Air Has Mass—Density of Air
Density of Gases
SCIENTIFIC
Introduction
Abstract concepts such as Archimedes’ Principle, conservation of matter, and gases have mass can be difficult ideas for students to grasp. This series of demonstrations will help solidify students understanding.
Concepts
•Archimedes’ Principle
•Buoyancy
• Chemical reaction
• Law of conservation of mass
Materials
Sodium bicarbonate, NaHCO3, about 30 g
Bottle, plastic, 16-oz
Vinegar, white, about 30 mL
Bottles, dropping, 30-mL, 4
Water, tap
Film canister
Bag, resealable, quart-size Spring scale, 200-g
Balance, 0.01-g precision
Test tube, to fit into plastic bottle
BB’s, small or other small sphere
Safety Precautions
Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Wash hands thoroughly with soap and water
before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Material Safety Data Sheets for additional
safety, handling, and disposal information.
Part A. In the Bag
Procedure
1. Place several grams of sodium bicarbonate into the film canister and snap the lid into place.
2. Add several milliliters of vinegar to a quart-sized resealable bag.
3. Place the closed film canister into the resealable bag.
4. Remove as much air as possible from the bag and seal it.
5. Mass the bag on a 0.01-g precision balance and record the mass.
6. Remove the lid from the film canister and shake the bag to react the vinegar with the sodium bicarbonate.
7. Once the reaction stops, mass the bag a second time.
8. Record the mass. Discuss the outcome and observations with the students.
Part B. In the Bottle
Procedure
1. Place several grams of sodium bicarbonate into a 16-oz plastic bottle.
2. Add several milliliters of vinegar into a small test tube.
3. Carefully slide the test tube into the plastic bottle and cap the bottle.
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Publication No. 91848
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Air Has Mass—Density of Air continued
4. Mass the bottle on a 0.01-g precision balance and record the mass.
5. Tip and shake the bottle to react the vinegar with the sodium bicarbonate.
6. Once the reaction stops, mass the bottle a second time.
7. Record the mass. Discuss the outcome and observations with the students.
8. Have the students listen as you loosen the cap of the bottle.
9. Retighten the cap and record the mass. Discuss the outcome and observations with the students.
Part C. Archimedes on a Spring Scale
Preparation
1. Place between 20 and 180 grams of BBs into each of four 30-mL dropping bottles with attached caps. Ensure one of the
bottles contains less than 30 g of BBs.
2. Fill a beaker with water.
Procedure
1. Hang one of the three heavier dropping bottles onto the spring scale and record the mass.
2. Place the hanging dropping bottle into the beaker and record the mass.
3. Repeat steps 1 and 2 with the two heavy dropping bottles.
4. Hang the lighter dropping bottle onto the spring scale.
5. Ask students to speculate as to the expected mass when the lighter bottle is placed into the beaker of water.
6. Place it into the beaker of water and record the mass.
7. Discuss the outcome and observations with the students.
Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing
the disposal of laboratory waste. The waste solutions from Parts A and B may be disposed of down the drain with plenty of
excess water according to Flinn Suggested Disposal Method #26b.
Tips
• The exact amounts of sodium bicarbonate and vinegar in Parts A and B are not important. Be careful not to add too
much in Part A or the bag will burst open.
• A string can be tied around the neck of any small bottle used in Part C if dropping bottles with attached caps are not
available.
Discussion
The law of conservation of mass states that the mass of the reactants in a chemical reaction equals the mass of the products.
Matter is neither created nor destroyed. In this demonstration, the reaction of sodium bicarbonate, NaHCO3, and acetic acid,
CH3CO2H, will be used to confirm this law. When mixed, the bicarbonate and acetic acid react to form carbonic acid and
sodium acetate.
NaHCO3 + CH3CO2H
sodium acetic carbonic sodium
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→ H2CO3 + NaCH3CO2
Equation 1
bicarbonate acid acid acetate
Air Has Mass—Density of Air continued
The carbonic acid decomposes to form carbon dioxide gas and water.
H2CO3
→ CO2(g) + H2O(l)
carbonic acid
carbon dioxide
Equation 2
In Part A, the reaction mixture is contained in a resealable bag. The carbon dioxide gas produced inflates the bag, changing the
total volume of the setup. The system is massed before and after the reaction showing the apparent “loss” of mass. This loss
is due to buoyant force acting on the inflated resealable bag. Air is a fluid. Just as in water, an object will be buoyed in air. The
degree to which objects are buoyant is related to their density and the density of the fluid. The smaller the density of the fluid,
the smaller the buoyant force.
Balances do not directly measure mass of an object. They measure the force an object exerts—its weight. Calibrated masses are
used to convert these weights to mass readings. As long as an object being massed has approximately the same density as the
calibrated masses, the mass readings will be accurate. This holds true for most solids and liquids but not for gases.
Air, like water, exerts a positive or upward buoyant force on all objects, which causes inflating systems, in this case a resealable
bag, to appear to weigh less than the original reactants. When “massing” liquids and solids, the balance compensates for the
minor amount of buoyant force. When massing gases, however, the buoyant force is no longer negligible. The apparent mass of
gas will be less than the actual mass of the gas.
True mass of gas = apparent mass of gas + mass of air displaced.
In Part C, Archimedes’ Principle is used to demonstrate the effect of buoyant force on an object. The apparent change in the
mass of an object in air versus water is due to the difference in the density of air and the density of water. Objects in fluids
(air or water) have two forces acting on them. For instance, in water there is the downward force due to gravity, mg, making the
object sink, and an upward force, called buoyancy, which pushes upward on the object. This buoyant force is equal to the volume
of the water displaced by an object, Vw, times the density of water, ρw, times the acceleration due to gravity, g.
FB = Vw . ρw . g
Equation 1
The force opposing buoyancy is due to gravity. Therefore the downward force Fg is equal to the mass, m, times the acceleration
due to gravity, g.
Fg = mg
Equation 2
If an object sinks, then Fg > FB. If an object floats just below the surface of the water, then the upward buoyancy force just
equals the force due to gravity in magnitude or FB = Fg. If an object floats, then Fg < FB.
In Part B of the demonstration, the setup is modified so that the reaction mixture is contained in a closed bottle. This allows for
the measurement of the mass of the reactants and the mass of the products without the loss of any material from the reaction
and with no change in the volume. Releasing the carbon dioxide from the closed system and massing again demonstrates that
the gas has mass and the apparent loss of mass in Part A was due to the buoyant force.
Connecting to the National Standards
This laboratory activity relates to the following National Science Education Standards (1996):
Unifying Concepts and Processes: Grades K–12
Evidence, models, and explanation
Constancy, change, and measurement
Content Standards: Grades 5–8
Content Standard B: Physical Science, properties and changes of properties in matter, motions and forces
Content Standards: Grades 9–12
Content Standard B: Physical Science, structure and properties of matter, chemical reactions, motions and forces
Flinn Scientific—Teaching Chemistry™ eLearning Video Series
A video of the Air Has Mass—Density of Air activity, presented by Annis Hapkiewicz and Peg Convery is available in Density of
Gases, part of the Flinn Scientific—Teaching Chemistry eLearning Video Series.
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© 2016 Flinn Scientific, Inc. All Rights Reserved.
Air Has Mass—Density of Air continued
Materials for Air Has Mass—Density of Air are available from Flinn Scientific, Inc.
Catalog No.
V0005
S0043
OB2096
AP1694
AP4836
Description
Vinegar, White, 3.78 L
Sodium Bicarbonate, 500 g
Balance, Flinn Scientific Electronic, 0.01-g Precision
Bottle, Dropping, Polyethylene, with Screw-On Cap
Spring Scale, Pull-Type, 250-g Capacity
Consult your Flinn Scientific Catalog/Reference Manual for current prices.
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© 2016 Flinn Scientific, Inc. All Rights Reserved.