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Chemistry - The study of substances, and in particular, the transformation of substances.

What is a substance?

For now, let's consider a substance to be one of two things: 

element - periodic table, composed of the same type of atom.


compound - collection of elements held together by chemical bonds. 

For instance:  see a model of glycine the simplest amino acid (building block of proteins such as hemoglobin )

or a mixture of them

We will return to elements and compounds latter. 

 

Why is Chemistry Important?

The machinery of life

The preservation and destruction of life

Ability to manipulate the world around us

[The practice of science is intended to be value free (i.e. not good or bad).  However, value can be applied to what is done with the results of scientific inquiry and also to the problems scientist chose to study and the methods they use]

How do scientists approach a problem? - the scientific method

Let's explore this by considering ice skating.

Step 1: Pick a problem: Why can we skate better on ice than on wood?

Step 2: Research: What do we know already? - Some observations:

You can skate on ice well below its freezing point of 0oC.
Wet surfaces are slippery.

Applying pressure to ice causes it to melt (this is unusual).

Step 3: Make a Hypothesis:
   The pressure from the skate melts the ice and makes it slippery. 

 

This turns out to be Bill Nye's hypothesis as well, and he constructed the following diagram to illustrate the idea. 

 

Sounds Reasonable! So we are done!

Reasonable is not enough
Step 4: We need to test our hypothesis.  Design an experiment to test the hypothesis 
measure the melting point of ice as a function of pressure and compare that to the pressure exerted by a skater.

An aside - temperature scales, scientific notation, unit conversion

Temperature Scales 

temperature measured on three different scales: Kelvin (K, "absolute temperature scale", no degree symbol), Centigrade (oC), and Fahrenheit (oF )

K = C + 273
C = 5 / 9 (F - 32)

Always good to check if an equation seems correct: use values or limits you know:  The boiling point (100oC = 212oF) or freezing point (0oC = 32oF ) of water for instance

C = 5 / 9 (212 - 32) = 100   checks out!

Some temperatures of interest:

0K - absolute zero - can't go colder
77K - boiling point of liquid nitrogen at atmospheric pressure (Question: can you raise the boiling point of liquid nitrogen above 77K?, or water above 373K?)
200K - sublimation point of dry ice
273K - freezing point of water
373K - boiling point of water

Scientific notation

Inconvenient to always write out very large or very small numbers so we use scientific notation:

50000 = 5 * 10000 = 5 *10*10*10*10 = 5x104 (move decimal 4 places right)
0.0005 = 5 / 10000 = 5 / (10*10*10*10) = 5 / 104 = 5x10-4 (4 places left)


Converting units:

dimensional analysis - factor label

Ex: using the tabulated conversion factor  1 atm = 1.01x105 Pa

 

Back to our experiment.

The melting point of ice as a function of pressure is given by the following graph:

How much pressure does a skater exert?

Pressure = Force / Area  (the smaller the area, the larger the pressure)

The weight of an object is the force it exerts due to gravity.

Need to make assumptions or define our system:

Scientists explanations of things generally only hold for a particular set of parameters - for a particular system.  Important to understand what parameters and assumptions were used.

Define the skaters weight and area of the skate:

Given the typical skaters weight (150lb) and the area of a skate blade (20cm2) this corresponds to a pressure of:

3.2 x 106 Pascals

Step 5. Draw a conclusion about the hypothesis.

Is the hypothesis correct?  NO, not enough pressure to melt the ice!

Step 6. Refine the hypothesis and design more experiments to continue testing it.

Theories and Laws

Theory - a physical or mathematical model designed to explain some observation or phenomenon.

Law - a general statement about a natural phenomenon whose validity is supported by substantial experimentation.

Note: Theories and laws are never proven.  It only takes one contrary example to disprove them.

So why can we skate on ice?

The "little green creature with fire hoses" theory

 

Theories and hypotheses must:

1. be falsifiable
2. make predictions that can be tested