What is Chemistry?

Chemistry is the study of matter and its transformations.

• "Classical" chemistry involves mostly electron transfers and/or interactions of charges (electron and nuclear). As we'll see only some electrons in atoms are involved - the outer or valence electrons of atoms.
• Nuclear chemistry is an extension of chemistry where nuclei are transformed changing one kind of atom (element or isotope) to another. This is a completely separate realm of phenomena, largely unimportant in everyday life (unless you work at a nuclear power plant!).

More specifically, chemistry is the scientific study of matter. So what do we mean by science? Two common "definitions":

• The body of knowledge and rules/laws/theories we have discovered regarding the natural world.
• The method of discovery and confirmation used by scientists. Classically we describe this process as the "Scientific Method" summarized in the steps below:
  • Identify a problem based on initial observations
  • Collect data via planned Observations and/or Experiments ("asking nature a question")
    • "Clean" simple experiments vs. statistical inference
    • Controls - everything the same except the variable of interest.
  • Analyze and Evaluate results
  • Hypothesis
  • Theory (model)
    • Law

Matter

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

Mass: 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 (the interaction with light), density etc. Note that physical changes such as melting, cutting, etc. do not change composition.

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 and different physical properties.

Measurements

Accuracy and Precision

First we need to define and distinguish between two terms: accuracy and precision. Consider the two targets below:

Which target is the work of the better marksman? I would say B, because she always hits nearly the same place - all we have to do to get all of her shots in the center is to adjust her sights. On the other hand, A is scattered all over. Sure he hit the center once, but, on average he needs lots of shots to do it, adjsuting his sights will do us no good!

• Looking at the average of the dots on each target (represented by an x) we see that A has very good accuracy, but lousy precision.
  • Accuracy refers to how closely we approach the "real" or actual value (center of the target). Note that ALL measurements have some scatter, so accuracy generally refers to an average value.
  • Precision refers to how closely values are repeated (are the hits "clustered"). Precision is often described as repeatability.
• B on the other hand, has poor accuracy, but very good precision.

For the most part, it is more important to be precise, than it is to be accurate, since we can always adjust our instrument, or our data, to bring the results to the proper value.

Exponential or scientific notation

It is often convenient to express numbers in exponential or scientific notation to indicate significant figures, and to just avoid writing the huge numbers of zeros we often run into in the natural world. [examples] See lab book exercises.

Significant Figures

For measurements we want to be sure we convey the precision (repeatability) of our measurements using significant figures. [examples] See lab book exercises.

• Look at example of making measurements with a ruler.
  • Which digits should we keep? Which did we measure?
  • For addition and subtraction, it turns out we look at the decimal place to determine if a figure is significant (if we should keep it).
    • Note that with addition and subtraction we can end up with more or fewer significant digits in an answer.

Significant Figures and Calculations: Two basic sets of rules:

1. Addition/Subtraction rule: Significant figures are determined by looking at the decimal place of the numbers being added or subtracted. The number with the "least decimal places" determines the decimal place of the answer, e.g. if we add 1,216,956 to 214.879, the first number has the fewest decimal places, so the answer is rounded off to the 1's place: 1217170.879 goes to 1217171.
• Note that this is based on the idea that the error in the "least decimal place" measured figure is larger than the subsequent decimal figures, so they are dropped after rounding.
• examples.
2. Multiplication/Division rule: In this case we count the digits. The number with the fewest significant digits determines the number of significant digits in the answer.
   • Note that in this case we are looking at how the error propagates as a fraction of the total (% error), for example
     • for the problem 2.0 x 201 = 402 should be written as 4.0 x 10².
     • This can be understood by doing the problem with upper and lower ranges of the measured number: 1.95<2.0<2.05
       • 1.95 x 201 = 391.95
       • 2.05 x 201 = 412.05
     • Obviously we can't claim agreement with more than 2 significant figures. In fact, 4.0 is stretching it!
   • examples.

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© R A Paselk

Last modified 28 August 2008