Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109	General Chemistry	Spring 2009
Lecture Notes:: 28 January	© R. Paselk 2002
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Matter

What is matter? Stuff. Has mass and occupies space.

Mass

Mass is the measure of quantity for matter. Mass is the property of matter resulting in its inertia and and attraction via gravity.

Do not confuse mass and weight. Weight is the force acting on an object due to gravity. We often interchange these terms in conversation, but they are quite different - you have the same mass whether you are weightless in space on here on Earth (taking a shuttle flight is no substitute for a diet!). To confuse us further we call the determination of mass "weighing"!

Matter has both physical properties and chemical properties. These are properties which do not depend on the quantity of substance and therefore they can be used to identify a substance (sometimes referred to as intensive properties).

• Physical properties of substances can be observed without, in principle, changing their compositions. Physical properties include mass, color, density etc. Note that physical changes such as melting, cutting, etc. do not change composition. When you perform a physical operation such as melting or cutting, you have changed the object, but you still have the same kind of stuff - chop a wooden stick in half, you have two sticks, not a newkind of thing like a rock or piece of metal. Physical operations are also often readily reversible - if you melt ice the water can be readily returned to ice, though not necessarily the same shape, it is obviously the same stuff!

States of Matter. Matter can exist in three states under earth-surface conditions:

• Solid: definite shape and volume (Crystals vs. super-cooled liquids or glasses)
• Liquid: definite volume, but no defined shape - will fit to container etc.
• Gas: no definite shape or volume - will fill whatever container they are in.
  • both liquids and gases are fluids.

A fourth state of matter commonly occurs under special conditions: a plasma. A plasma is an ionized fluid - can be contained by magnetic fields.

• Chemical properties of substances describe behaviors which lead to changes in composition. Chemical properties describe reactivity under various circumstances (does it burn in air, react with acids or bases, corrode in sea water etc.) Note that chemical changes result in different compositions. If you burn a stick you no longer have wood - you have created a new set of kinds of stuff such as carbon dioxide and water, a gas and a liquid completely unrecognizable as even related to wood.  

Chapter 2: Atoms Molecules & Ions

Chemistry has a very long history, however progress was limited until quantitative measurements became the accepted norm for lab work in the 17th and 18th centuries. A fundamental observation was (and is) that mass is unchanged in chemical processes. This observation is summarized in the:

Law of Conservation of Mass

Mass is neither created nor destroyed during a chemical change. (Strictly speaking there is no measurable change.) For example, if we burn gasoline (octane) in air we will get carbon dioxide and water:

2 C₈H₁₈ + 25 O₂ 16 CO₂ + 18 H₂O

If we were to weigh (determine the mass) of the carbon and oxygen vs. the carbon dioxide and water we would find them to be identical - the masses are the same on both sides of the equation (that's why its called a chemical equation, the two sides are "equal"). Looked at another way, if you count the atoms, the numbers of each kind of atom on each side are identical - so we can also say that atoms are conserved in chemical processes.

This is really the fundamental assumption of chemistry, and thus the measurement of mass is the fundamental process underlying much of chemical work.

Another fundamental concept is that of elements. Elements are substances which cannot be broken down further into simpler substances by chemical or (non-nuclear) physical means.

Once conservation of mass was accepted it was noted that combinations of various elements to form compounds nearly always result in the formation of compounds with constant proportions by mass. This set of observations is summarized in "Proust's Law," now known as the

Law of Definite Proportion:

a given compound always contains the same proportion by mass of its constituent elements.

But of course there is more than one way to combine many elements to give compounds. For these compounds a new set of observations applies:

The Law of Multiple Proportions:

When two elements combine to form a series of compounds, the ratio of the mass of the second element which will combine with 1 gram of the first will always be reducible to a small whole number. (Similarly, with multi-element compounds {other than macromolecules}, the ratios of the elements also reduce to small whole numbers.)

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Last modified 28 January 2009