The only way out is to beat this test.

My MCAT Schedule

Hi All,

I decide to tackle the new MCAT and try my hardest on this one. This came up because when I was working last Thursday, I realize that I have been working here for 6 months already. Time seems to fly very fast when you don't notice it. I brought this topic up when I was talking to one of my patients and he told me that you need to do what you want to know now because when you look back in 10 years, you will ask yourself why you didn't go for whatever you want to do when you had time. He was right; I am still young and I should focus on my career as early as possible. I decide to create my own schedule for my MCAT study and decide to make a post about my study everyday.

Here is my schedule:

Making a post everyday forces and allows me to focus and complete my task on time.

Understand Why Charged Molecules are Soluble in Water

Have you ever wondered why charges ions and molecules dissolved in water?

What you have learned at school is polar dissolves polar and nonpolar dissolves nonpolar, but why?

Water molecules are partially positive  on the hydrogen and negative on the oxygen. These two atoms play an important part of dissolving NaCl.

In liquid, NaCl goes through dissociation, a reversible process that breaks the compound (NaCl) into ions (Na+ and Cl-). At this very moment, that is when partial negative charged Oxygen "captures" Na+ and partial positive charged Hydrogen captures Cl-



These water molecules essentially devour or dissolve these ions, not allowing them to form the compound, NaCl again. Therefore, NaCl is dissolved in water.

Finally Understand Signs of U, Heat, and Work

A few weeks ago, I published a blog regarding the sign of work, which can be seen in here: http://understandhowthingworks.blogspot.com/2014/06/understand-sign-of-work.html

Today, I would like to wrap up all of the confusions regarding the sign.

Some of the notes are taken from LINK. Much thanks to their helpful illustrations and notes.

Let's begin with our universal first law of thermodynamic. U=Q+W

Sign of Q: 

When Q is positive, Heat is taken into the system, increasing the temperature, increasing the internal energy (U) which becomes positive.

When Q is negative, Heat is taken away from the system, decreasing the temperature, decreasing the internal energy U which becomes negative.

Sign of W:

When W is positive, the surrounding does work on the system, increasing the internal energy U which becomes positive. This means that temperature of the system will also increase because internal energy U is directly related to temperature.

This is the so-called Work is being done on the system (by the surrounding--to make it clear). This process is called Compression. As a result, the equation becomes U= E+W (Note: you can think of it as you need positive work to increase the internal energy U, that's why W HAS to be positive).

When W is negative, the system is doing work to the surrounding, decreasing the internal energy U which becomes negative. As a result, temperature of the system also decreases.

This is the so-called Work is being done by the system. This process is called expansion. As a result, the equation becomes U= E-W (Note: you can also think of this as you need work to be negative to decrease the internal energy U).

These observations explain why gas cools when expands and gas heats up when being compressed.

I hope this blog post has cleared out any confusion that any student might have had in their school.

Understand the Effect of Impurity on Boiling point and More Information on Melting point

As previous post has mentioned, adding impurity to solid would decrease the melting point due disrupted arrangement of the particles. For example, when you add impurity (say, MgF2) to NaCl, you are disrupting the ionic bonds between Na+ and Cl- ions. Putting F- next to Cl- will cause repulsion and same result will happen to Mg2+ and Na+. Because NaCl is a solid, they cannot rearrange themselves to avoid the repulsion. Due to this, the ionic bonds of NaCl will get weaken and thus, it requires less energy to melt them.

how about boiling point?

Impurity actually increases the boiling point of the liquid.

Let's use the same example from above. Imagine NaCl is in a solution now; adding impurity to it would actually stabilize the bonds! This is because in liquid, ions are free to rearrange themselves in order to achieve a maximum stability. Now, Mg2+ and Na+ are no longer repelled against each other. Rather, Mg2+ will rearrange itself to find his partners: Cl- and F-. Because of the additional stability from impurity, it requires more heat and energy to break the bonds! Boiling point increases as a result.

Understand Freezing point/Melting point Part II : Freezing point/Melting Point Depression and Effect of Impurity

Melting point or freezing point depression  is basically the reduction of melting point or freezing point due to impurity.

The presence of impurity has two effects on melting and freezing point

1. lowers melting/freezing point ( If it is in range, it lowers end of range. For example, if the melting point is 188-189C, the mixture with impurity will be lower than 188C.

2. Melting point/freezing point will be in a range now instead of a sharp peak. For example, if the melting point were 188C, now it could be in a range of temperatures.

Why does this happen?

Adding impurity to the solution basically messes up the lattice (the structure of the molecule). This causes the structure of the molecule to break easily, meaning you need LESS temperature to melt it now. If it requires 180C to melt the solid, now it requires 100C. Pretty cool isnt it?

The opposite is true for freezing point

To form a structure of molecule from liquid, the correct lattice or structure is needed. Impurity basically impedes the formation of the correct structure due to difference in size, shape or charge. To allow the formation of solid to happen, you need lower entropy = low temperature. Therefore, when you add salt to water, it will remain as liquid at temperature at or below 0C, which is the freezing point of water.

Quoted from my previous post at this link

"Many people use salt to lower the freezing point of the ice. What does it mean? Imagine the temperature for today is 0 C. This is also the temperature when ice forms. Adding salt to the ice will lower it to -6 C. This means liquid will be frozen into solid at now -6 C (used to be 0 C). Since today's temperature is 0 C, the ice on the road after addition of salt is going to melt because we haven't reached -6 C (the new freezing point) yet. Until then, everything will be stayed at liquid."

This still remain true.

Understand the sign of work in electricity

Imagine the plates of a capacitor has a potential difference of 10V. How much work must be done in order to move the electron from the negative plate to positive plate?

Given that electric field goes from positive plate to negative plate. If you want to move the electron from negative to positive plate, you need to invest some energy to make that happen (positive work) because you are going up against the electric field.

The equation to solve that is W= qV. I was very confused about the sign of work due to the sign of V and q. However, I realize that no matter what you do, your work must be POSITIVE! Therefore, you can do whatever you want to q and V to make work positive :)