5.1.3 What is a Color?

In the study called "Light: Waves and Photons," you explored the relationship between light and color. Within the visible spectrum, the shortest wavelengths of light (roughly 400 nm) are blue, and the longest wavelengths (around 700 nm) are red. These numbers represent the wavelengths of light not absorbed by compounds having these colors. For instance, a solution of copper sulfate appears blue because when it is exposed to electromagnetic radiation it transmits light with wavelengths near 400 nm and absorbs all other wavelengths. It's really the reflection of blue light, and the absence of colors at wavelengths that are far from 400 nm that our minds interpret as blue.

Now let's look at the color green. According to the visible spectrum presented in the study, green light has a wavelength of about 500 nm But does this make sense? Perhaps when you were younger and just learning to paint and draw, someone told you that yellow and blue make green. You figured out how to mix yellow and blue paints to get green paint. So is green a single wavelength or a combination of two different wavelengths?

Consider three green objects: a green sweater, the leaves on a tree, and the coating on green M & M's™. Imagine that these are all the exact same shade of green. Why are they green? Are they "pure" green or a mixture of yellow and blue? Is there any way you could tell the difference? Follow the directions below to go online and share your ideas with your classmates!

Scoring

This discussion is worth a maximum of 15 points. You'll get 10 points for participating. Your instructor will give you another 5 points if you post a follow-up comment or question that furthers the discussion.

4.2.7 Heat and Temperature: What Is the Difference?

Discussion Topic

Heat and temperature are often used interchangeably in everyday life. It won't come as a surprise to anyone that when you increase the heat of an object, the temperature will go up. But do heat and temperature really mean the same thing? Should the terms be used interchangeably?

We have introduced the concept of heat energy. Heat is the energy that flows into or out of a system because of a difference in temperature between the system and the surroundings. Heat always flows from a hot object to a cooler object. Heat transfer occurs until all objects are the same temperature. If a hot object is put in surroundings that are the same temperature, no heat energy flows, because there is no temperature gradient.

The temperature of an object is due to the internal kinetic energy of the particles in it. When heat flows from a hot object to a cooler object, the particles within the hot object slow down. This means the object's kinetic energy decreases. So temperature reflects the amount of heat energy in the system.

One striking difference between heat energy and temperature is their dependence on the amount of substance in the system. The amount of heat energy required to change the temperature of a substance depends on the amount of substance in the system. Imagine you have a 100 gram sample of pure water at 50C which has a heat energy of q. If you divide the water into two equal portions, what will the temperature and heat energy of each of the portions be? Consider the definitions of heat and temperature as you formulate your answer, then go online to share your ideas with your classmates.

Scoring

This discussion is worth a maximum of 15 points. You'll get 10 points for participating. Your instructor will give you another 5 points if you post a follow-up comment or question that furthers the discussion.

3.2.10 Discussion

Great job on the last discussion! Very interesting comparisons for the size of a mole. Here is the next discussion question, with one difference: you only need to pick only 1 thing to discuss for your first post (I broke them into 9 separate questions)--try to choose one that has not already been picked and list which number you choose (#1-9)--10pts. Then comment on somebody else's post (is it easy to understand? did they make it sound more confusing? is there anything else to add to it? etc.) 5pts.

In this unit, you have heard about many different types of stoichiometry problems. This discussion gives you an opportunity to share your insights about solving stoichiometry problems with your classmates. Generally, if you are having trouble with a particular topic, someone else in the class is having the same problems. More importantly, it is likely that someone in the class can explain the problem in a way that you can understand. Use this opportunity to get help on the subjects you find difficult and give advice to others about problems they are having.

1. Talk about how to convert from grams to moles, and
2. how to determine the percent composition by mass of elements in compounds using both laboratory data and molecular formulas.
   
3. Tell your classmates how you go about determining the empirical and molecular formula of a compound given combustion data and the molar mass of the compound.
   
4. Discuss the simplest way to relate the amount of reactants and products in a chemical equation by looking at mole ratios.
   
5. Solution stoichiometry is often confusing at first, so take a minute to talk about how to make a solution of a given molarity and the how to make dilutions from it.
6. Discuss how to determine the limiting reagent in a reaction and how to find the theoretical and percent yield.
   
7. Gravimetric and volumetric analysis can both be used to determine the amount of a substance dissolved in a solution. Talk about why you might choose one method over the other.
   
8. Discuss how a titration works and what types of things need to be considered before doing a titration.
   
9. Finally, remember to discuss how to calculate how many particles there are in a mole.