Lecture Notes – Physical vs Chemical Properties and Changes –  Mixtures & Pure Substances Chapter 3.1 - 3.4 (Std 2d & 3e)

 

After you read these note, it is important for you to go to this link Chemical and Physical Properties & Changes w practice quiz .  Read the additional background material on physical and chemical properties and answer the practice questions.  When you get to the bottom of this page, take the link there to learn about “different types of matter”, and take the practice quizzes there to check your understanding.

 

Physical Properties ­–

Properties of an element or a compound that can be observed without a chemical reaction. These include:

mass

volume

density

color

odor

shape

melting point

boiling point

state (solid, liquid, gas)

      The state can be changed, but the substance will still be the same when you return to the original state.

      Example – We think of water as only being the liquid state.  Actually water can be a solid which we call ice.  Water can also be a gas, which we call steam.  Steam can be condensed to form liquid water and ice can be melted to form liquid water.  Water in any state is still water, specifically H2O.

hardness

softness

 

Chemical Properties –

Properties of an element or compound can will be observed when a chemical reaction occurs.  Examples of  chemical properties could be:

      1.  how sulfuric acid reacts with sugar

      2.  how electricity reacts with water to form hydrogen and oxygen

      3.  how magnesium reacts with water to form magnesium oxide

 

Physical Change –

The stubstance(s) involved in physical changes are not changed chemically in physical changes.  They may change state (solid to liquid to gas) or they may be separated or mixed.  However, they can always be seaparated or changed back to their original state.  Examples are:

      1.   water is frozen or evaporated (changed to ice or to steam)

      2.  a piece of paper is torn in two

      3.  a diamond is used to cut a ruby

      4.  salt dissolves in water, when the salt water is evaporated the salt is left behind

      5.  two different colored paints are mixed to form a new color that is different from either of the original colors

      6.  acetone evaporates rapidly at room temperature

 

Chemical Change –

Two substances (reactants) react with each other and one or more new substance(s) (products) are created that is/are different from the original substance(s).  Products and reacts have different physical properties.  Examples are:

1.  when sulfuric acid (H2SO4) reacts with sugar (C12H22O11) and forms carbon (C) and water (H2O) reactants and products are very different in both chemical and physical properties

2.  when magnesium (Mg) and oxygen (O2) react they form magnesium oxide (MgO)

3.  when electricity is passed through water (H2O) two new substances are formed, hydrogen (H2) and oxygen (O2)

     

TYPES OF MATTER

 

Pure Substances –

Constant defined composition and cannot be separated into simpler substances by physical means.  There are two types of Pure Substances:

Elements – Are pure substances made out of only one kind of atom.  The periodic table contains all the known elements.

Compounds – Pure substances that are made of two or more different atoms.  These substances follow the law of constant composition.  That means that there is a set ratio of each atom in the compound to every other atom in the compound ( i.e. H2O ) .  These substances can be broken down by chemical means into the atoms that make them up.  However, they cannot be separated by physical means.

 

Mixtures –

These substances are made up of more than one element    and / or   compound.  Different mixtures can be almost any proportion of one element or compound to another.  A good visualization of this can be seen at Elemnt, cmpd, mix (molecular level w MDL models you can rotate)- applets (you will need Chime to view this, see instructions on the home page for how to do get a free download).  Salt water is a good example of a mixture.  You could have 15% NaCl and 85% H2O, while another mixture could have 70% NaCl and 30% H2O.  The two types of mixtures are:

Homogeneous mixtures – There is the same composition of elements    and / or    compounds everywhere.  So, homogeneous mixtures are exactly the same everywhere.  When a substance dissolves in water (like salt water) it will be homogeneous. 

Heterogeneous mixtures – Substances in heterogeneous mixtures are not evenly distributed.  Try to visualize sand and rocks in water.  The sand and rocks will be at the bottom, while the water will be above.  The sand will tend to settle under the rocks and between them. 

 

Look at this tutorial.  It has questions for you to test yourself with and be sure you understand Classifying Matter with practice quiz and answers