Wednesday, September 26, 2012

Course grades...

A few people have asked "What was my grade on Exam 1?". I know I mentioned some estimated letter grade ranges in class, but you really have to be a little careful about thinking in letters. Think about your grade in numbers and the total points you have accumulated. I do not determine your final course grade by looking at the letters you may have earned on exams, I look at the numbers. Why is this an important distinction? Here are a couple examples. Let's say you are taking a class where your grade is determined by your score on 4 exams, and to make the math a little easier, let's say that each exam is worth 100 points and the class is using a typical 90/80/70 percent scale for A/B/C and there are no "+/-" grades.

Example 1: Your exam scores are 90, 91, 82, 92. If we assign letters to those individual exams, you got A, A, B, A. Three "A" grades sounds pretty good! What grade would you get for this course? You've earned 355 points out of 400 possible points, that's just under 89%. Look's like a B.

Example 2: Your exam scores are 78, 87, 79, 78. If we assign letters to those individual exams, you got C, B, C, C. Hmm, looks like a "C" is in your future. But wait! You've earned 322 points out of a possible 400 points, that's 80.5%. The result is a "B" and everyone cheers with delight!

This is why I'm always a little hesitant to assign grades to individual exams or assignments, it can lead to incorrect expectations.

Tuesday, September 18, 2012

Key and corrections posted...

I posted the answer key for those sample questions on my website ( I also posted an updated version of the sample questions, I spotted a couple typos that have been corrected.

I have one other clarification from class today... Any topic that has been mentioned in class can appear on the exam. There are details within a topic that might have been part of your reading assignments that were not explicitly mentioned in class, but those may still appear on the exam. You are responsible for the reading assignments. The reason this came up in class is that someone specifically asked about ice cream... we didn't get into ice cream in class, but it was part of your reading assignment... that's a topic that we didn't mention in class, so I'm not including ice cream on the exam. Hmm, that may have been more confusing than the original confusion... OK, to be (hopefully) clear, ice cream specifically will not be on this exam. All the other parts of your reading and the things we did in class might be on the exam.

Friday, September 14, 2012

Thursday, September 13, 2012

Intermolecular Forces and Dissolving

Question from email------------------------------
I have a question in relation with our today's lecture, yogurt comes out from milk then why yogurt can not dissolve in water like milk, that completely mixes with water.
The short answer is that yogurt contains networks of proteins that are solid enough to make yogurt thick, but not so solid that yogurt is just a big solid lump. To explain better, we need to think about why things dissolve. Let's start with a lump of sucrose (table sugar) and a glass of water. The sucrose is solid because the intermolecular forces between the sugar molecules are very strong. The water is liquid because the intermolecular forces are also quite strong, but the individual water molecules can slide past each other. When the lump of solid sugar is dropped into the glass of liquid water, the sugar dissolves. Energetically, we can think of this as a series of interactions being broken and formed, with the result being whatever state bring us to the lowest energy. If the sugar is going to dissolve in the water, we need to break sugar-sugar interaction (requires energy) and we need to break water-water interactions (requires energy). At the same time, we need to form water-sugar interactions, which liberates energy. If the energy we get back from forming water-sugar interactions is greater than the energy required to break the sugar-sugar and water-water interactions, then the sugar will dissolve in the water. This is the part of chemistry called thermodynamics, which looks at how changes in energy affect chemical reactions. Looking at the following figure:

Going from “A” to “B” represents breaking sugar-sugar and water-water interactions, both of which require energy to be added to the system, noted by the skinny red arrow. When water-sugar interactions are formed, energy is taken out of the system, represented by the skinny green arrows. If the amount of energy we get back from forming water-sugar interactions is relatively small (going from “B” to “C” in the figure), then the net change in energy for the whole process is positive, and the sugar will not dissolve. This overall change is represented by the fat red arrow. If, on the other hand, the amount of energy we get back from forming water-sugar interactions is relatively large (going from “B” to “D” in the figure), then the net change in energy for the whole process is negative (fat green arrow), and the sugar probably will dissolve.
Now back to the yogurt question. The intermolecular interactions we have to think about in yogurt are protein-protein, water-water, and protein-water. {Proteins make this a little trickier because proteins have portions that are more hydrophilic and portions that are more hydrophobic.} The protein-protein interactions in yogurt are pretty strong, so the proteins stick together to form nets, BUT there are also parts of the protein molecules that have fairly strong protein-water interactions. The proteins do not form a hard, compact, crystalline solid like a sucrose crystal because they can form a lot of protein-water interactions, but the parts of the protein molecules where the protein-protein interactions are strong prevent the whole molecule from dissolving in water.
The even deeper part of this question actually shows up in the words that were used. yogurt does not “dissolve” in water, but milk “completely mixes” with water. Remember, milk is an emulsion, so although it does mix with water, it's not really “dissolving”. In that sense, milk and yogurt are similar, the difference being that the parts of milk that do not dissolve are tiny little droplets and clusters that can freely float around in the aqueous part of the milk, while the part of yogurt that doesn't dissolve is a large, extended network of proteins that makes yogurt thick and clumpy. And delicious.

Thursday, September 6, 2012


Carbohydrates. Some people think they're the enemy, but they're really just an innocent little (or not so little) food molecule. Carbohydrates are a class of food molecules that consist of carbon (“carbo”) and hydrogen & oxygen (“hydrate”).
These are simple sugar molecules with a single ring. There are quite a few possibilities, but the 3 main monosaccharides in food are glucose, galactose and fructose.
If two monosaccharides react to liberate a water molecule (a dehydration or condensation reaction), they form a disaccharide. There are many possible combinations of monosaccharides, but again, when we're looking at food and cooking, there are 3 main disaccharides: sucrose (table sugar, made from glucose-fructose), maltose (grain sugar, made from glucose-glucose) and lactose (milk sugar, made from glucose-galactose). To get the energy out of a disaccharide, it's usually necessary to break the two halves apart again by adding a water molecule (a hydrolysis reaction, the reverse of a dehydration reaction). This can be accomplished a couple different ways, one of which is by enzymes. The enzymes that break up disaccharides are named to reflect the disaccharide they hydrolyze: sucrase hydrolyzes sucrose, maltase hydrolyzes maltose, and guess what lactase hydrolyzes?
If many glucose molecules react to form a glucose polymer, one possible polymer is starch. There are 2 kinds of starch; amlyose is a single chain of glucose molecules that usually forms a helical structure, and amylopectin is a branched chain of glucose molecules. Both are present in plants, the relative amounts of amylose and amylopectin vary, although there's almost always more amylopectin than amylose. Amylose can by hydrolyzed by an enzyme called... amylase. Is there a pattern? I think so...
Plants make glucose polymers to store energy rather efficiently and compactly, so it would make sense that animals would also use a glucose polymer to store energy. The animal glucose polymer is called glycogen and is even more branched than amylopectin.
With a very small change in structure, alpha-glucose becomes beta-glucose. There's a very nice side-by-side animation of these two molecules at {}. Polymers of beta-glucose are called cellulose, and this tiny structural change means that it is MUCH more difficult to hydrolyze cellulose that polymers made of alpha-glucose. Cellulose is what is typically called “dietary fiber” and passed through the digestive tract relatively intact.
There are a LOT of fascinating details in the structure, function and reactivity of carbohydrates, this is just a little taste.

Saturday, September 1, 2012

Atomic Structure - Nuclear Chemistry

From email:
I was reading a book of "on Food and Cooking" and i came up with the question that, why proton in an atom does not repeal each other or why electrons of same atom does not attract it's own proton? i have read it's answer also but i am still unclear. could you please explain this
Atoms are pretty amazing things for the exact reasons you point out. Protons are positively charged, so if like charges repel each other, the protons in an atom should be trying to get as far apart as possible. But all of the positively charged protons in an atom are crammed into the tiny space of the nucleus.This was pretty confusing to the scientists who originally discovered the structure of the atom, but their data was conclusive, so additional research was required to explain their observations.
There are four basic forces in the universe: gravitational forces, electromagnetic forces, the weak nuclear force, and the strong nuclear force. The strong nuclear force is extremely strong, but it only acts over a very small distance, probably about the size of a proton or neutron. When 2 protons are brought very close together, the strong nuclear force is able to act and stick the protons together to form a nucleus.
The electrostatic/electromagnetic forces that cause protons to repel each other ("like charges repel") can act over a much longer distance than the strong nuclear force. This is why atoms don't just melt into each other under normal conditions; when two nuclei approach one another, the repulsive force between these positive charges pushes the nuclei apart. If the two nuclei are smashed together hard enough, the protons can get close enough to allow the strong nuclear force to take over and the nuclei fuse together. This is what happens in the sun. The extremely high temperatures make nuclei move very fast and the high pressure leads to a lot of collisions, so nuclei smash together and undergo nuclear fusion. The reverse of this process, nuclear fission, is what provides the energy in nuclear power plants.
What about the electrons? Electrons have very little mass and are moving very fast. The negatively charged electrons are attracted to the positive charged protons in the nucleus, but are moving fast enough to prevent them from crashing into the nucleus. Because electrons are so small and moving so fast, their motion is a little more complicated (due to quantum effects, but that's a little beyond this course...), but in a very basic way it can almost be thought of as the way planets move around the sun. The planets are attracted to the sun by gravity, but the motion of the planets keeps them from crashing into the sun.
In almost all chemistry and physics, properties and behavior are determined by the balancing of force. The fascinating part of science is figuring out how those different forces interact with each other.