Transcript Examples of Chemical Changes

Chemistry
CHEMICAL CHANGES
• In a chemical change, a new substance or
substances appear.
• Chemical changes are not reversible by
normal methods.
PHYSICAL CHANGES
• In a physical change, the substance does not
change.
• Physical changes can be reversed.
Examples of Physical Changes
• Cutting a piece of paper:
The shape has changed, but it is still paper.
• Boiling water:
Water changes to steam. Steam is not a new substance; it is just a different form of
water.
• Dissolving salt in water:
It’s hard to tell whether a new substance forms when salt dissolves in water. If we
evaporate the water, we would get the salt back. That makes it a physical change,
because it is reversible.
All changes of state are physical changes.
• Water turning to ice or to steam is always a reversible
process. Ice and steam are not new substances. They
are still the same water particles, just arranged a bit
differently.
Examples of Chemical Changes
• A chemical change will involve a chemical reaction.
• Cooking and burning are always chemical changes.
• A chemical reaction occurs when two or more chemicals combine
to produce different chemicals.
• How can you identify a chemical change?
- The color might change
- Energy can be released
- The smell might change
- A gas may be produced
- A precipitate may form
Question 1
When a cloud forms in the sky, is it a physical or
a chemical change?
A) Physical change
B) Chemical change
Question 2
Is rusting a physical or a chemical change?
A) Physical change
B) Chemical change
An intensive property is one that does
not depend on how much material is
present.
An extensive property is one that does
depend on the amount of substance.
For example, temperature is an
intensive property. If a beaker of water
has a temperature, all the water
has the same temperature.
It doesn’t depend on how much
water there is.
So, mass and volume are both extensive
properties. They both change if the amount of
material changes. It is interesting to note that
density (defined as mass/volume) is an
intensive property, but it is defined in terms of
two extensive properties.
Examples of intensive properties
include:
Temperature, density, chemical
potential, viscosity, electrical resistivity,
specific heat capacity, melting point and
boiling point, pressure
Examples of extensive properties include:
Energy, mass, volume, weight, length
Question 1
Which of the following is an intensive property?
A) Weight
B) Energy
C) Freezing point
D) Width
Question 2
Which of the following is an extensive property?
A) Freezing Point
B) Density
C) Temperature
D) Width
Matter can be anything that occupies space and has mass.
State of Matter
Solid
Liquid
Gas
Plasma
Shape
Definite
Indefinite
Indefinite
Indefinite
Volume
Definite
Definite
Indefinite
Indefinite
Particle
Movement
Packed
closely
together
More free to
move
Move
independently
Move
independently
Examples
Diamonds
Ice, Powder
Water, Milk,
Coke
Helium,
Hydrogen,
Sun, Plasma
TV
Demonstration
All matter possesses a certain amount of energy. However, energy is not mass, as it cannot be weighed,
and it does not occupy space.
Question 1
What happens to a substance at the
temperature known as the melting point?
A) It changes from a liquid to a gas
B) It changes from a solid to a liquid
C) It changes from a liquid to a solid
D) It changes from a solid to a gas
Question 2
"The shape of a liquid can change to fit the
container it fills." Is this statement true or
false?
True
or
False
Question 3
Which of the following statements applies to
gases?
A) They have no fixed shape or form.
B) Their atoms or molecules move
independently.
C) Their atoms or molecules have lots of energy.
D) All of the above.
Question 4
Which of the following states of matter has the
least energy?
A) Solid
B) Liquid
C) Gas
D) Plasma
Physical properties do not change the chemical nature
of matter.
Physical Property
Description
Example
States of Matter
changes
Energy increases/decreases to change
solid to liquid to gas and vice versa
Draw the States of Matter
Changes Triangle
Volume
amount of space occupied
Draw a graduated cylinder
with liquid
Mass
amount of matter it contains
Draw the a triple beam
Density
amount of mass per unit of its volume
D = m/v
Draw a marble inside a
container of water
Compressibility
extent that the volume of a substance
will change in response to pressure.
Draw a hand squeezing a ball
Solubility
amount of it that will dissolve in a
liquid under certain conditions.
Draw Alkaselter dissolving in
water
Specific Heat Capacity
amount of heat energy that
is needed to raise the temperature of
1 kg of the substance by 1 K
Chemical properties are only visible in a chemical
reaction.
A chemical property is one that will cause an
irreversible change in a substance when it occurs.
Examples of chemical properties include:
- sulfur burns in air
- iron rusts in water
- hydrogen peroxide decomposes to oxygen and
water
- vinegar reacts with baking soda
Question 1
Beeswax melts at 64° C. It is often used to make
candles. Why would it be impractical to make
a cooking pot from beeswax?
A) Water boiled in a beeswax cooking pot would
taste bad
B) It would attract bees
C) The beeswax would melt before the water
boiled
D) It would be difficult to shape the beeswax
Question 2
What is a substance's melting point usually the
same as?
A) Its freezing point
B) The point at which it evaporates
C) Its boiling point
D) None of the above
MATTER
Can it be separated
by physical means?
NO
PURE SUBSTANCES
MIXTURES
Can it be decomposed
by ordinary chemical
means?
NO
ELEMENTS
YES
Is the composition
uniform?
YES
COMPOUNDS
YES
HOMOGENEOUS
MIXTURES
NO
HETEROGENEOUS
MIXTURES
MATTER
Can it be separated
by physical means?
NO
PURE SUBSTANCES
MIXTURES
Can it be decomposed
by ordinary chemical
means?
NO
ELEMENTS
YES
Is the composition
uniform?
YES
COMPOUNDS
YES
HOMOGENEOUS
MIXTURES
NO
HETEROGENEOUS
MIXTURES