Course Lecture Topic Information
Chem 105 Web Site
Dr. Kimberly Lawler-Sagarin
Elmhurst College
This discussion will take place the week of August 30 through September 5
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Assignments for the discussion board Post Introductions (Q1) by Wednesday evening | Student Numbers |
This week's discussion questions are as follows:
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These will remain the same throughout the semester
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Required Readings for This Week
(CfCT = Chemistry for Changing Times and Colour = Colour: Why the World Isn't Grey)
The student will
What is Chemistry?
Chemistry is the study of matter and its changes from one substance to another.
The energy gained or released in a chemical change is included in the study of chemistry. In this course, we will be focusing on the theme of color. This relates to chemistry in many ways. The color or colorful dyes, paint pigments, neon signs, fireworks, gemstones, and autumn leaves are all the result of light interacting with matter (in particular, chemicals).
This week, we will introduce the topic of light and color. But first, we will review a number of fundamental ideas about using measurements in science. This will give us a basis to talk about different colors of light.
Measurement and Significant Figures
When we make a measurement, such as measuring our height with a tape measure, or our weight with a scale, there is some uncertainty in the number we obtain. Even if our measurement technique is perfect, we are limited by the
precision of the measurement device used.
Learning Goals for this Discussion
To achieve these goals, it is essential that you read and study the
Discussion 1 Virtual Lecture notes (below), the required sections of the
texts, and all the discussion posts for discussion 1. The Concept Quiz is based on the reading and the virtual notes.
For example, if I were to place one of my cats on a scale, as in the cartoon on the left, I may estimate that weight of the cat is 17.4 pounds. You might say it is 17.3 pounds, whereas another person still might say the cat weights 17.5 pounds.
The last digit is said to be estimated. The last estimated digit in a measured number is considered significant. We know the cat is a least 17 pounds (there is a marker at 17), but less than 18. This means we know the first two digits (17) with certainty. The last digit (4 - or 3 or 5) is estimated because it is beyond the last calibration marker.
The number of significant figures in a measured number is all the digits in the number that are known with certainty plus one uncertain digit. Any digits beyond the last uncertain digit are not significant.
Below are some rules for determining whether a digit is significant or not:
Selected Rules for Significant Figures
| value | significant figures | rule |
| 16.74 g | 4 sig figs | non zero digits are significant |
| 0.003 m | 1 sig fig | a zero at the beginning of a decimal number is not significant |
| 2500 km | 2 sig figs |
a zero at the end of a large number without a decimal point after is not significant |
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25.0 mL 50. |
3 sig figs 2 sig figs |
a zero at the end of a decimal number is significant |
| 405 | 3 sig figs | zeros between non zero digits are significant |
Scientific Notation
In chemistry, we often need to use very large or very small numbers. For example, the number of particles in a liter of air at room temperature is about 27,000,000,000,000,000,000,000. Scientific notation is a compact way of writing such numbers. The quantity 27,000,000,000,000,000,000,000 becomes 2.7 x 1022 in scientific notation.
To illustrate this, let's look at a simpler example:
Six million, or 6,000,000 can be written in scientific notation as 6 x 106. Here's why:
Numbers smaller that 1 may also be written in scientific notation. In this case, negative exponents will be used.
For example, 0.001 is written as 1 x 10-3. 10-3 = 1/1000 or 1 divided by the quantity (10 x 10 x 10).
Here are some more examples:
We can write a number in scientific notation simply by moving the decimal point over until there is one non-zero digit in front of the decimal - this gives us the first part of the number. Counting how many places we moved the decimal gives us the exponential portion.
| Conventional Notation | Scientific Notation |
| 0.012 | 1.2 x 10-2 |
| 0.000541 | 5.41 x 10-4 |
| 0.000007 | 7 x 10-6 |
| 0.0010 | 1.0 x 10-3 |
The decimal point is moved to the right until there is one nonzero digit before the decimal place.
Trailing zeroes that occur after a decimal point are significant and are therefore retained when the number is put in scientific notation, as in the last example. This brings us to a rule for significant figures in scientific notation.
| value | significant figures | rule |
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4.8 x 10-5 M
3.50 x 10-5 M |
2 sig figs 3 sig figs |
any digits in the coefficient of a number written in scientific notation are significant |
SI Units, Metric Units and U.S. Units
In chemistry, we tend to work primarily with the metric system.
The International System of Units (SI) is a modification of the metric system that is also often used in the sciences, especially physics and engineering. However, in our daily lives, most of us tend to use the U.S. system. Many commercial products list quantities in both systems.
Here are some of the common units used in each of the systems, along with those you are likely to see in chemistry:
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Many of the common units used in chemistry are metric units in which the base unit is modified with a prefix. A prefix in the metric system (or the SI system) increases or decreases the unit by a factor of 10. For example, the prefix centi- decreases the unit by a factor of 100. A centimeter is 1/100th of a meter (100 times smaller). Below are some common prefixes you may come across in this class.
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Unit Conversions
Read: CfCT, Appendix A.3
Please download the following reference sheet containing relationships between commonly used units: Download a help/reference sheet on unit conversions
For example, to convert 11 centimeters into inches, I need to know the equality:
From this, I can make the following conversion factors:
To convert 11 centimeters (cm) into inches (in), I multiple the quantity by the appropriate conversion factor.
Here, I choose the conversion factor that has centimeters on the bottom to cancel out the centimeters in my original number.
Here's another example:
So far we have: 500 mL
1 fl oz = 29.6 mL
Concept Check
Light and the Electromagnetic Spectrum
What is light? There are many different ways to describe light. Here we will discuss just a few of them.
Light is a form of energy that is also called electromagnetic radiation. An obvious source of light is our sun. Mass in the Sun is converted into energy. This energy radiates outward, with some of it eventually traveling to Earth (hence the term "radiation").
Light has a dual nature. Sometimes light behaves like a wave. Light "waves" can travel through empty space (a vacuum) as well as other mediums like air. The way these waves behave is similar (but not identical) to how water waves behave.
Light is also capable of forming wave-like interference patterns, just as waves in water. Consider dropping two pebbles into a still pond. The pattern that results is due to interference - peaks and valleys meeting and canceling each other out, while peaks coming together reinforce one another. A great picture can be found here from physicist Paul Doherty of the Exploratorium.
Although it is often convenient to describe light as a wave, in many cases, light behaves more like a particle. It turns out that radiant (light) energy travels in discrete packets. These packets of energy, or "particles of light", are called photons. In the 1800's the wave-like view of light was dominant. However, in the early 1900's, the particle-like nature of light was developed to explain certain experimental observations that could not be accounted for by the wave model.
For this week, we will focus on the wave-like nature of light.
Light waves can be characterized by wavelength. The wavelength is generally given the Greek symbol lambda, and is the distance between successive peaks in the wave as shown below.
Different colors of light have different wavelengths, and also contain different amounts of energy.
In the example above, the light wave is moving toward the right, indicated by the arrow. The figure above is just a moment in time, or a snapshot, of the wave as it travels. And light travels very fast. The speed of light in a vacuum (empty space) is given the symbol c. c has a value of 3.00 x 108 m/s (meters per second) corresponding to about 186,000 miles per second!
Frequency is often used to characterize light, just as the wavelength is.
Imagine you are standing at one point in the wave's path.
The number of peaks that pass by you in 1 second is referred to as the frequency (Greek symbol nu). The frequency of light is related to its wavelength in an inverse fashion:
A higher frequency of light will have a shorter wavelength. In the same amount of time, more peaks will pass you if the wavelength is shorter.
The frequency can be calculated from the wavelength as in the following example:
Then use the equation relating frequency to wavelength and the speed of light c:
Final units are listed as "/ s" meaning "per second" or "cycles per second."
The final answer is read as "five point 1 times 10 to the 14th per second."
We generally think of light as the visible light that we can see.
However, the light that our eyes are able to see is just a very narrow region of the electromagnetic spectrum. The electromagnetic spectrum consists of the entire range of radiant energy, including very long wavelengths to very short wavelengths. Only a small range of these wavelengths are visible to the human eye. This range is called the visible region or visible spectrum.
The entire electromagnetic spectrum is broken into several different regions by wavelength, including the visible region. This is schematically shown in the diagram below. Note how narrow the visible region is compared to some other regions. In this course, we will be focusing primarily on this visible region, but we will occasionally need to consider other regions as well.
Visible Light and Color
Visible light is the region of the electromagnetic spectrum from about 4.00 x 10-7 m to over 7.00 x 10-7 m. These units are more conveniently expressed in nanometers (nm). 1 nanometer = 1 x 10-9 meters, or one one-billionth of a meter.
Concept Check
White light from the sun contains all the wavelengths in the visible region. The wavelengths can be separated from one another through the use of a prism. The following table lists the regions of the visible spectrum associated with different colors.
Next week, we will explore how light from the sun, which is white, gives us blue skies.
Optional:
A fun Dr. Quantum video from YouTube and FreeScienceLectures details the complex ideas surrounding wave-particle duality. This is more in detail than we need to go here, but the wavelike interference patterns shown in the first minute or so may be very helpful:
Dr. Quantum and the Double Slit Experiment: http://www.youtube.com/watch?v=DfPeprQ7oGc
1 in 2.54 cm ------------ or ------------ 2.54 cm 1 in
1 in 11 cm x ------------ = 4.3 in 2.54 cm
A recipe from a metric cookbook calls for 500 mL of milk. You have a measuring cup that measures fluid ounces. How many ounces do you need?
1 fl oz
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29.6 mL
1 fl oz
500 mL x ---------- = 16.9 fl oz
29.6 mL 
Question: What is the frequency of light associated with a wavelength of 590 nm?
Answer: First, convert the wavelength to meters:
1 x 10-9 m 4.00 x 10-7 m x ------------------ = 400 nm 1 nm 1 x 10-9 m 7.00 x 10-7 m x ------------------ = 700 nm 1 nm
Color Wavelength Range (nm) Red 620 nm - 790 nm Orange 600 nm - 620 nm Yellow 580 nm - 600 nm Green 490 nm - 580 nm Blue 460 nm - 490 nm Violet 390 nm - 460 nm
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