Transcript Explaining the density of gases

Explaining the density of gases
Aseel Samaro
Introduction
 Have you ever wondered why helium balloons float
or why carbon dioxide gas sinks?
 The answer lies in understanding the density of
gases.
How dense are gases?
 As for solids and liquids, the density of gases depends on the mass of the
gas particles and the volume they occupy.
 This table shows the densities of different gases.
 All gases less dense than air will float in it, while gases with a density higher
than that of air, like carbon dioxide, will sink in it.
Gas
Density (mg/cm3)
hydrogen
0.089
helium
0.18
air
1.28
carbon dioxide
1.977
ammonia
0.73
chlorine
3.2
Which gases will sink in air?
Applications of gases based on density
 Gases, just like solids and liquids, become less dense when heated.
 This principle was used in the development of hot-air ballooning.
 The air has to remain hot for the balloon to stay afloat.
 This led to many fires because early balloons were made from paper.
 Nowadays air balloons use nylon (melting temperature 250 °C) and
air at 120 °C.
 Most modern balloons use helium instead of air (density 0.18 g/l).
 Carbon dioxide gas is nearly twice as dense as air.
 When it is released, it sinks to the floor, causing any air (and oxygen)
to rise up above it.
 This is a useful property for fighting fires.
Air temperature (°C)
Air density (g/l)
20
1.20
99
0.95
120
0.90
Density of air at
different
temperatures
Why is it preferable to use helium in a balloon rather than hot air?
helium is lighter than air; does not need to be heated to make it less dense
Chlorine damages the lungs. It was used as a chemical weapon in
World War I by firing it into the trenches. Why was this an effective
method?
Chlorine is denser than air; it filled the trenches instead of floating away
Working out density
 We can work out the mass of different elements and compounds
using the Periodic Table.
 We do this by: finding the atomic mass of the element, multiplying it
by the number of atoms of that element in one unit of the
compound, and adding all the masses together.
 For example, the atomic mass of N = 14 and the atomic mass of H = 1.
 Atomic mass: The mass of an atom is primarily
determined by the number of protons and
neutrons in its nucleus.
 atomic number of a chemical element (also
known as its proton number): is the number of
protons found in the nucleus of an atom of
that element
 So the mass of one unit (one molecule) of the compound ammonia
(NH3) is (1 × 14) + (3 × 1) = 17 in atomic mass units.
 Scientists discovered that if you work out the atomic mass of any
gaseous element or compound, and scale it up from atomic mass
units to grams, all gases occupy very nearly the same volume of 22.4
litres (at standard temperature and pressure).
 So if you had 17 g of ammonia gas, it would have a volume of 22.4
litres. The approximate density can then be calculated as:
mass
density =
––––––––
volume
17
=
–––––
22.4
= 0.8 g/l
Use the Periodic Table to work out the approximate density of the
following gases:
 oxygen (O2)
 carbon dioxide (CO2)
 sulfur dioxide (SO2)
 camping gas – butane (C4H10)
 C2H2
 argon (Ar)
 nitrous oxide (N2O)
 propane (C3H8)
Did you know…?
 The first human flight in a hot-air balloon took place in 1783 by the
Montgolfier brothers in France.
Thank you