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Topic: How do solutions work?  (Read 2050 times)

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Offline MITaylor

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How do solutions work?
« on: March 18, 2015, 10:25:34 PM »
I thought solutions meant that molecules were broken up; if so, why does salt water still taste like table salt? If not...how do solutions work?
Thanks!

Offline Corribus

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Re: How do solutions work?
« Reply #1 on: March 18, 2015, 11:24:52 PM »
Analogy time.

Suppose you have two groups of people. Most of them wear red shirts. A minority wear blue shirts. Now tell them all to go stand in a football field. 

Scenario 1.  The people with red shirts like people with blue shirts less than they do other people with red shirts, and vice-versa. In this scenario, people with red shirts will tend to stick together, and people with blue shirts will tend to stick together. So you'll have groups or clumps of people in blue shirts awash in a sea of red shirts. Given time, the blue and red shirts will completely self-segregate, with all blue shirts on one side of the field and all red shirts on the other.

Scenario 2. The people with red shirts LOVE people with blue shirts, and people with blue shirts LOVE people with red shirts. In this scenario, people will want to mingle, such that each person with a blue shirt tends to be completely surround by red-shirted people, and few blue-shirted people are close to each other.

Scenario 1 is a suspension; scenario 2 is a solution. Notice that you haven't cut anyone in half here. You could, for example, remove all the people with red shirts from either scenario and you'd still be left with the same blue-shirted people you started with. In a solution, molecules of the solvent interact favorably with solvent molecules, such that solvent molecules surround each individual solute molecule and separate them so the sample becomes a single, homogenous phase.  In a suspension, solute molecules and solvent molecules don't get along. Solute molecules like to stick together and stay as far away from solvent molecules as possible. They form two completely different phases, large clumps or groups of solute molecules (blue-shirted people) surrounded by solvent molecules (red-shirted people) that want nothing to do with them. Sugar, for example, loves water and will dissolve readily in it so that each molecule of sugar is completely surrounded by water molecules. Only one phase is evident = solution. Fat molecules, however, hate water molecules, and so the phases stay separate. Not a solution.
« Last Edit: March 18, 2015, 11:42:54 PM by Corribus »
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Offline Corribus

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Re: How do solutions work?
« Reply #2 on: March 18, 2015, 11:52:20 PM »
One other thing since I didn't address your specific question fully. Table salt is not made of molecules. It is made of ions stuck together by mutual attraction of electric charges. 

The only thing that changes about the analogy is that the solute in this case consists of two different "molecules" (ions) bound tightly together. In a solution, the ions are more happy to be mingling with the solvent molecules, so they separate and disperse. If the ions are more happy to be attached to each other than mingling with the solvent molecules, they stay solid and settle to the bottom of the container.

Table salt is made of sodium and chloride ions, and they happen to like mingling with water more than they do with each other, so they form a solution by separating and dispersing in the water.  If you put table salt on your tongue, it dissolves in the water in your mouth, so there is no difference between imbibing salt water and solid salt - both cases result in a salt solution in your mouth. Your tongue has ion receptors - it is the dissolved ions (specifically, sodium and to a lesser extent other positively charged ions) that result in a salty taste. If salt didn't dissolve at all, you wouldn't taste anything.
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Offline Borek

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Re: How do solutions work?
« Reply #3 on: March 19, 2015, 03:55:18 AM »
I thought solutions meant that molecules were broken up

Dissolved sugar molecules are just dispersed.

Dissolved kitchen salt dissociates - so (while this is technically not correct) you can try to describe it as if the molecules were broken up.

When you put kitchen salt on the tongue it dissolves in your saliva, so the taste you feel is a taste of the solution, not of the solid.
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Offline Arkcon

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Re: How do solutions work?
« Reply #4 on: March 19, 2015, 08:17:19 AM »
I thought solutions meant that molecules were broken up; if so, why does salt water still taste like table salt? If not...how do solutions work?
Thanks!

Do try to realize, solid sodium chloride doesn't taste like anything.  If you make a solution of NaCl in water, or dissolve a few crystals in the saliva on your tongue, you always make a solution first, then the ions interact with taste receptors.  No one has ever tasted NaCl -- as sure as no NaCl crystal conducts electricity.  Yes, molten or dissolved it does, but the chemical properties of ionic solids are different from solutions.
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