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.

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.

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.

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.

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.

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.

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.

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 s^h means soluble if the solvent is hot. The "v" means very soluble.

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 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.

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)

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..

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.

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.)

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)

Zumdaul: 6th Edition: pages, 133-144: 7th Edition: pages 127-137

Tro: 2nd Edition: pages 146-150

Zumdaul: 6th Edition: pages, 148-155: 7th Edition: pages 140-146

Tro: 2nd Edition: pages 150-155