A few random questions

[byeh2004]

G-Chem
1) When does a substance cause a volatile liquid to decrease in boiling point?
2) When does a substance cause a volatile liquid to increase in boiling point?
3) Do we consider the delta H of oxygen when trying to calculate the heat of rxn for a combustion reaction? (This might be a silly question, but for some reason one of the answers on a Kaplan test I took didn't have to O2 into the equation)
4) How do you relate the doppler effect to an earthquake? How is the frequency of the shake different from someone standing closer to the epicenter vs someone far from it?

Physics
1) What factors will cause something to develop a magnetic field?
2) When does the force and electric field go in the same direction?
3) What is the difference between conduction and convection?

Biology
1) What's the biggest difference between B and T-Lymphocytes?
2) Is blood flow faster in capillaries or arteries? Is blood flow independent of blood pressure?

Thanks!

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[niranjan162]

1. A substance causes a decrease in boiling point when it is not volatile or has a lower vapor pressure then wut its being added to because the substances molecules block potential space for the original liquid.

2. same concept.

3. im not good with delta H stuff.

4. you can just imagine the shockwave vibrating the earth as similar to sound waves. It really depends on the other forces, but i would guess that the waves further from the epi center would be further apart since they are being slowed by travelling through the medium, friction, etc.

1. electricity moving through a wire creates a magnetic field.

2. it depends on whats being pushed. so you have an e-field, pointing up. if you have a negative charge its gonna feel an attraction towards the center of the e-field, whereas a positive charge will feel a force in the opposite direction.

3. conduction is passing of heat through touching. Convenction is passing of heat energy through a medium such as air.

1. google?

2. this is kinda of a weird question. Capillaries are very small, so much so that single cells squeeze through them, whereas arteries will have much more cells and plasma and such. So I would say arteries. Blood pressure is not independent of blood flow. The heart pushes blood through arteries. blood pressure is basically of measure of how much force is being exerted by the heart.

 

[BRRF]

About the blood flow...don't forget fluid dynamics, specifically flow.

Picture all the capillaries in the body combined into one vessel. The capillaries will have a far greater cross sectional area than the arteries. Therefore velocity of blood will be slower in them. That being said, total 'flow' in the true sense of the word has to remain constant throughout the body. ie. the amount of blood leaving the heart must equal the amount entering.

As an extension we can also conclude that blood pressure will be higher in the capillaries based on Bernoulli's equation.

 

[ladyoflies]

Physics
1) What factors will cause something to develop a magnetic field?
moving charge...whether it's in a wire or just on it's own.
2) When does the force and electric field go in the same direction?
I think the previous reply covered this one. It depends on the charge of the particle. Electrons will feel force in the opposite direction of the E-field, positive charges will feel force in the same direction.
3) What is the difference between conduction and convection?
I always thought of conduction associated to solids and convection to only liquids and gases (and ovens). The official definition is something like, conduction is transfer of energy through collisions among molecules and convection is transfer of energy through flow of heated material.

 

[byeh2004]

About the blood flow...don't forget fluid dynamics, specifically flow.

Picture all the capillaries in the body combined into one vessel. The capillaries will have a far greater cross sectional area than the arteries. Therefore velocity of blood will be slower in them. That being said, total 'flow' in the true sense of the word has to remain constant throughout the body. ie. the amount of blood leaving the heart must equal the amount entering.

As an extension we can also conclude that blood pressure will be higher in the capillaries based on Bernoulli's equation.

I always thought that the aorta had the highest blood pressure out anything vascular cavity. There's a chart in my EK book that lists it to be so.

 

[baller99]

G-Chem

3) Do we consider the delta H of oxygen when trying to calculate the heat of rxn for a combustion reaction? (This might be a silly question, but for some reason one of the answers on a Kaplan test I took didn't have to O2 into the equation)

elements in their standard state have an enthalpy of 0, since thats just how they are

Biology
1) What's the biggest difference between B and T-Lymphocytes?

i would say that b (or at least b derivatives) can produce antibodies and t can not. but just know the functions for all types. it seems to be kinda complicated.

 

[byeh2004]

I found something on a webpage about the doppler effect of earthquakes.

"The simple answer is no. The Doppler effect is produced when the source of
a set of waves is moving relative to the observer of those waves. Which
part of the system is in motion doesn't matter. If you move past a
stationary siren or a siren moves past you, you will hear the same thing.
The change in pitch you hear results from the relative movement - the sound
coming out of the source is always at a constant pitch. The pitch change
happens when you pass the source because you suddenly switch from
moving toward the source to moving away from the source.
In an earthquake, the source is stationary and generally the observers are,
too, therefore there would be no Doppler effect. If you were moving toward
the epicenter, the seismic waves would have a higher frequency (or pitch -
but pitch is misleading since seismic waves are too low a frequency to be
heard)and if you were moving away from the epicenter, the waves would have
a lower frequency. To my knowledge, no one has tried to observe this,
although it could be of some importance in looking at the response of
moving objects (like trains or cars) to earthquake waves.
Another issue is that unlike horns and sirens which put out noise over a
long time, allowing you to pass the source and hear the change, seismic
waves are only produced for a very short time (a second or two). The waves
spread out as they move away from the epicenter, giving rise to the wavy
patterns seen on seismogaphs. At the epicenter, the siesmograph of most
earthquakes would be a single spike in the record and all you feel is one
big jolt, not the long shaking people feel farther from the center. The
frequency shift produced by the Doppler effect would not change the shape
of this spike and so no real effect would be noticed, even if you just
happened to be speeding by the epicenter just as an earthquake happened to
happen."

 

[silverlion]

1. A substance causes a decrease in boiling point when it is not volatile or has a lower vapor pressure then wut its being added to because the substances molecules block potential space for the original liquid.

WRONG!!! In fact it is the opposite. If a solution experiences a decrease in boiling point then it means that the vapor pressure of the solution equals the atmospheric pressure at a temperature lower than the boiling point of the pure solvent. This can happen only if the solute (the liquid in the solution which has a lesser proportion than the other) is more volatile...

 

[byeh2004]

I found something on a webpage about the doppler effect of earthquakes.

"The simple answer is no. The Doppler effect is produced when the source of
a set of waves is moving relative to the observer of those waves. Which
part of the system is in motion doesn't matter. If you move past a
stationary siren or a siren moves past you, you will hear the same thing.
The change in pitch you hear results from the relative movement - the sound
coming out of the source is always at a constant pitch. The pitch change
happens when you pass the source because you suddenly switch from
moving toward the source to moving away from the source.
In an earthquake, the source is stationary and generally the observers are,
too, therefore there would be no Doppler effect. If you were moving toward
the epicenter, the seismic waves would have a higher frequency (or pitch -
but pitch is misleading since seismic waves are too low a frequency to be
heard)and if you were moving away from the epicenter, the waves would have
a lower frequency. To my knowledge, no one has tried to observe this,
although it could be of some importance in looking at the response of
moving objects (like trains or cars) to earthquake waves.
Another issue is that unlike horns and sirens which put out noise over a
long time, allowing you to pass the source and hear the change, seismic
waves are only produced for a very short time (a second or two). The waves
spread out as they move away from the epicenter, giving rise to the wavy
patterns seen on seismogaphs. At the epicenter, the siesmograph of most
earthquakes would be a single spike in the record and all you feel is one
big jolt, not the long shaking people feel farther from the center. The
frequency shift produced by the Doppler effect would not change the shape
of this spike and so no real effect would be noticed, even if you just
happened to be speeding by the epicenter just as an earthquake happened to
happen."

Nevermind about this. This question pertains to a question I saw while taking AAMC 7

I found an explanation on the Study Q&A section

The Doppler shift can impact any wave. In this case, the wave is an impact wave travelling through the ground away from the rupture. The source of the wave is the rupture, and given that the rupture is moving, the wave it emits will be Doppler shifted. You can thus treat it as a source moving north emitting a wave, much like a car travelling north emitting a sound.

 

[BRRF]

I always thought that the aorta had the highest blood pressure out anything vascular cavity. There's a chart in my EK book that lists it to be so.

I think there is a difference between the pressure of blood that causes a push forward and the pressure exerted on the wall.
Not sure...but I guess use the facts that aorta has highest blood pressure AND highest blood velocity.

 

[byeh2004]

In basics: B-Lymph. produce antibodies, and T cells primarily destroy antigens that are coated by antibodies.

Both contain memory cells that can recognize past antigens.

T Lymph. has something called a Suppressor T cell which basically shuts off the immune response after the threat has been eliminated. A defect in the gene that produces Suppressors causes autoimmune disease (where your immune system attacks your own cells).

 

[silverlion]

The blood pressure is the highest in the aorta...and least in the vena cava...The capillaries have very low BP....

A simple way of remembering it is that as the blood moves away from the source of pressure (that is the heart) it keeps on losing pressure....owing to the friction offered by the walls....more the surface area of the walls the greater the loss of pressure...in the case of the arterioles, capillaries and venules...the combined cross sectional area is way larger than that of the arteries of the aorta...so the pressure is really low.......The pressure os even lower in the veins because there is nothing that is increasing the pressure anymore...the blood is simply reaching the heart via the Vena cava through the squeezing action of the muscles due to movement...The least pressure is in the Vena cava

 

[byeh2004]

I think there is a difference between the pressure of blood that causes a push forward and the pressure exerted on the wall.
Not sure...but I guess use the facts that aorta has highest blood pressure AND highest blood velocity.

When we're relating it to physics what do we use in fluid dynamics?

I know Q=Av, but I'm not sure if thats the right pressure. Because capillaries are way smaller in diameter, shouldn't the velocity be faster (considering if we are dealing with the same flow rate)?

How do we relate the Bernoulli equation to the vascular system?
Atm P is the same so we only have (1/2)*rho*v^2 + rho*g*h = (1/2)*rho*v^2 + rho*g*h

Does P=F/A come into any of this? How about rho*g*h? Is hydrostatic pressure related to blood pressure or blood flow?

 

[byeh2004]

WRONG!!! In fact it is the opposite. If a solution experiences a decrease in boiling point then it means that the vapor pressure of the solution equals the atmospheric pressure at a temperature lower than the boiling point of the pure solvent. This can happen only if the solute (the liquid in the solution which has a lesser proportion than the other) is more volatile...

The higher the vapor pressure, the lower the boiling point is going to be. Because if its closer to the atm pressure, then its going to take less temperature for the vapor pressure to rise to the atm pressure (temp increase vapor pressure).

The vapor pressure equation for a nonvolatile solution is

Pv= Xa*Pa (where Xa is the mole fraction of the liquid and Pa is the partial pressure of the pure liquid)

We know that if we add salt to a pot of water, the water is going to boil faster (or at a lower temperature). How do we relate it to the equation? Is Xa or Pa increasing in the equation in this situation?

 

[silverlion]

We know that if we add salt to a pot of water, the water is going to boil faster (or at a lower temperature). How do we relate it to the equation? Is Xa or Pa increasing in the equation in this situation?

This is NOT TRUE! When you add salt to water it does not boil faster. The boiling point DOES NOT DECREASE HERE....it actually increases...This is because the vapour pressure of the solvent (Water) multiplied by the mole fraction of water (which has decreased on addiction of salt) will give a lower value of Pv (Vapor pressure of solution)....

If Pv is lower than the pure solvent....You would have to increase the temperature to bring it up to be equal to the atmospheric pressure as compared to the temperature the pure solvent will boil at...

 

[byeh2004]

Oh man, I'm so confused now.

Does salt really lower boiling point or raise it? I know it makes water boil faster but what the heck does that mean !

If it raises boiling point, it makes more sense because the higher the boiling point the lower the vapor pressure.

Pv= Xa*Pa

Our Xa will be smaller due to the added solute (higher denominator) hence our Pv will decrease causing a higher boiling point.

Therefore nonvolatile solutes usually increase the boiling point.

So I guess lets go back to the original question, what causes boiling point to lower?

Well we know that the higher the vapor pressure, the lower the boiling point.
- A nonvolatile solute will never cause the boiling point to lower because it causes the vapor pressure to lower, which then causes the boiling point to be higher.
- Raoult's equation for volatile solutes is
Pv= Xa*Pa + Xb*Pb (where a and b denote each volatile solute)
for example: Let's mix water and ethanol together.
- Hypothetically, let's say we mixed 5 moles of water and 5 moles of ethanol. Partial pressure of water can be 400 torr and ethanol can be 200 torr.
- Let's see what happens if we were just comparing the individual solutes.
- Water Pv = (5/5)(400) = 400 wow, by itself is the vapor pressure really its partial pressure?
- Ethanol Pv = (5/5)(200) = 200
- Combine them together: (5/10)(400)+(5/10)(200)= 200+100= 300.
- Conclusion: It seems that the vapor pressure of the combine solutions is lower compared to water whereas its higher compared to pure ethanol.
- 300<400, 300>200: Just like salt in water, does that mean water will boil at a higher temp than when its by itself. And for ethanol, does that mean ethanol will boil at a lower boiling point than by itself because the partial pressure of the 2 solutions is higher than pure ethanol?

 

[byeh2004]

This is NOT TRUE! When you add salt to water it does not boil faster. The boiling point DOES NOT DECREASE HERE....it actually increases...This is because the vapour pressure of the solvent (Water) multiplied by the mole fraction of water (which has decreased on addiction of salt) will give a lower value of Pv (Vapor pressure of solution)....

If Pv is lower than the pure solvent....You would have to increase the temperature to bring it up to be equal to the atmospheric pressure as compared to the temperature the pure solvent will boil at...

Haha, I was typing up this last post up there when you were writing this one. Cheers! Could you tell me if the logic I did on the top is right?

 

[spyderracing32]

Does salt really lower boiling point or raise it? I know it makes water boil faster but what the heck does that mean !

Adding salt works to:
A) Season your noodles (not on the MCAT)
B) If you're really close to boiling and really using some big salt crystals they could act like boiling chips (provide nucleation site)

Raoults law, combined with the law of partial pressures, describes the nature of a solution of components with differing volatilities.

P_vap_tot = Xa*Pvap_a + Xb*Pvap_b

It should make sense that if you add a solute with a significantly high vapor pressure in a sufficient quantity then the Pvap of the solution would go up in relation to the Pvap of the solvent. Same goes for BP depression and adding a component with a lower vapor pressure than the pure solvent.


Edit: And why did you make ethanol a lower Pvap than water? That's not the case in real life. Yes the solution will boil at that point, however up to that point the more volatile component will boil out before the less volatile component (hence, fractional distillation, boiling point azeotropes, etc.).

 

[silverlion]

.
- Conclusion: It seems that the vapor pressure of the combine solutions is lower compared to water whereas its higher compared to pure ethanol.
- 300<400, 300>200: Just like salt in water, does that mean water will boil at a higher temp than when its by itself. And for ethanol, does that mean ethanol will boil at a lower boiling point than by itself because the partial pressure of the 2 solutions is higher than pure ethanol?

You're right when we stick to the parameters you've mentioned...This is how it would be...

Spyderracing is also right that in reality the vapor pressure of ethanol is more than that of water (so your assumptions are just off the fact)...but yes...the logic is right...

Spyderracing is also making another good point. Think about it...If there is a substance that is more volatile than your original solvent (which means that the vapor pressure for this new substance is more than what the vapor pressure for the original solvent would be)...then in a solution the new substance would only increase the no. of gaseous molecules above the solution (which would contribute to the vapor pressure of the solution) as compared to the pure solvent....If there are more no. of gas molecules at the same temperature as compared to the pure solvent this simply means that the vapor pressure of the solution would become equal to the atmospheric pressure quicker than the pure solvent would...

THis in other words means that the boiling point has been decreased...

BUT...It all depends on the solvent and the solute .....If the solute is less volatile then the solvent (as is the case with salt in water) then the vapor pressure of the solution is less than that of water (solvent) and in such a case the boiling point would go up...as you would have to heat the solution more than what you would have for the pure solvent to reach the boiling point....

I think this should make sense...Its a bit long...sorry about that...this is not a hard concept...just think about it for a while...let your intuition guide you and you will see it clearly in your head!!

 

[silverlion]

I am not trying to confuse you...but I think it would be interesting to point it out here...

the fact that when you add some salt to water the solution tends to absorb heat...this is the heat of hydration of the salt molecules..It is an endothermic reaction...

From this viewpoint it may seem that there would be an effect on boiling point....I however dont think that is to any considerable amount....

Anyway...I don't think this is important for the MCAT in any way...I'm just messing with my own head and writing **** down for no real good reason!!

byeh....as far as the lowering and raising the boiling point is concerned there is one thing that might help in clearing things a bit...and that is figuring out why things are more volatile than others in the first place...if you have time to kill then look into that...It related pretty well with this concept

 

[niranjan162]

WRONG!!! In fact it is the opposite. If a solution experiences a decrease in boiling point then it means that the vapor pressure of the solution equals the atmospheric pressure at a temperature lower than the boiling point of the pure solvent. This can happen only if the solute (the liquid in the solution which has a lesser proportion than the other) is more volatile...

o sorry i confused myself, i meant to put increases boiling point.

 

[byeh2004]

Thanks guys that cleared up alot of stuff.