Chapter 2 Notes

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Ch. 2 Learning Objectives
1. I can classify pure substances. This means that I
know the difference between an element and a
compound.
2. I can describe the characteristics of an element.
This means that I know how elements are identified
by using chemical symbols.
3. I can describe the characteristics of a
compound. The means that I know the difference
between a compound and other pure substances.
4. I can distinguish a pure substance from a
mixture. This means that I know what makes them
different.
Learning Objectives Continued
5.
6.
7.
8.
9.
I can classify mixtures. This means that I know what a
homogeneous and heterogeneous mixture is. I also know
what a solution, suspension, and colloid is.
I can describe the physical properties of matter. This
means that I know what each property is and how we can
identify them.
I can describe how to separate a mixture. This means
I can differentiate between filtration and distillation.
I can describe the chemical properties of matter.
This means I can list the different properties.
I can describe how to tell that a change has a
occurred. This means I can state the three ways to tell a
change has happened.
Section 2.1:
Classifying Matter
Pure Substances
 Matter that always has exactly the same composition is
classified as a pure substance.
 Examples: salt and sugar
 **Every sample of a given substance has the same
properties because a substance has a fixed, uniform
composition.
 Substances are classified into two categories: elements
and compounds.
Elements
 An element is a substance that cannot be broken down
into simpler substances.
 An atom is the smallest particle of an element.
 **An element has a fixed composition because it
contains only 1 type of atom.
 No two elements contain the same type of atom.
Examples of Elements
 Solid elements at room temperature
 Aluminum
 carbon
 Gas elements at room temperature
 Oxygen
 Nitrogen
 Liquid elements at room temperature
 Bromine
 Mercury
Symbols for Elements
 Jons Berzelius, a Swedish chemist, suggested that
chemists use symbols to represent elements.
 Characteristics of symbols:
 1 or 2 letters (First letter is always capitalized)
 Still used on the periodic table today.
 Why is using symbols important?
 So there is a universal symbol for all languages.
Compounds
 Water is composed of the elements hydrogen and
oxygen
 Compounds
 Are made from two or more simpler substances that
can be broken down into those simpler substances.
 Always contain 2 or more elements joined in a fixed
proportion.
Mixtures
 **Properties can vary because the composition of a
mixture is not fixed. (this means the composition can
change)
Mixtures
 In a heterogeneous mixture, the parts of the mixture
can be classified and noticeably different from one
another.
Mixtures continued
 In a homogeneous mixture, substances are so evenly
distributed that it is difficult to distinguish one
substance from another in the mixture.
 Appears to be only one substance.
Solutions, Suspensions, and
Colloids
 Based on the size of its largest particles a mixture can
be classified as a solution, a suspension, or a colloid
Solutions
 When substances dissolve and form a homogeneous
mixture, the mixture that forms is called a solution.

Example: sugar and water
 Liquid Solutions
 Do not separate
 Can not be filtered
 Clear
**The particles in a solution are too small to settle out of
the solution, be trapped by a filter, or scatter light
Suspensions
 Is a heterogeneous mixture that separates into layers
over time.
 Example: sand and water
 Can a suspension be filtered?
Colloids
 A colloid contains some particles that are intermediate
in between small particles in a solution and the larger
particles in a suspension.
 Do not separate into layers
 Can not be filtered
 Examples: Fog
Physical Properties
 A physical property is any characteristic of a material
that can be observed or measured without changing
the composition of substances in a material.
Examples of Physical Properties
 Viscosity
 Conductivity
 Malleability
 Hardness
 Melting point
 Boiling point
 Density
Viscosity
 The resistance to flow
 The greater the viscosity the slower the liquid moves
 High viscosity liquids: honey, corn syrup
 Low viscosity liquids: vinegar , water
 Viscosity will decrease when the liquid is heated.
Conductivity
 Ability to allow heat to flow
 Materials with high conductivity are called conductors.
 Example: Metals
Other Properties
 Malleability – the ability of a solid to be hammered
without shattering.
 Hardness – diamond is the hardest know material
 Density – is used to test purity of a substance
 Melting and Boiling Points
 When a material has reached its melting point the
material changes from a solid to a liquid
Using Physical Properties
 How are physical properties used:
 To identify a material
 To choose a material for a specific purpose
 To separate substances into a mixture
Using Properties to Identify
Materials
 What are the three steps used to identify a material?
 Decide which properties to test
 Test on the unknown sample
 Compare results with data for known materials
 Why is identifying a material important?
 To solve crimes
Using Properties to Separate
Mixtures
 What are the two separation methods?
 Filtration- the process that separates materials based on
the size of the particles.
 Distillation- process that separates the substances in a
solution based on their boiling points.
 Example: providing fresh water
Recognizing Physical Changes
 When does a physical change occur?
 When some of the properties of a material change, but
the substances in the material stay the same.
 Two examples: ice melting and hair styles
Observing Chemical Properties
 What is a chemical property?
 Any ability to produce a change in the composition of
matter.
 When can a chemical property be observed?
 Only when the substances in a sample of matter are
changing into different substances.
 What are the two examples of chemical properties?
 Flammability – material’s ability to burn in the presence
of oxygen
 Reactivity – how readily a substance combines
chemically with other substances.
 Oxygen is a highly reactive element and
nitrogen has an extremely low reactivity.
Recognizing Chemical Changes
 When does a chemical change occur?
 When a substance reacts and forms one or more new
substances.
 Examples of a chemical change
 Baking a cake
 Ripening bananas
 Leaves on a tree changing color
Evidence of a chemical change
 Change in color
 Production of gas
 Formation of precipitate
Ask yourself…
Are different substances
present after the change takes
place?
Key Point
 When matter undergoes a chemical
change, the composition of the matter
changes. When matter undergoes a
physical change, the composition of
the matter stays the same.
Homework
Page 58
Section 2.3 Assessment
Questions # 4-8
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