Where Does Sugar Go When It Dissolves in Water?
― It Only Becomes Invisible. Not One Particle Is Lost
Drop white sugar into hot water and stir, and it soon disappears. Yet when you drink it, it's still sweet. The sugar hasn't vanished. It has split into pieces too small for the eye to follow and spread through the water. And it isn't invisible just because the pieces are small. It's invisible because the "boundaries" that bounced light back are gone.
In the morning, you add a spoonful of sugar to your tea. The white grains sink to the bottom of the cup, and as you stir, they slowly get smaller.
Before long, not a single grain is visible. The colour of the tea and how clear it looks are the same as before.
If a child asked, "Where did the sugar go?", what would you say? "It dissolved" isn't really an answer. What does dissolving actually mean?
There Are Two Reasons It Disappears
A grain of sugar is a lump of trillions of sugar molecules packed neatly in rows. Water molecules pull molecules off its surface, one at a time.
The grain looked white because its surfaces bounced light back. Once the molecules are scattered, there are no surfaces left to bounce it, and light passes straight through.
So sugar disappears for two reasons at once: it becomes "too small to see," and it no longer "gets in the way of light." Let's take them in turn.
Water Molecules Pull Sugar Off, One by One
Sugar molecules have many spots that water molecules are drawn to. So when a grain goes into water, water molecules stick to the molecules on its surface, one after another.
A sugar molecule surrounded by water molecules leaves the grain and moves out into the water. When a surface molecule comes off, the one beneath it becomes the new surface. This repeats, and the grain shrinks from the outside in.
Stirring speeds things up because it pushes away the "already sweet" water around the grain and brings in water that isn't sweet yet. Hot water works faster because the molecules move more vigorously, so they break away with more force.
Look at Figure 1. The left shows the grain before dissolving, the middle shows it partway through, and the right shows it fully dissolved.
The Gap Between Molecules Is Only 1/150 of a Light Wave
A sugar grain looks white for the same reason snow does, even though snow is a pile of ice grains. Each grain is clear by itself, but there are so many surfaces that light bounces off in all directions.
When the sugar dissolves, those surfaces vanish. All that's left are molecules scattered through the water. A single molecule is thought to be about 1 nanometre across (one millionth of a millimetre).
Dissolve a teaspoon of sugar (about 3 grams) in a glass of water, and the average gap between molecules comes to about 3 nanometres. The waves of visible light are 400 to 700 nanometres long. To light, sugar molecules are too fine to add up to a "surface." It is almost as if nothing were there.
Use the slider in Figure 2 to change how much sugar you dissolve. Add more, and the gap shrinks only slowly, staying far smaller than a light wave. That's why even strong sugar water looks clear.
The Proof It Hasn't Gone: Weight and Sweetness
If the sugar had really vanished, the weight would drop too. But if you leave the water and sugar on a scale together and let it dissolve, the reading doesn't change at all. Only the shape of the grains is gone. Not one molecule has been lost.
The sweetness stays for the same reason. Your tongue picks up the separated sugar molecules one by one and registers them as sweet. If anything, dissolved sugar reaches the tongue more easily.
If you let the water evaporate, the sugar appears again as grains. The molecules move close together and line up once more.
If you leave it unstirred, the dissolved sugar becomes heavy sugar water and collects at the bottom. Molecules spreading by their own motion alone is very slow, and is estimated to take about a full day to travel 1 centimetre. That's why only the last sip is sweet.
About 200 grams of sugar is said to dissolve in 100 grams of water at room temperature. That's twice the weight of the water. It stays clear because the molecules are scattered one by one.
Summary
When sugar dissolves in water, the grain breaks into single molecules. The surfaces that made it look white are gone, and the gaps between molecules are far smaller than a light wave, so light passes straight through. Even so, the weight and the sweetness remain.
Dissolving doesn't mean disappearing.
It means spreading out so finely that light can't find it.
How a pile of clear grains looks white is explained in "Ice is clear, so why does snow look white?", and how dissolved material turns back into grains is covered in "Why do alum crystals grow into such beautiful shapes?"
- Put water in a glass and set it on a kitchen scale. Place about two teaspoons of sugar next to it and read the weight of both together.
- Add the sugar to the glass and stir until it dissolves. When it's fully dissolved, look at the scale again. The weight should not have changed.
- Put sugar in a different glass and set it down gently without stirring. After half a day, taste a little from the top and a little from the bottom and compare.
If you spread sugar water thinly on a small plate and leave it for a few days, the water evaporates and sugar crystals appear again. For the tasting test, use a clean spoon.
For Those Who Want to Know More ― Terms, Formulas, and Links to TextbooksEach part is labelled by level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Chemistry" and "Chemistry"
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity-level courses (physical chemistry, optics) not taught in high school
- ResearchNot yet settled even at university; questions researchers are studying now
Junior highTerms: This Phenomenon Has Names
- Solute, solvent, aqueous solution: The thing dissolved (sugar) is the solute, the thing doing the dissolving (water) is the solvent, and the liquid that results is the aqueous solution. An aqueous solution is clear and has the same concentration everywhere.
- Law of conservation of mass: The total weight doesn't change, whether you dissolve something or make it react. The scale experiment checks this.
- Solubility: The most a given amount of water can dissolve of a substance. Sugar dissolves in larger amounts as the temperature rises.
Junior highHigh schoolChecking with a Formula: How Far Apart Are the Molecules in a Teaspoon of Sugar?
For one teaspoon (3 grams) of sugar dissolved in 200 millilitres of water, we find the number of molecules and the average gap between them.
| In symbols | N ＝ ( m ÷ M ) × NA d ＝ cube root of ( V ÷ N ) |
| In words | Number of molecules = mass ÷ mass of one mole of molecules × Avogadro constant. Average gap = cube root of the volume per molecule |
| Where it comes from | One mole (6.02×10²³ molecules) has a fixed mass for each kind of molecule, so we convert mass into a count. If each scattered molecule has its own small cubic room of equal size, the side of that room is the average gap |
| N | Number of dissolved sugar molecules |
| m | Mass of sugar (grams) |
| M | Mass of one mole of sugar (sucrose) (grams) |
| V | Volume of the solution (nanometres cubed) |
| d | Average gap between molecules (nanometres) |
| One teaspoon of sugar | about 3 grams |
| Mass of one mole of sucrose | 342 grams |
| Avogadro constant | 6.02×10²³ molecules |
| Volume of 200 millilitres of water | 2×10²³ nanometres cubed (1 millilitre = 10²¹ nanometres cubed) |
| Wavelength of green light | about 500 nanometres |
| Moles of sugar | 3 ÷ 342 ≒ 0.00877 mol |
| Number of molecules (in units of 10²¹) | 0.00877 × 602 ≒ 5.28 (that is, about 5.3×10²¹) |
| Volume per molecule (dividing by the count in units of 10²¹) | 200 ÷ 5.28 ≒ 37.9 nanometres cubed |
| Its cube root (the average gap) | The cube root of 37.9 is about 3.36 nanometres |
| How many times the gap fits in a light wave | 500 ÷ 3.36 ≒ 149 times |
Even one teaspoon holds about 5.3×10²¹ molecules (5.3 sextillion), yet the gap per molecule is only about 3.4 nanometres. To a light wave, they are so fine that about 150 molecules fit along the length of a single wave, and light passes through without sensing any unevenness.
| Mass of one mole of water | 18 grams (200 millilitres of water is 200 grams) |
| Moles of water | 200 ÷ 18 ≒ 11.1 mol |
| Water molecules per sugar molecule | 11.1 ÷ 0.00877 ≒ 1266 |
Even in tea that tastes sweet, there is only one sugar molecule for roughly every 1,300 water molecules.
High schoolHigh school+Why Can Water Pull Sugar Apart?
High schoolA sucrose molecule has eight spots where oxygen and hydrogen are joined (hydroxy groups). These spots and water molecules attract each other through hydrogen bonds. The force pulling molecules together in the crystal and the force of water pulling them apart compete, and the water side wins, so the sugar dissolves.
High school+Solubility changes a lot with temperature. According to values in chemistry handbooks, sucrose dissolves at about 200 grams per 100 grams of water at 20℃ and about 480 grams at 100℃. If you dissolve a lot while hot and then cool it, the part that can no longer stay dissolved comes out as crystals. This is the principle behind making rock candy and alum crystals.
UniversityHow Fast It Spreads, and How Light Scatters
The speed at which molecules spread from the concentrated side to the dilute side in unstirred sugar water is described by Fick's law. The diffusion coefficient of sucrose in water is about 0.5×10⁻⁹ square metres per second at 25℃. The square of the distance spread is proportional to "2 × diffusion coefficient × time," so spreading 1 centimetre takes about 100,000 seconds, roughly a full day.
The scattering of light by particles much smaller than its wavelength is called Rayleigh scattering, and its strength drops off sharply, in proportion to the sixth power of particle size. Moreover, scattering from sparsely scattered molecules cancels out, leaving only weak scattering from "fluctuations" in concentration. That is why sugar water looks almost perfectly clear.
📖 Derivations and further reading: Fick's laws of diffusion (Wikipedia, Japanese) ／ Rayleigh scattering (Wikipedia, Japanese)
ResearchWhat We Still Don't Know
- How do water molecules arrange themselves around sugar? How the water around a sugar molecule is arranged, and how quickly that arrangement gets swapped out, is being studied both with computer simulations of molecular motion and with experiments.
- How does the very first molecule come off a crystal surface? We know dissolving starts at corners and steps, but how it proceeds molecule by molecule is still being studied as observation techniques improve.
- How molecules move in concentrated sugar water. In syrup-thick solutions, molecular motion slows sharply. This slowing is studied as a problem linked to how glass solidifies.
So the content of this article is also an "explanation within what is known today." Even a familiar phenomenon like dissolving still leaves open questions at the molecular scale.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Grade 7 science: "Properties of aqueous solutions," "Solubility and recrystallization" | Why it turns clear when dissolved; why the weight doesn't change |
| High school | Basic Chemistry: "Amount of substance"; Chemistry: "Dissolution and solutions" | Calculating the number and spacing of molecules; hydrogen bonds |
| High school+ | Chemistry: "Solubility curves" | How the amount that dissolves changes with temperature; crystal making |
| University | Physical chemistry (diffusion), optics (scattering) | Fick's law, Rayleigh scattering |
| Research | Solution chemistry, molecular simulation | Water arrangement around molecules; the first step of a crystal dissolving |
| ― | Connection to daily life | Stir a drink well before drinking; boiling down sugar water brings the crystals back |
- The Chemical Society of Japan (日本化学会), ed., Kagaku Binran, Kiso-hen (化学便覧 基礎編), Maruzen Publishing (solubility of sucrose)
- Solubility (Wikipedia, Japanese)
- Avogadro constant (Wikipedia, Japanese)
- Hydrogen bond (Wikipedia, Japanese)
- P. Atkins, J. de Paula, Atkins' Physical Chemistry, Oxford University Press (diffusion and Fick's law)
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), Commentary on the Junior High School Course of Study (announced 2017), Science volume (中学校学習指導要領(平成29年告示)解説 理科編) (properties of aqueous solutions)
※This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the mechanism. The molecular spacing is an estimate that ignores the change in volume caused by dissolving.