Supersaturation names the force that makes crystals. Because it does, we measure supersaturation to understand why a patient makes stones, and we reduce risk of more stones by lowering supersaturation.

Fortunately, universal and quantitative laws govern how atoms and molecules form crystals so we can calculate supersaturation and predict the risk of crystals using equations that apply everywhere, even in the kidneys and urine.

Supersaturate a solution yourself, and see how it works.

Supersaturate your own sugar solution

Saturate Your Sugar Solution

Find a heat resistant glass container, fill it with water, and stir in table sugar until no more will dissolve. You will know when no more will dissolve because extra sugar remains at the bottom even if you shake it up, or let it stand. At that point you have ‘saturated’ the water with sugar. It holds all the sugar it can hold. The concentration of sugar in the water has reached its solubility.

What Do ‘Saturate’ and ‘Solubility’ Mean?

Pause a minute here and think about what you have done.

You added so much sugar some could dissolve in the water yet leave extra sugar behind at the bottom of the glass.

Even if it looks pretty simple, a lot happens in your glass, every second.


We say you saturated your solution because solid sugar remained at the bottom though you shook it up, or let it stand. No more sugar could dissolve because more remained.

Molecular Motion

Imagine yourself the size of a sugar molecule swimming in the water. Sugar molecules leave the surface of the solid sugar like divers off the edge of a pool or rock and swim with you. Sugar molecules leave the water for the sugar surface.

If you sat on the solid sugar you would feel the other molecules in the solid pulling you toward them. Sugar molecules have electrical charges on their surfaces that attract each other. For those who want more about sucrose crystallization, here are details. 

The molecules  dart about randomly in the water, at a speed determined by temperature. Those on the solid sugar surface wriggle, some violently enough to swim away even against the attraction from deeper molecules below the solid sugar surface. Were you a sugar molecule you too would dart about in the water and wriggle, on and off the solid sugar.


When water and solid sugar mix together, we name that concentration of sugar in the water the ‘solubility’ when sugar molecules leave the solid sugar and return to the solid sugar at the same rate – so the amount of solid sugar stays constant. In other words, at solubility the amount of solid sugar at the bottom of the glass neither grows nor shrinks.

Dissolve the Extra Sugar

Now, heat the water; you need not make it very hot.

Shrink again. You and the other sugar molecules will dart and wriggle more and faster. Sugars will wriggle more violently off the solid surface. Because they dart faster those in the water have less chance of staying on the surface. Heating raises the temperature which means the speed of molecular motion and shifts the balance away from resting on the solid surface and toward swimming in the water. This means heating raises the solubility of the sugar in the water.

The solid extra dissolves. The concentration in the water rises.

All the sugar at the bottom will eventually dissolve as heating raises the solubility of sugar.

Make Supersaturation by Cooling

Let the solution cool.

If you are careful, the water will remain clear.

The darting and wriggling of molecules goes back to where you started. But, the water has a low more sugar dissolved in it. You know that because all that was at the bottom is not in the solution. It is supersaturated with sugar. The sugar concentration is above the solubility.

Your kidneys do what you have done.

They create a liquid filtrate from blood containing a variety of kidney stone forming materials like calcium, oxalate, phosphate and uric acid. In blood their salts (calcium oxalate, calcium phosphate as examples) are below saturation. Kidneys leave some of what has been filtered to go out in the urine, but extract most of the water. Extraction of the water concentrates the stone forming salts. The urine becomes supersaturated depending on the balance between the amounts of salts and water left to go out.

Get your solution to crystallize

Your sugar solution is clear at room temperature, but being supersaturated can collapse into crystals, letting go of all the ‘extra’ sugar until it reaches solubility.

Tap the glass, or shake it to make some bubbles, and you will see.

Urine is a lot more stable than your solution. Many of the same molecules that glue crystals together to make stones slow their formation. So even when supersaturated, urine in a glass will not make crystals just because you tap on the side. But it can make crystals. We can estimate the supersaturation of urine in patients, and that value is one way of gauging the risk of new kidney stones.

If you work at it, you can dip thin wood sticks into your supersaturated sugar solution and make pretty rock sugar lollipops like in the lead picture of this post. Food dyes help. To do this you would want the solution to crystallize slowly, so put the sticks into the solution in the refrigerator, and let things happen without disturbances such as shaking.

Why would kidneys create supersaturated urine?

You might wonder why evolution created urine supersaturation. I consider it the inevitable dark side of water conservation.

We evolved on an earth not so rich in fresh water as one might like, so thriftiness fostered survival. Not only that. In a world with little salt to eat, urine contained very much less calcium than it does in our world of abundant salt intake. Water extraction would not so much supersaturate urine then as it does now.

What can patients do about supersaturation?

Think a lot about water

Physicians hector patients about drinking water to lower urine supersaturation. Steady and copious water drinking makes a lot of sense when you consider that crystals are always waiting to form, and those already present are always waiting to grow.

Help your doctor

Modern 24 hour urine analyses present your doctor – and you if you wish to look – with supersaturations for all of the main stone forming crystals except struvite. But to use these intelligently requires three things be known, and patients can help with all three.

What are the crystals in your stones?

In your home experiment we spoke of supersaturation with respect to sucrose – table sugar. For urine we speak of supersaturations with respect to each of the main stone forming crystals individually. We view supersaturations through the lens of stone analysis. Lost analyses, lost stones – lost opportunities to understand urine supersaturation with respect to the actual crystals you form.

The stone analysis in a dresser drawer, or in some dusty filing cabinet in a local hospital or the office files of a prior physician, those analyses might just as well never have been made if your current physician cannot see them. Take those stones out of your dresser drawer and get them analysed.

There is No Normal Range for Urine Supersaturation

Given all the complexity of urine chemistry, supersaturation and crystal formation are not so tightly linked as in your experiment. The stone matrix proteins and urine proteins alter rates of crystal formation so much that one cannot simply set a ‘normal’ supersaturation value and work to lower your values to that supposed normal point.

Instead physicians need to work with a more rough and ready rule: the supersaturations of of a person making new stones are too high with respect to the crystals in those stones and need to be lowered.

Are you really making new stones now?

But, are you making new stones?

That judgment requires your physician compare the stones you pass or have removed with the stones seen on your x rays and decide if stones are forming or old stones are causing new symptoms.

You cannot do this. But you can keep careful track of when stones pass, and keep all records from emergency rooms where stone attacks have been treated. Your physician may practice at other hospitals from where your emergency visits occur and will not have access to the information. If x rays are done, get the images on a CD so your physician can view them and count the stones.

Are your 24 hour urines valid?

Everyone likes to show off.

Everyone likes to collect 24 hour urines over a weekend.

Put those two facts together and everyone can see the problem.

My honest if wordy memoir about how I practice stone prevention presents a patient whose work as a nurse raised her weekday supersaturations above those during weekends.

Most patients feel inclined to drink specially well while they collect their 24 hour urines. Why? Human nature.

The physician must use what you offer. If weekends and showing off lower urine supersaturations who can plan adequate treatment from inadequate information?

No one but you can prevent this problem.

I am telling you this so you are informed and can act in your own best interests.

Return to Walking Tour on Supersaturation

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