy green liquid. What is the
density of the liquid?
2. What is the mass of an object that has a density of 8.0g/cm3 and a volume of 64.0cm3?
3. Diamonds have a density of 3.5 g/cm3. How big is a diamond that has a mass of 0.10g?
4. Two liquids, A and B, have densities of 0.75g/mL and 1.14g/mL respectively. When both liquids
are poured into a container, one liquid floats on top of the other. Which liquid is on top?