Unit 5: Conservation of Mass as
an Example of a Fundamental
Law
Core 270
Spring 2008
Dr. Sharon Fredericks
Matter and mass
• Matter
• Mass = measure of the amount of matter; relies on
inertia (resistance to changing velocity)
– Units: gram or kilogram, slug
• Weight = depends on the strength of gravitational
force; technically not the same as mass but ok for us
– Units1: Newton, pound
• An astronaut’s weight changes as he/she goes from
Earth to the moon, but her/his mass doesn’t.
• Close to the Earth, 1 kg of something has a weight of
2.2 lb
2
Figure
Studying Matter
• Properties
– Size, color, temperature
– Chemical composition: what matter is made of
– Chemical reactivity: how matter behaves
• Transformations
– Physical change: one that does not alter the
___________________make-up of a substance
• Phase change (melting, freezing, evaporating),
dissolving
– Chemical change: one that does alter the
___________________ make-up of a substance
• New substance is formed
Physical
Figure 2.2
2
Change
Chemical
Figure 2.5
2
Change
What do you think?
• In the boxes below, each solid circle
represents an atom of the same element. Each
hollow circle represents an atom of a 2nd
element.
• The transformation from left to right is best
viewed as a chemical change or physical
change? Why?
Sample problem
Identify which are chemical changes and which are
physical changes:
• Mixing salt and water to make salt water
• Steam condensing to water
• Milk souring
• Ignition of matches
• Breaking of a dinner plate
• Leaves turn red in Autumn
States of Matter
• 2 broad categories: solid & fluid
– Solid:
– Fluid:
• Conversion of one state to another is called
_______________________________________
• Solid
Liquid = melting/freezing
– Melting point = freezing point
• Liquid
Gas = boiling/condensing
– Boiling point = vaporization temperature
• Solid
• Liquid
Vapor = subliming/condensing
Vapor = evaporating/condensing
Changes of
2
Phase
Heat is added in order to pull atoms or molecules a part for melting & evaporation.
Heat is released during freezing and condensation as atoms & molecules “bond”.
Sample Problem
• Sulfur dioxide (SO2) is a compound produced when
sulfur burns in air. It has a melting point of –72.7oC
and a boiling point of –10oC. In what state does
SO2 exist at room temperature (25oC)? How about
at –11oC?
Solution to Problem
• Note that the scale is in Celsius and both the mp and
the bp are negative numbers
• As you go to the left, you increase in the negative
direction.
Your Turn
• The melting point of the element bromine is
–7.2 oC. Its boiling point is 58 oC. In what
state does Br2 exist at room temperature
(25oC)? How about at –11oC?
Solution to Problem
• Note that the scale is in Celsius.
• The mp is negative; the bp is positive. So zero degrees
Celsius is somewhere in between.
• As you go to the left, you increase in negative
numbers. As you go right, you increase in the positive
numbers.
Melting vs. Dissolving
Melting
Dissolving
• Involves a solid to liquid
transition at a specific
temperature.
• Only ONE substance (no
solvent)
• Ice  water at 0 degrees
C
• Sugar(s) molten sugar
at 365 degrees F (186 deg
C)
• One substance is mixed
with another to form a
solution
• At least TWO substances
involved
• Salt + water salt water
at all temperatures above
room temperature
– Aqueous solution
Sample Problem
• What is happening
when you put an ice
cube in your mouth?
• What is happening
when you put a sugar
cube in your mouth?
Conservation of Mass
•
• It is Fundamental Law
• To be tested, the type of system has to be
determined1: closed or open
• Closed: matter can not enter or leave the system;
• Open: Matter can enter and leave the system; most
systems are open
Concept Problem
A piece of dry ice (solid carbon dioxide) is
placed in an air-tight container and sealed
shut. At room temperature, the solid
changes to a vapor or sublimes.
Is the mass of the carbon dioxide and
container the same, less, or more after all
the solid evaporates?
Closed and Open Systems with
respect to matter
• Closed
– Sealed glass bottle
– Closed zip-lock bag
• Open
–
–
–
–
A room with an open window or door
Car
Ocean, sea, lake, pond, river
Anything in which matter (remember gases like air) can
flow in and out
The
1
Earth
• It can be considered an open system because we
know that material from space can fall to Earth.
Furthermore, the space shuttle, rockets, etc can
leave and sometimes return
• It can be considered a closed system for all
intensive purposes because a fraction of 1% of
matter is crossing the boundary.
• This is a good ___________________________
and demonstrates that sometimes we settle for a
description or law that is ___________________
_______________________________________
Law of Conservation of Mass for
an Open System
• mfin = mori + min – mout
–
–
–
–
mfin = total matter in final condition of system
mori = total matter in original condition of system
min = matter that has been put into the system
mout = matter that has been taken out of the system
• If Dm = min – mout
then
• If morig is constant, then
• Rearranging:
• By tracking changes,
Problem
• A person has a checking account balance of
$515.20. He took out $80 from the ATM, which
he used for daily expenses. He bought $35.90
worth of groceries using his debit card (from
checking) and paid his Visa bill ($52.88) and rent
($400). He deposited his weekly paycheck of
$350. What was mori, min, mout, Dm, and mfin?
Assume the amount of money=mass and use the
formulas introduced.
Your Turn
A candy factory made 100 dozen chocolate truffles. It sent 30
dozen to the local candy store, 45 dozen to the candy store
in the next town, and shipped out 4 dozen to a private
customer. It placed the rest in its own factory store. The
factory store sold 6 dozen during the first week of sales,
but the private costumer sent back a dozen because she
only wanted 3 dozen.
What was the original amount (morig), the amount of candy
that left the factory (mout), the amount of candy that came
into the factory (min), the change of amount (Dm), and the
final amount (mfin) of candy in the factory?
Solution
Exception to the law3
• In ____________________reactions, mass is not
conserved and is converted to energy according to
Einstein’s famous equation E=mc2.
• In ________________________reactions mass is
conserved, since in every case the mass-energy of
the reactants is huge in comparison to the energy
absorbed or released when they react.
– “By way of example, a gram of TNT releases 4.16 kJ of
energy when exploded. However, the rest-energy of a
gram of TNT is 90 TJ, or about 20 billion times as
much. This means that even if the products of a TNT
explosion were stopped and allowed to cool to the
original temperature, they would only lose 1 part in 20
billion in weight. This amount would be very difficult
to measure.”3
Amalie (Emmy)
1
Noether
• Brilliant mathematician – considered one of the
most important in the 20th century
• Established role of symmetry to laws of
conservation with respect to the General Theory of
Relativity
• Could not officially teach in universities in
Germany and was given a token salary after 10
years
• Could not present any of her original work,
including a ground breaking paper – a male
colleague took her place
References
1. Kelinsteuber, et al., Natural Science 5th
edition, King’s College, PA, 2004.
2. Suchocki, J., Conceptual Chemistry, 2nd
edition, Benjamin Cummings, 2004.
3. Wikipedia web site:
http://en.wikipedia.org/wiki/Conservation
_of_mass