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Offline Lana

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Science Project
« on: 25/01/2004 21:35:48 »
I need help in science. Why do the substances that conain suger take a shorter amount of time to boil then just paine water and why do other substances stirred in water like salt, vinigar, and milk take a longer amount of time to boil.


 

Offline Ylide

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Re: Science Project
« Reply #1 on: 26/01/2004 00:18:46 »
This should be explained in your chemistry textbook...if it's not, I question the credentials of the author.

First, you must know that the condition for boiling is when the vapor pressure of the water (the pressure of the gaseous water coming from the liquid) is equal to the atmospheric pressure.

The elevation of the boiling point (and conversely the depression of the freezing point) for water is affected by the presence of solutes, such as sugar, salts, etc.  When you dissolve a substance in water, water molecules arrange themselves in an ordered way around the polar (positively or negatively charged) areas of the solute.  There is a small attractive force that holds the water around the solute.  When you add heat to boil water, the energy is absorbed by the water molecules and some of it is converted to kinetic energy.  You're essentially adding enough energy to cause the average velocity of the water molecules to exceed the intermolecular forces keeping it a liquid.  The more water molecules that absorb this requisite energy, the greater the vapor pressure of the water.  Those extra attractive forces caused by the presence of the solute increase the energy requirement for water to be converted to water vapor because they are held in place by the solute.  This is also why more dilute solutions will be closer to the real boiling point than more concentrated solutions...more water molecules are able to move freely about the container and have a greater chance of meeting the energy conditions to escape the liquid.





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Offline chris

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Re: Science Project
« Reply #2 on: 26/01/2004 12:29:37 »
Brilliant explanation !

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Offline Quantumcat

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Re: Science Project
« Reply #3 on: 26/01/2004 15:20:23 »
Ah, the joys of year 9 chem ...

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Offline tweener

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Re: Science Project
« Reply #4 on: 28/01/2004 04:36:52 »
Great explanation.  I've never heard it put so clearly.


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Re: Science Project
« Reply #4 on: 28/01/2004 04:36:52 »

 

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