Saturday, May 12, 2012

Final Exam Review Time!

Question: What makes a bond polar? What makes a molecule polar? How does this compare to ionic bonds? Please give examples of each.

             Polarity can be very confusing if not fully understood. Polarity can occur in three ways, Polar Covalent Bonds, Dipoles, and the Bond Continuum. What makes bonds polar is a difference in electronegativity of atoms. The electronegativity of an atom is the ability of that atom to attract electrons in a bond. Given this definition the more electronegative the atom, the more it will keep its electrons to itself and will not "share" them with other atoms and vice versa. If one atom is more electronegative than the other, the more electronegative atom will hog the electrons of the other atom for itself, creating a slightly more negative side of the bond. An example of this Polar Covalent Bond is the acid bond of HCl. The electronegativity of Hydrogen is about 2.1 while that of Chlorine is about 3. The chlorine atom naturally pulls the Hydrogen's electrons towards itself, which creates a greater concentration of electrons around the chlorine atom which means there is a slight negative charge on the chlorine and a slight positive charge on the hydrogen. The Bond Continuum occurs when a covalent bond becomes too polar. When this happens, the more electronegative atom will actually steal the electron from the other atom, thus essentially turning into an ionic bond. Polarity in covalent compounds also creates dipoles. Dipoles are two electronegative poles, like in the world, of a bond, one positive and one negative. When polar bonds combine in molecules with multiple bonds and at least two sigma bonds, an overall polarity can occur. An example of this is water, H2O, in which the Oxygen atom, being more electronegative, draws the Hydrogens' two electrons towards it, creating a positive side where the hydrogens are and a negative where the oxygen is due the the increased/decreased electrons there. 

Wednesday, November 9, 2011

Exam Review Question!!

Question: What is the difference between a solution and a heterogeneous mixture? A pure substance and a mixture? A solution and compound? Give examples of each. Overall give 5 examples of heterogeneous mixtures and 5 examples of homogeneous mixtures?

             There are many ways to distinguish between mixtures of elements, including homogeneous, heterogeneous, solutions etc. A heterogeneous mixture is a mixture whose composition is not uniform throughout. A solution, also known as a homogeneous mixture, is a mixture where the composition is uniform; either all solid, all liquid or all gaseous. An example of a heterogeneous mixture is Campbell's chicken noodle soup and an example of a solution is gasoline. The difference between a pure substance and a mixture is that a pure substance has a fixed composition and a mixture's composition varies depending on the product. An example of a pure substance would be something like a bar of soap and a mixture would be milk. Milk can have different amounts of calcium and fat while soap usually will have the same amount of all its ingredients. A solution is different than a compound in that a solution can only be homogeneous while a compound can only be heterogeneous because they are substances. A solution would be water while a compound could be something like gushers, a blueberry bagel, or body wash with moisture beads. A compound could be anything from salt to cotton.


Sunday, September 18, 2011

Grocery Store Chemistry!!

For our first Chemistry project, we were given the task to find an object at home and to come up with at least five chemical and 5 physical properties. I chose an EZ Freeze lunchbox icepack. I had recently broken the plastic covering of one so I decided to use the stuff inside for my experiments!

The Physical Properties:
As you can tell from the picture, when frozen, the substance acts like ice and is cold and solid. From what i could tell, the freezing point is -5 degrees celsius. It also is a pink color and it's melting point is 33 degrees celsius. I found the melting point by boiling some water in a metal pan over the stove and putting the gel in a cup and placing the glass measuring cup into the hot water. When the substance changed from a solid, it looked and felt like a gel, and I took the temperature. When the pink stuff is at room temperature, it is a gelatinous substance and has a high/low viscosity. The gel reacts much like water to heat; when it gets colder it freezes into an ice-like substance and when it gets warmer, it changes phases into the liquid phase, and is more like a gel than a liquid. I know all of these are physical properties because the definition of physical properties is stated as the quantity or condition that can be observed or measured without changing the substances composition. None of the above descriptions changed the composition of the substance, only the phase. The picture below shows me heating up the frozen substance to find the melting point.

The Chemical Properties:

In order to find the chemical properties of the gel, I had to perform a series of experiments. First, I had been using gloves to handle the gel because I wasn't sure if it was corrosive and would burn my hands or not. So I decided to see if the gel was corrosive by putting it on a piece of cloth. I cut out my cloth and poured some gel on it. As you can see below, the gel does not corrode the cloth, but it does stain it slightly.

Next, I wanted to see if it would react with acid. Since I had to do all of my experimentation at home, our teacher told us to use vinegar instead of actual acid. The only vinegar in my pantry was Apple Cider Vinegar. So I put some of the gel into a glass cup and added 3/4 teaspoon of apple cider vinegar. Nothing exciting happened, it only mixed together and over a few minutes the vinegar and the gel turned into a very light pink liquid that smelled like apple cider vinegar. The picture below shows the mixture of gel and Apple Cider Vinegar.

Many chemical properties have to do with burning or their ability to catch fire in air or so. So I decided to see what would happen if I used a lighter to try and burn the gel. I soon realized that since I think it is water based, it would not catch fire. I tried to burn it by putting some of the gel on a notecard and burning the card. I realized that after the card burned up, the gel had a thicker consistency and was still pink. The picture below is me trying to catch the gel on fire using the notecard.

Next, I was trying to think of substances that react often and are used often in cooking. I thought about bread and how baking powder is what makes the bread rise, why not see if it does anything to my gel? I got a disposable cup and poured some gel into the cup. Then I added one heaping tablespoon of baking powder to my gel. At first it just kind-of sat on top of the gel so I decided to mix them. It started bubbling a little then all of a sudden it started foaming. It created a frothy substance and all gelatinous properties were gone.

For my last experiment, I chose to add some salt to my gel. Salt is an interesting compound and usually reacts well with things. So I took another disposable cup and put some gel into it. Then I got my bag of sea salt and poured some in. I began mixing immediately. The salt absorbed all of the gelatinous substance and left a dark pink liquid. The salt turned a pink-ish color and the liquid was easily decanted fro the salt. The pictures below are of the liquid and the salt separated.

I know all of these are chemical properties because the definition of a chemical property is the ability of a substance to undergo a specific chemical reaction. I got all of the above chemical properties by putting the gel through several chemical reactions involving changing the composition. Some red flags of chemical properties/reactions are if it gives off heat,light or sound or requires heat to react, changes in color, production of gas, or forming precipitates.

 In conclusion, the physical properties of the gel are:
1) it forms a solid when frozen
2) it is pink
3) it's melting point is 33 degrees celsius
4) when at room temperature, it is a gelatinous substance
5) it reacts to heat similarly to water

And the chemical properties are:
1) it is not corrosive, only staining
2) when mixed with vinegar, it turns into a very light pink liquid
3) when exposed to open flames, its consistency gets thicker
4) when mixed with baking powder, it foams and creates bubbles
5) when mixed with salt, the gel is absorbed by the salt and leaves a dark pink liquid