Last updated 10-6-09
<-CHM151

Water, Solubility, Concentration

Solubility is a measure of how much a gas, liquid, or solid becomes dissolved in a liquid.

The lava lamp is a good icon for solubility. It took them years to develop formulas for the globs and the liquid, so the globs would not dissolve in the liquid but be almost the same density.

If you buy gas, you've probably seen the additive MTBE: Methyl Tertiary Butyl Ether. The oxygen in the molecule helps gasoline burn more completely. It is very soluble in gasoline. Unfortunately, it's also partially soluble in water. 42 g dissolve in a liter. It can be tasted at a very low 0.0001 g / liter, which means it would not take much to make drinking water in wells or aquifer undrinkable.

Over 20,000 of these storage tanks are estimated to be leaking in the state of Virginia alone. Again the problem comes from the fact that it is soluble in water.

Applications of Solubility Knowledge

Cleaning

Separation (purifying)

Identification

Understanding solubility will help you do cleaning because you can make an educated guess of what dissolves what.

By dissolving one substance and not another, you can separate two substances in a mixture.

Knowing what dissolves and what doesn't can be used to identify an unknown substance.

What if you got your hands dirty with some grease and there was no soap around?   One thing to remember is that "Like Dissolves Like". So if you want to dissolve the grease on your hand, use something greasy or oily. Cooking oil or butter are two examples. One student said he heard that potato chips can clean greasy hands because there's a lot of oil the chips. I haven't tried it but I bet it would work.

 

This child has a clean face now, but after eating the cotton candy, it won't be. To clean off cotton candy we know that water would good for that, not oil. Remembering "Like Dissolves Like" we guess that water and sugar must be alike in some way.

 

To understand how things dissolve, let's first look at something you are familiar with--- magnets.  If you were to drop a small bar magnet into a group of marbles, you might suspect it to simply be sitting amidst a group of marbles.  That is true, but now...

 

Let's throw in several of the small bar magnets. At first they will just be randomly mixed with the marbles. However, since magnets have north and south poles that attract each other, magnets will start to stick to other magnets. In a little while a different arrangement will occur.

 

The magnets pull toward each other and push the marbles out of the way as they come together. The glass marbles have no magnetic poles, so the magnets have no attraction to them. The magnets will find an arrangement that allows for opposite poles to get closer to each other and like poles to stay as far away from each other as possible. Also, since metal magnets are more dense than glass, gravity would pull them to the bottom.

 

Instead of magnets, consider what happens when mixing in wood sticks. Wood has no magnetic poles so it doesn't attract other pieces of wood. The wood sticks would stay randomly mixed with the marbles.

 

Now let's throw in a mixture of bar magnets, horseshoe magnets, wood sticks, and wood balls. As you might expect, only the items that have magnetic poles will try to reorganize.

 

The items that have magnetic poles pull on each other and squeeze the items they don't attract out of the way. If the magnetic items are more dense than the non-magnetic items, they will end up below (like shown). If the magnets have the same density as the non-magnetic items, they end up as a ball in the middle. In every case, they separate themselves from the items that don't have any magnetic poles. It' not that the magnets repel the wood or glass, it's just they are very attracted to only things magnetic.

The above picture explains Like Dissolves Like. Items that act like magnets will attract each other and become mixed or dissolved in each other. Items that do not act like magnets will stay mixed or dissolved in each other.
To explain why water and oil do not mix, we will find out why water is like a magnet, but instead of magnetic north and south poles, it has plus and minus electrical poles. The effect is the same. The plus end of water is attracted to the negative end of a different water molecule. The plus end is where the two hydrogens attach to the oxygen atom.
That's worth repeating. The plus end of water is attracted to the negative end of a different water molecule. They align themselves + to - . The motion in the water would keep them more random than is shown here, but this picture is close to what water would do when brought to freezing.

Why is water positive toward the hydrogen end? If we look at all of the protons (+) and electrons (-) of both hydrogen and oxygen, this is what we see. If you count the bottom half you get 4 protons and 5 electrons (a minus 1 net charge). If you count the top half, you get 6 protons and 5 electrons (a plus 1 net charge). Even though this is a simplication because electrons are in motion, we still get the idea why the top half (hydrogen half) is positive and the bottom half (oxygen only half) is negative.

Molecules like water that have plus and negative poles (like magnetic poles) are called POLAR. Molecular that do not have any negative or positive poles are called nonpolar.

Water is known as the universal solvent. The reason is has that power to dissolve is because of its charge. The oxygen side of water is negatively charged because the protons in oxygen's nucleus pulls the shared electrons closer to the oxygen atom.

The protons in the two hydrogen atoms are then more exposed. So the hydrogen side of water is positively charged.

Like a magnet that pulls on things that are magnetic, water pulls on things that are electrically charged. Magnets have north & south poles, water has positive and negative poles and thus called a polar solvent.
Since unlike charges attract, the negative end of water will be attracted to the positive sodium ion. The positive end of water will be attracted to the negative chloride ion.
Since water is always in motion, it will pull on the ionic compound and move the ions away from each other. This dissolves the ionic compound. (Roll cursor over image to see animation).
This might be a good time to introduce a new word: Electrolytes. Electrolytes are compounds that are pulled apart by the charges on water. In other words, electrolytes are compounds that dissolve in water. "Electro" comes from the electrical charge on both water and on the compound being dissolved. Their + and - charges pull on each other. The word "lyte" comes from a Greek word "lytos" meaning loosen, dissolve, or untie. So together, an "electrolyte" is a compound that is dissolved by the electrical push and pull from water or some solvent like water. In this drink, the electrolytes are probably salts such as NaCl, KCl, CaCl2, MgCl Na2HPO4.
The ions formed when water pulls them apart would be Na+, K+, Ca2+, Mg2+, Cl-, and HPO42-.
acids
The above electrolytes are consumable, but there are others that aren't so healthy. Those are acids. They also qualify as electrolytes because they too are pulled apart by the charge of water. For example, hydrochloric acid (HCl) is pulled apart into H+ and Cl-. Sulfuric acid (H2SO4) gets pulled apart into H+ and HSO4-. Nitric acid and phosphoric acid also break apart.
Acids: Weak versus Strong Electrolytes: When water is added to the above acids, virtually all of the acid molecules are pulled apart to form ions. They are called "Strong acids" because they are strong electrolytes. The H+ ion produced gives them their acid strength. On the left we have citric acid (in the lemon) and acetic acid (in the vinegar). These acids do not completely come apart in water. So we call them weak electrolytes and therefore weak acids. Approximately only 1% of these acids are pulled apart (disassociated) when in contact with water. That's why these are weaker, and why we can consume them.
lye
Bases: Weak versus Strong Electrolytes: To be thorough we should also mention that bases that dissolve in water are also electrolytes. Here is lye (sodium hydroxide), which disassociates into Na+ and OH-.
conductivity tester
Quick check for electrolytes: The presence of electrolytes can easily be checked by the measuring the resistance of the solution. The opposite of electrical resistance is conductance. So a meter that measures either one tells you if electrolytes are present and how many are present. If the solution allows electrical current to flow, then that means there must be some ions present that are moving the electrons through the solution. This simple test gives us insight into what is happening with the compounds in the solution.
tester Cheap Conductivity Tester:  You can buy a bonafide conductivity tester like shown above, or you can make your own with a battery and a light bulb. Dunk the red and black wires into the water and if the bulb comes on, there are electrolytes present. In other words, if ions are present, then the bulb will light up. If the bulb is bright, then there is a high concentration of ions, and you have a strong electrolyte present. If the bulb is dim, then it's a low concentration of a strong electrolyte or there is a weak electrolyte present. If bulb is dark, then very little or no ions are present. In other words, a non-electrolyte is present or the water is pure.
test pool Detective Work:  Hummingbird feeders are filled with sugar water. Sugar (sucrose) dissolves in water but it releases no ions. The negative end of water (oxygen end) is attracted to the hydrogen atoms in the "OH" groups. And the positive end of water (hydrogen end) is attracted to the oxygen atoms in the "OH". This is why water dissolves sugar; however, none of the hydrogen atoms are pulled off as H+ ions. So a conductivity test shows no ions (no electrolytes). With the cheaper version, the light does not come on. However, let's say the hummingbirds are dying. You suspect that something other than pure sugar water was used. You test the water for electrolytes and the light comes on meaning something is in the water that has formed ions. So this isn't pure sugar water.
Water Myth
We’ve heard that wax or oils repel water. But that isn’t true. Water is so attracted to other water molecules that anything between them is squeezed out of the way.(Roll cursor over image to see animation).

Water is always trying to pull itself into a tight ball as long as there is nothing nearby that has a charge on it. Therefore, this surface is not repelling water; it’s simply not attracting it and keeping water from doing what it does naturally.

Again, wax on a car's paint doesn't repel water, it simply isn't attracting it. Water pulls itself into beads on its own.

Many plants have a waxy coating on their leaves. Wax is long chains of carbon and hydrogen that have no charge on them. So water isn't attracted to the wax, allowing water to pull itself into beads. This same attraction forms a "skin" on the water as the water molecule interlock with each other. This "skin" is also called surface tension. Some insects take advantage of the surface tension and are able to walk on the surface of the water.

Water molecules pull on each other so strongly they bunch up into spheres. Because of this, we are able to witness rain and raindrops.

There's probably no other liquid that can fall the height of clouds and reach the ground as drops. Most liquids would break up into an aerosol or fog.

So without water's built-in + and - charge holding it together, there would be no rain, just fog.


papertowel
Surfaces where water is attracted: Water is attracted to plant fibers because plant fibers are made from cellulose, which is a chain of glucose molecules. The "OH" groups have a partially negative charge on the oxygen and a partially positive charge on the hydrogen. Water, as you know, also has these charges so will be attracted to the cellulose (That's why paper towels soak up water).
meniscus
Glass is made from silicon dioxide, sodium carbonate, calcium oxide, magnesium oxide, and aluminum oxide. All of these compounds are polar because their oxygen atoms pull electrons away from the metals (and silicon) and towards the oxygen atoms leaving oxygen atoms partially negative and the metals partially positive. That means that water will be attracted to glass. In a glass graduated cylinder (top image), you see water creep up the sides of the glass walls producing a curved surface, which is called the meniscus. If this was filled with mercury, there would be no curve because mercury isn't attracted to glass.
glass beaker with drops of water on it
This is a glass beaker yet there are drops of water on the beaker. Water is strongly attracted to glass, so there should never be drops of water on glass beakers or even glass windows. The water should just form a sheet and slide down the glass. However, I'm sure you've seen water drops on glass many times. The reason for the drops is that the glass has a thin film of oil (too thin to even see). There's a strong glass cleaning solution made from sulfuric acid and potassium dichromate that decomposes the film of oil on glassware. After that treatment, you will see water does not form drops on clean glass.
Let's return our attention as to what is soluble and what isn't. To the left is a chain of carbon and hydrogen atoms. We call these hydrocarbons. This one is about 7 carbons long and is typical of gasoline. The bottom image shows the charges of a carbon atom attached to two hydrogens. You may notice that unlike water the charge is divided evenly. Also the electrons going around hydrogen do a good job of hiding the hydrogen's proton. That way water isn't attracted to it. Therefore, if water and gasoline are mixed, water senses no charge on the gasoline and simply squeezes the gasoline out of its way as it tries to attach to other water molecules.

There is a way water can dissolve gasoline or oil. Soaps and detergents are chains that have one end that is like oil (or gasoline), which has no charge, and the other end of soap is charged. Sulfate is commonly attached to the end of those long carbon/hydrogen chains which depicted as zig-zag lines.

The long chains will mix with the oil because there's a slight attraction between the long chains in the soaps with the long chains in the oil. Water will be strongly attracted to the charged ends. Once the soap is attached to the oil droplet, it will dissolve in water because water surrounds it as it holds on to the sulfate ends (Roll cursor over image to see animation. Realize they wouldn't move one by one, but all at the same time).

Water is called the universal solvent because it is everywhere; however, it doesn't dissolve substances that have no charge on it like wax, oil, or fat. Acetone, however, can dissolve substances that have a charge and those that don't. So it's a good all around solvent. The drawback? It's more expensive than water and it easily catches on fire.

Measuring the concentration of a solution
A solution consists of something dissolved in water or another solvent.

There are two ways we talk about the concentration of a solution. One is an approximation of the concentration and the other gives numbers.

A solution contains the solvent and a substance dissolved in that solvent. That substance that got dissolved is called the solute.

For concentration, we might say a solution is dilute or concentrated (weak vs. strong). Or we call it unsaturated (meaning a substance is dissolved in the solvent, but not as much as could be dissolved). Saturated means the water (or another solvent) has dissolved the maximum amount of some substance (the solute). Supersaturated means that the solution has more of a substance dissolved in it than it normally can hold, which means that substance (the solute) is likely to come out of solution and form solid particles.
Making rock candy is an example of dissolving sugar until you get a supersaturated solution (more sugar than water is suppose to hold). That's done by heating the water. By submerging a stick with a coating a small crystals of sugar into the supersaturated sugar solution, the dissolved sugar will come out of solution and cause the small crystals to grow.

Salt (NaCl) is very soluble in water; 350g per liter.    However, if water evaporates, there will be too much salt for the water to hold in solution. The salt begins to form crystals.

A lake near Death Valley (near town of Trona) is supersatured with salt causing the salt to crystallize out.

The salt looks like snow from a distance.

The instructors at Mesa Community College usually take their geology club students to Trona to collect salt crystals from this lake.

Here's an example of the salt (NaCl) crystals collected. They call these halide crystals.

The red color comes from a red algae that somehow is able to grow in this supersaturated solution of salt.

When I want to know the solubility of different substances in water or some other solvent, I turn to the CRC Handbook of Chemistry and Physics. This reference book indicates the solubility of many inorganic and organic compounds.

Here's a scan of one of its pages showing solubility. What do you think w, al, eth, ace, and bz stand for?

(Roll cursor over image to see answers).

By the way The "s" means soluble, the sh means soluble if the solvent is hot. The "v" means very soluble.

I mentioned that a knowledge of solubility can be used to separate a mixture. Let's say you needed some aspirin but the only kind you found was chewable aspirin that had sugar in it also. You are a diabetic, so you can't eat the sugar. The chart says aspirin (acetylsalicyclic acid) is soluble in H2O, ether, & chloroform; very soluble in ethanol; and slightly soluble in benzene. Not shown is the data for sucrose; however, it says that sucrose (sugar) is only slightly soluble in ethanol. So that means if you use some strong alcohol like 190 proof (95%) Everclear, it will dissolve the aspirin and leave most of the sugar undissolved. If the alcohol is cold, even less sugar will dissolve. So crush the aspirin tablets, pour in ethanol and stir. The sugar will settle to the bottom and the aspirin will dissolve in the ethanol. Now you can drink the ethanol and you will get your aspirin without the sugar.

So far we've mentioned the ways we approximate concentration. Now let's look at quantitative ways of measuring concentration.

Here's the list but let us go through them one by one.

Mass (weight) per Volume (w/v)
Here's another segment from the CRC handbook. This is the entry for sodium phosphide (Na3P). The heading says the solubility is in grams per 100cc (that's a weight to volume or w/v). The solubility of Na3P in cold water (0°C) is 5.41 grams per 100cc and in hot water (100°C) is 93.11g/100cc. This shows that heating up water really makes things dissolve more. If these values are grams per 100cc, you can easily change these values to 5.41% w/v and 93.11% w/v. This is the Mass/Volume Percent format. So "% w/v" means grams per 100cc (or 100mL)

Here is the information on a tube of Close-Up toothpaste. The list the active ingredients as having a concentration of 0.14% wt/vol. "wt/vol" is often abbreviated (w/v). When you see a "%" sign you are suppose to think "per hundred". So 0.14% wt/vol means 0.14 grams per hundred milliliters (cc). In other words, for every 100mL of toothpaste, there's 0.14 grams of the fluoride ion, which is the ion that fights cavities. They didn't give the concentration of the salt where the fluoride came from, which is sodium monofluorophosphate (Na2PO3F)

Here we have Crest toothpaste. They list two concentrations. One is for sodium fluoride (0.243% w/v and one is for just the fluoride ion (0.15% w/v). So that translates to 0.243 grams of NaF per 100mL of toothpaste and 0.15 grams of F- per 100mL of toothpaste.

It's nice that they both give the concentration of the fluoride ion, because you can compare the two. It wouldn't help as much if one gave the concentration of NaF and the other Na2PO3F.

Insecticides are often dissolved in solvents (other than water). Their concentrations are usually given as the active ingredient weight divided by solvent weight, then converted to percent.

Mass Percent (w/w)
Here's a close up of the ingredients shown on the bottle above. Notice they call it "By Wt." which of course is "by weight". The active ingredient is 25% by weight, which means that whatever weight you measure, the active ingredient is 25% of that weight. So one pound of insecticide solution contains 1/4 pound of the active ingredient (Diazinon). So, again, this is a weight over weight percentage (% w/w)
When medicines are dissolved in water, they usually use the Mass/Volume percent (% w/v) format. This particular medicine shows concentrations in two ways: 5mg/mL and 0.5%. Let's start with 5mg/mL (5 milligrams per mL) and see if we can get 0.5%, which means 0.5grams/100mL.
Notice the dimensional analysis starts with 5mg over 1mL. Since we need 100mL, we multiply by 100/100 (which is like multiplying by 1). To get rid of the "m" we multiply by 0.001 over "milli" The "milli" cancels and we end up with 0.5 g over 100mL, which is 0.5% w/v. They don't always show the "w/v" but they should.
 
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1 Concentration on bottle To get 100mL
To get rid of "m"
           
2
5
mg
100
0.001
=
0.5 g = 0.5 % w/v
3
1
mL
100
milli
  100 mL      

Here's an example where they don't indicate "w/v" which is poor practice but commonly done. We will assume the % means % weight to volume.

Let's calculate how much glycerin is in this bottle. At 0.2% that means 0.2g/100mL. By multiplying by the volume of the bottle (15mL), the milliliters (ml) cancels and we get 0.03 grams of glycerin in the bottle. Actually there's very little of anything in the bottle, which makes you wonder why they charge so much.


Here's a company that sells IV solutions. A common one used for dehydration and to give a patient calories is Dextrose 5%. It lists the ingredients as 50 grams of Dextrose monohydrate diluted with water to 1000mL. So that is 50g per 1000mL. Our dimensional analysis starts with that fraction and multiplies by 0.1 over 0.1 in order to get 1000mL down to 100mL. This also turns 50g into 5g. So we end up with 5g per 100mL, which is another way of saying 5% (w/v). Now we know why it's called Dextrose 5%. They also sell Dextrose 10%, which is close to putting Coca-Cola into our veins (as far as sugar concentration is concerned)

Speaking of Coca-Cola, let's see how to figure out the mass/volume percent of the sugar. It lists sugars as 39grams and the volume as 355mL. Concentration is weight per volume, so that 39g over 355mL, which reduces to 0.11g/1mL. Concentration as %w/v needs the volume in 100mL. To get that we just multiply by 100 over 100. That gives us 11g per 100mL which is 11% w/v. That's almost the same as the Dextrose 10% used in some IVs.
 
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1
sugar concentration
To get 100mL
conc. in g/100mL
  Now in % w/v
2
39
g sugar
100
=
11
g
=
11
% w/v
3
355
mL soda
100
100
mL      

The tricky part of the calculation is not to divide by the 100 in cell C3. That allows the 100 to end up in E3. So the formula for E2 is "=A2*C2/A3" That gives you "11" grams. As before, the % means "per 100" so we replace the 100 from the denominator with a % sign, and we replace grams (g) with a "w" and the "mL" is replaced with "/v".

So far, we've talked about a solid dissolved in a liquid. Here we have a liquid (alcohol) dissolved in another liquid (water). The concentration in these cases is often volume percent (%v/v). On this bottle it says, "Alcohol 10% by vol." That means 10% of any volume of this champagne is alcohol. So if the whole bottle is 750 milliliters, then 10% of that is alcohol giving us 75 milliliters of pure alcohol.

Other liquors list the alcohol concentration as "proof". 100 prove is actually 50% v/v alcohol. Here the Kahlua is 53 proof which is 26.5% v/v alcohol. So 26.5% of the 750 ml is alcohol.


In chemistry, it's more convenient to list the concentration in moles per liters. Remember moles is how we count molecules. With a count we can use a chemical equation to count other products or reactants in that equation. So that's why we often see chemicals dissolved in water measured in moles per liter . This is given the name "molarity or molar" which is abbreviated as a capital M.
In other words, a liter of the solution on the right that says "1 M K2CrO4" contains 1 mole of K2CrO4 (potassium chromate). One liter of the solution on the left (KOH) contains 0.80 moles of KOH. So this is better than giving the concentration in grams. If in grams per 100mL, you would have to look up the molecular weights to calculate moles. Molarity saves that step.

Here's a 0.1 molar solution of sodium arsenate. That means 1 liter of this solution contains 0.1 moles of sodium arsenate.

To make up 2 liters of this solution you would have to know the molecular weight of sodium arsenate (Na3AsO4), which means consulting the Periodic Table to find the molar mass of sodium (times 3), the molar mass of arsenic, and that of oxygen (times 4). Those add up to 208 grams per mole of Na3AsO4. This solution is only 0.1M, so that means we only need one tenth of a mole for every liter. So we would use 1/10 x 208 grams=20.8 grams for each liter that we want. This is a 5 liter jug, so that means 20.8g/L x 5L = 104 grams. So 104 g of Na3AsO4 will make us 5 liters of 0.1M sodium arsenate solution.

While you have the above calculations fresh in your mind, let's do a few sample problems of that type.

Here is 0.1M (0.1 moles per liter) of sodium phosphate.
Example 1: This bottle has 2.5 liters of solution. How many moles of sodium phosphate are in those 2.5 liters?
We first just realize that 0.1M means 0.1moles per liter. We start with that. We simply multiply by the liters to get moles because the liters will cancel..

 
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1
Molar Concentration
times volume in liters
 
Gives moles
2
0.1
moles
2.5
liters
=
0.25
moles
3   liter          
bottle 0.1 molar sodium phosphate

Example 2: If you wanted to make up 2 liters of this 0.1M solution, how many grams of sodium phosphate (Na3PO4) would you weigh out?
We start off the same as the problem above except we end up changing the moles into grams by using the molar mass of sodium phosphate that we have to figure out.

 
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I
                   
 
0.1
moles Na3PO4
2
liters
163.94
g Na3PO4
=
32.79
g
    liter    
1
mole Na3PO4      


Example 3: To the left is a 1 liter bottle of Everclear liquor. How many milliliters of alcohol (ethanol) is in this bottle?

This is easy. 1 liter is 1000mL. So just take 95% of 1000 mL. That gives you 950mL of pure ethanol.

Example 4:

You are working in a rural clinic where you have to make up your own IV solutions. You are asked to make up a 3.5 liters of 5% w/v solution of this anhydrous Dextrose. How many grams of this dextrose do you weigh out? (Notice I first change 5% w/v into what that means, which is 5 grams per 100mL. Multiply by the liters wanted to cancel "L" in "mL". Get rid of "m" in "mL" by multiplying by milli over 0.001.)

 
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K
1                      
2
5
% w/v
=
5
g
3.5
Liters
milli
=
175
g dextrose
3      
100
mL
   
0.001
     

Example 5: The ingredient states 0.14% wt/vol of fluoride ion. Note: "wt/vol" is another way to write "w/v". If you use 1.5 cc of toothpaste to brush, how many grams of fluoride ion is in that 1 cc? (Note: cc=mL so they cancel)

 
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1
Given conc. of fluoride
 
equals g/100mL
volume
     
2
0.14
% wt/vol
=
0.14
g
1.5
cc
=
0.21
g fluoride
3      
100
mL
         
For students in my CHM151 class, send your answers to chm151@chemistryland.com. Use a subject line of "Solutions".
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Since Oct., 2009