Why does sugar dissolve in water? Sugar dissolves in water because water molecules are polar and can interact strongly with the many hydroxyl groups on sugar molecules. Water molecules surround the sugar, form hydrogen-bonding interactions, pull individual molecules away from the crystal surface and disperse them throughout the liquid. The sugar has not vanished or been destroyed; its molecules are still present, but they are separated and mixed at molecular scale.
People searching for why sugar dissolves in water, why sugar dissolves faster in hot water, whether stirring changes solubility, why sugar does not dissolve in oil, what saturation means and how sugar crystals form again are asking about polarity, intermolecular forces, diffusion and thermodynamics at the same time. Dissolving is a physical process governed by interactions between solute and solvent, not a chemical reaction that converts sugar into a different substance.
For students, sugar water is a useful molecular model. A crystal is an ordered structure held together by attractions among sugar molecules. Water competes with those attractions. If the interactions between sugar and water are favourable enough, molecules leave the crystal, become hydrated and spread through the solution until a dynamic balance is reached.
The Short Answer: Water Pulls Sugar Molecules Apart
Sugar crystals contain molecules packed together in an ordered lattice. Water molecules interact with exposed sugar molecules at the surface and begin separating them from neighbours.
Once detached, each sugar molecule becomes surrounded by water and can diffuse through the liquid, making the crystal appear to disappear.
Why Water Is Polar
A water molecule has an uneven distribution of electrical charge because oxygen attracts shared electrons more strongly than hydrogen.
That polarity lets water form strong attractions with many other polar molecules, including sugars.
Why Sugar Is Polar
Common sugars contain many oxygen-rich hydroxyl groups. These groups carry partial charges that interact strongly with polar water.
The numerous interaction sites make sugar much more compatible with water than with nonpolar liquids such as cooking oil.
What Hydrogen Bonding Does
Hydrogen bonds form between partially positive hydrogen atoms and electronegative oxygen atoms on nearby molecules.
Water can form hydrogen bonds with sugar’s hydroxyl groups, helping stabilise sugar molecules after they leave the crystal.
Why the Crystal Does Not Stay Intact
Molecules at the crystal surface are held by attractions to neighbouring sugar molecules, but water molecules collide with the surface continuously.
When sugar-water interactions compensate for breaking sugar-sugar attractions, molecules can detach and enter solution.
Why Sugar Seems to Disappear
Dissolved sugar molecules are too small and too widely dispersed to see individually. The visible crystal vanishes because its organised structure has been dismantled.
The mass remains in the liquid, which is why sugar water weighs more than the same amount of pure water.
Why Dissolving Is Not Melting
Melting turns a solid into a liquid through heating while keeping it as the same pure substance. Dissolving disperses one substance among molecules of another.
Sugar can dissolve in cold water without ever becoming liquid sugar.
Why Dissolving Is Not a Chemical Reaction
Sucrose molecules remain sucrose molecules in ordinary sugar water. No new substance must be formed for dissolution to occur.
The process can usually be reversed by removing water, which allows sugar crystals to form again.
Why Hot Water Dissolves Sugar Faster
Higher temperature increases molecular motion. Water molecules move and collide with the crystal surface more rapidly, while dissolved sugar diffuses away faster.
The faster transport reduces the concentration build-up at the surface and speeds the visible disappearance of the crystal.
Why Hot Water Can Dissolve More Sugar
For sucrose, equilibrium solubility rises strongly with temperature. Hotter water can therefore hold a larger amount before becoming saturated.
This is a thermodynamic property of the sugar-water system, not merely a consequence of faster stirring or mixing.
Why Stirring Speeds Dissolving
A layer of sugar-rich solution forms near the crystal surface. Stirring replaces that layer with less concentrated water.
This maintains a stronger concentration gradient, allowing sugar to leave the crystal more rapidly.
Why Stirring Does Not Change Final Solubility Much
At a fixed temperature, stirring changes how quickly equilibrium is reached but usually does not change the equilibrium amount that can dissolve.
Once saturation is reached, continued stirring cannot make unlimited sugar disappear.
Why Smaller Sugar Crystals Dissolve Faster
Crushing sugar increases total surface area exposed to water. More molecules can leave the solid at the same time.
Powdered sugar therefore dissolves faster than a large sugar cube of equal mass under similar conditions.
Why a Sugar Cube Takes Longer
A cube has less exposed surface area relative to its mass and water must progressively reach deeper material as the outside dissolves.
The cube can break apart during dissolution, which increases area and accelerates the later stages.
What Diffusion Does
Once sugar molecules enter solution, random molecular motion spreads them from high-concentration regions toward lower-concentration regions.
Diffusion helps make the final solution uniform even without vigorous stirring, although the process is much slower.
Why Sugar Eventually Spreads Through the Whole Glass
Molecules are in constant thermal motion. Random collisions gradually redistribute dissolved sugar until concentration differences become small.
Gravity does not make dissolved sugar simply sink because molecular mixing dominates at this scale.
Why Sugar Water Can Still Look Clear
Individual sugar molecules do not scatter visible light strongly enough to make an ordinary solution cloudy.
A clear appearance therefore does not mean the water is chemically pure.
Why Sugar Water Tastes Sweet Everywhere After Mixing
Once concentration becomes nearly uniform, any sip contains similar numbers of dissolved sugar molecules.
The tongue detects those molecules when they bind sweet-taste receptors.
Why Saturation Happens
At a given temperature, there is a limit to how much sugar can remain dissolved at equilibrium.
Beyond that point, molecules enter and leave crystals at equal average rates, so adding more solid leaves visible sugar undissolved.
What a Saturated Solution Is
A saturated solution contains the maximum equilibrium concentration of dissolved solute under the stated conditions.
It is not necessarily thick or visibly crystalline; saturation is a molecular balance, not a texture description.
Why More Sugar Collects at the Bottom
Once the liquid is saturated, additional sugar cannot all enter solution. Gravity causes the undissolved solid to settle.
The layer at the bottom is excess solid coexisting with saturated liquid above it.
Why Temperature Changes Saturation
Solubility depends on the energy and entropy changes associated with separating crystal molecules and hydrating them in water.
For sucrose, higher temperature favours a much larger dissolved concentration.
Why Cooling Can Make Sugar Crystallise
A hot saturated solution can hold more sugar than the same solution after cooling. As temperature falls, the excess dissolved sugar becomes unstable.
Molecules then return to an ordered solid phase if suitable crystal nuclei are available.
Why Rock Candy Forms
A concentrated hot sugar solution is allowed to cool slowly around a string or seed crystal.
Sugar molecules leave solution and attach to an ordered lattice, building large crystals over time.
Why Seed Crystals Matter
Starting a new crystal requires molecules to arrange into a stable ordered nucleus. That nucleation step can be difficult.
An existing crystal surface provides the correct structure, making further growth easier.
What Supersaturation Is
A supersaturated solution temporarily contains more dissolved sugar than is stable at that temperature.
It can remain clear until a disturbance or seed crystal triggers rapid crystallisation.
Why Candy Making Depends on Sugar Solubility
Heating sugar syrups removes water and allows very high sugar concentrations. Cooling conditions then determine whether sugar crystallises or remains glassy.
Fudge, fondant, caramel and hard candy differ partly because cooks control water content and crystallisation differently.
Why Caramelisation Is Different From Dissolving
Caramelisation occurs when sugar is heated strongly enough to undergo chemical reactions that create new compounds, colours and flavours.
Dissolving in water merely disperses intact sugar molecules unless additional chemistry occurs.
Why Sugar Dissolves Poorly in Oil
Cooking oils are mostly nonpolar hydrocarbons and triglyceride regions that cannot form favourable interactions with sugar’s many polar hydroxyl groups.
Because sugar-oil attractions are weak, the crystal gains little advantage from leaving its own lattice.
Why “Like Dissolves Like” Is Useful
Polar solutes often dissolve well in polar solvents, while nonpolar substances often mix better with nonpolar solvents.
The rule is a simplification, but it captures the importance of compatible intermolecular forces.
Why Salt Also Dissolves in Water
Salt dissolves for a different molecular reason. Water stabilises positive and negative ions through ion-dipole interactions.
Sugar remains neutral molecules in solution, while salt separates into ions.
Why Sugar Does Not Conduct Electricity Like Salt
Dissolved sucrose molecules carry no net charge and do not create a large population of mobile ions.
Salt solutions conduct much better because sodium and chloride ions move independently through the water.
Why Sugar Lowers Water’s Freezing Point
Dissolved sugar changes the chemical potential of water and makes formation of an ordered ice phase less favourable.
The solution therefore freezes at a lower temperature than pure water, an effect important in ice cream and frozen desserts.
Why Sugar Raises Boiling Point Slightly
Dissolved nonvolatile sugar lowers water’s escaping tendency, so a somewhat higher temperature is required for vapour pressure to equal external pressure.
The effect increases with concentration.
Why Sugar Changes Density
Adding dissolved sugar increases mass substantially while volume does not increase by the same proportion.
Concentrated sugar solutions are therefore denser than pure water.
Why Dense Sugar Solutions Can Form Layers
Solutions with different sugar concentrations have different densities. If poured carefully, a concentrated layer can sit below a dilute one temporarily.
Diffusion eventually mixes the layers unless temperature or other conditions maintain the gradient.
Why Sugar Changes Refractive Index
Dissolved molecules alter how electromagnetic waves propagate through the liquid.
Refractometers exploit this effect to estimate sugar concentration in fruit juice, brewing and food processing.
Why Brix Is Used
Degrees Brix is commonly used to describe soluble solids in solutions, especially sugar-rich liquids.
In simple sucrose solutions it closely tracks sugar concentration, while real juices contain other dissolved materials too.
Why Honey Dissolves in Water
Honey contains high concentrations of glucose, fructose and other polar molecules that interact strongly with water.
Adding water lowers viscosity and separates the sugars further, allowing the mixture to disperse rapidly with stirring.
Why Syrup Mixes More Slowly Than Granulated Sugar Sometimes
A thick syrup has high viscosity, so bulk mixing can be slow even though the sugars are already dissolved.
The limitation becomes fluid motion rather than crystal dissolution.
Why Cold Drinks Take Longer to Sweeten
Lower temperature slows molecular motion and often lowers sugar solubility.
Stirring helps, but very concentrated cold drinks can still retain undissolved crystals longer than hot ones.
Why Powdered Sugar Can Clump
Very fine particles absorb moisture and can stick together. Starch is often added commercially to reduce caking.
Clumps may initially dissolve more slowly than separate particles despite the powder’s high potential surface area.
Why Brown Sugar Dissolves Easily
Brown sugar contains sucrose crystals coated with molasses. Both sucrose and many molasses components are water-compatible.
The sticky coating changes flavour and handling but does not prevent dissolution.
Why Sugar Can Preserve Jam
Very high sugar concentration lowers water activity, making water less available for microbial growth.
The preservation effect is related to dissolved sugar interacting with water, not to the sugar acting as a disinfectant.
Why Too Much Water Shortens Jam Shelf Life
Dilution raises water activity and makes the environment more favourable for yeasts and moulds.
Food preservation therefore depends on concentration, acidity, heat treatment and storage together.
Why Sugar Dissolution Is a Dynamic Equilibrium
In a saturated solution, molecules continually leave the crystal and others continually rejoin it.
The visible amount stays constant because the opposing molecular rates are equal on average.
Why Equilibrium Is Not Stillness
Molecular motion continues even when macroscopic concentration no longer changes.
This distinction is central to chemistry: equilibrium describes balanced rates, not frozen particles.
Why Dissolution Can Be Endothermic or Exothermic
Different solutes absorb or release different amounts of heat when dissolving because lattice breaking and hydration have competing energy effects.
Sugar dissolution has its own thermal signature, while salts can behave very differently.
Why Entropy Matters
Dissolving usually increases the number of possible molecular arrangements because ordered crystal molecules become dispersed through the solvent.
That increase in configurational freedom can help drive dissolution even when some energy is required to separate the solid.
Why Solubility Is Not Just About Bond Strength
A crystal may be strongly held together, but strong solute-solvent interactions and entropy can still favour dissolution.
Predicting solubility therefore requires comparing the entire free-energy balance rather than naming one bond.
Why Sugar Molecules Do Not Sink as Molecules
A dissolved molecule is constantly struck by surrounding water molecules. Brownian-scale thermal motion overwhelms any simple downward settling of individual sucrose molecules.
Only undissolved crystals large enough to behave as particles settle visibly.
Why Evaporation Makes Sugar More Concentrated
When water molecules escape into the air, sugar remains behind because sucrose is essentially nonvolatile under ordinary conditions.
The remaining solution becomes more concentrated and can eventually reach saturation.
Why Dried Sugar Water Leaves Crystals
As nearly all water evaporates, dissolved sugar molecules are forced closer together and ultimately form solid material again.
The residue proves that sugar remained present throughout the period when the liquid looked clear.
Why Sugar Dissolution Is a Good Molecular Lesson
The process shows how invisible intermolecular attractions control visible behaviour.
A crystal disappears macroscopically because billions of molecules change neighbours from sugar molecules to water molecules.
Why Sugar Dissolution Is a Good Thermodynamics Lesson
Temperature, concentration and equilibrium determine how much dissolves and whether crystals grow.
The familiar kitchen process therefore contains the same free-energy principles used throughout chemistry.
Why Sugar Dissolution Is a Good Kinetics Lesson
Stirring, temperature and particle size change the rate of dissolution without necessarily changing equilibrium solubility.
This distinction between rate and final state is one of the most useful ideas in physical chemistry.
Common Myths About Dissolving Sugar
Sugar does not disappear, stirring does not allow infinite sugar to dissolve and hot water does not chemically destroy sucrose under ordinary conditions.
Dissolution is molecular mixing governed by solvent interactions and equilibrium.
Frequently Asked Questions
Why does sugar dissolve in water? Water forms favourable polar and hydrogen-bonding interactions with sugar. Why does hot water work faster? Molecular motion and transport increase, and sucrose solubility also rises. Why does oil not dissolve sugar well? Oil is largely nonpolar.
Can sugar come back out? Yes. Cooling or evaporating a concentrated solution can produce crystals.
Where to Go Next
Sugar dissolution connects naturally to the chemistry, food-science and water topics across eduKateSingapore.
Sugar dissolves in water because water molecules replace the intermolecular neighbours that held sugar in a crystal. The visible solid disappears, but the molecules remain present—separated, hydrated and moving through the solution.
Why Sugar Dissolves Molecule by Molecule
The crystal does not usually break apart into visible chunks first and then become liquid. Water molecules attack the crystal surface at molecular scale, surrounding exposed sucrose molecules and allowing them to detach one by one or in very small groups.
The shrinking crystal we see is the macroscopic result of countless microscopic departures happening over the entire wetted surface.
Why the Crystal Surface Matters Most at First
Only molecules exposed to water can leave directly. Molecules buried deep inside the crystal remain shielded until outer layers dissolve or cracks expose new surfaces.
This is why increasing surface area by crushing the crystal changes dissolution rate so strongly.
Why Water Molecules Reorient Around Sugar
Water’s partial charges allow individual molecules to rotate so that favourable regions face the oxygen and hydrogen atoms on sucrose hydroxyl groups.
That local organisation creates a hydration shell around the dissolved sugar molecule, stabilising it inside the liquid.
Why Hydration Shells Matter
A dissolved sugar molecule is not floating naked in empty space. It is constantly surrounded and struck by water molecules arranged transiently according to intermolecular attractions.
This dynamic shell helps explain why sugar remains dispersed instead of immediately rejoining the crystal.
Why Hydrogen Bonds Are Temporary
Individual hydrogen bonds in liquid water form and break rapidly. The important point is not one permanent bond but a constantly changing network of many favourable interactions.
Sugar remains soluble because new water-sugar interactions keep replacing old ones as molecules move.
Why Sugar Molecules Still Collide With One Another
Dissolved molecules are not isolated completely. They diffuse randomly and sometimes approach each other closely.
In dilute solution those encounters are brief, while in concentrated solution they become frequent enough that crystallisation becomes more likely under suitable conditions.
Why Concentration Matters
Concentration describes how much solute is present relative to the amount or volume of solution. As more sugar dissolves, the average spacing between sucrose molecules decreases.
High concentration changes viscosity, density, sweetness, freezing point and the likelihood that crystals will reform.
Why Dilute Sugar Water Is Easy to Mix
When only a small amount of sugar is present, many water molecules are available to surround each sucrose molecule and viscosity remains close to that of pure water.
Diffusion and convection can therefore redistribute the solute relatively quickly.
Why Concentrated Syrup Is Viscous
At high sugar concentration, molecules crowd one another and interact frequently. Molecular movement becomes more hindered, increasing resistance to flow.
The syrup pours slowly not because sugar has become solid again, but because the liquid phase is highly crowded.
Why Viscosity Slows Further Dissolution
In a thick concentrated solution, dissolved sugar moves away from the crystal surface more slowly and fresh water reaches the surface less efficiently.
This transport limitation helps explain why the last portions of sugar can dissolve much more slowly near saturation.
Why Stirring Is Most Helpful Before Saturation
When unsaturated water surrounds a crystal, stirring continually brings solvent capable of accepting more sugar into contact with the surface.
Near saturation, mixing still speeds approach to equilibrium but cannot create unlimited capacity because the thermodynamic limit remains.
Why Boiling Can Dissolve Very Large Amounts of Sugar
Sucrose solubility rises sharply with temperature, so near-boiling water can hold far more sugar than cold water.
Candy makers exploit this by dissolving large sugar masses hot, then controlling cooling and water loss to determine final texture.
Why Hot Syrup Can Crystallise on Cooling
As temperature falls, the solution’s equilibrium solubility decreases. The liquid can temporarily contain more sugar than is stable.
Once nucleation begins, excess molecules join growing crystals until concentration returns toward the new equilibrium value.
Why Scratching a Pan Can Trigger Crystals
Rough surfaces or stray crystals provide nucleation sites where sucrose molecules can arrange into an ordered lattice more easily.
Confectioners therefore control agitation and surface cleanliness carefully when they want a smooth glassy candy rather than crystals.
Why Fudge Wants Small Crystals
Fudge is intentionally crystallised, but the goal is to create many tiny crystals rather than a few large ones. Fine crystals produce a smooth texture.
Temperature and beating are timed so nucleation occurs in a controlled way after the syrup has cooled sufficiently.
Why Hard Candy Avoids Crystals
Hard candy is cooled into an amorphous glass in which sugar molecules become immobilised without forming a regular crystal lattice.
Interfering sugars, acids and rapid cooling can help suppress crystallisation and preserve the glassy state.
Why Corn Syrup Can Prevent Sucrose Crystallisation
Glucose and other sugars interfere with the regular arrangement required for a pure sucrose crystal.
Adding a mixture of different molecules makes orderly lattice formation more difficult, which is useful in confectionery.
Why Invert Sugar Behaves Differently
Hydrolysing sucrose creates glucose and fructose. The mixture has different sweetness, hygroscopicity and crystallisation behaviour from pure sucrose.
Bakers and confectioners exploit these differences to control texture and moisture retention.
Why Lemon Juice Can Affect Sugar Syrup
Acid can catalyse sucrose hydrolysis during heating, creating some glucose and fructose. Those additional sugars interfere with sucrose crystallisation.
The effect is chemical and should be distinguished from the purely physical process of dissolving sugar in water.
Why Sugar Solutions Can Be Supersaturated Without Immediate Crystals
Nucleation has an energy barrier. Molecules must assemble into a sufficiently ordered cluster before a stable crystal can grow.
Without dust, scratches or seed crystals, a supersaturated solution can remain metastable for surprisingly long periods.
Why a Seed Crystal Can Cause Rapid Growth
An existing lattice removes much of the organisational difficulty. Dissolved sucrose molecules can attach directly to a matching crystal surface.
Once growth begins, the seed becomes larger as the solution sheds its excess dissolved sugar.
Why Rock Candy Crystals Grow Slowly
Slow evaporation or cooling supplies sugar to the crystal gradually, giving molecules time to attach in ordered positions.
Fast uncontrolled nucleation would create many smaller crystals instead of a few large transparent ones.
Why Sugar Dissolves Differently From Flour
Flour contains starch granules and proteins that do not form a true molecular solution in cold water. They suspend, hydrate and sometimes clump instead.
The visual disappearance of sugar therefore reflects true dissolution, while flour-water mixtures are largely dispersions.
Why Cocoa Does Not Dissolve Like Sugar
Cocoa powder contains many insoluble particles and fat. It forms a suspension when mixed with water or milk rather than a simple molecular solution.
This is why cocoa can settle or form clumps while dissolved sugar remains evenly distributed.
Why Milk Can Dissolve Sugar
Milk contains mostly water, so sucrose can dissolve in its aqueous phase. Proteins, fat droplets and minerals add complexity but do not remove water’s ability to hydrate sugar.
Warm milk therefore dissolves sugar readily while remaining an emulsion and colloidal system at the same time.
Why Alcohol Changes Sugar Solubility
Ethanol is less polar than water. As alcohol concentration rises, the solvent becomes less favourable for highly polar sugar molecules.
Sugar therefore dissolves less readily in high-proof alcohol than in pure water, which matters in liqueurs and food extraction.
Why Sugar Can Crystallise Out of Liqueurs
Temperature changes, evaporation or high alcohol content can reduce sugar solubility enough for crystals to form.
The process is the same equilibrium principle seen in water, but the mixed solvent changes the solubility limit.
Why Ice Cream Uses Dissolved Sugar
Sugar lowers the freezing point of the water phase, so not all water freezes at the same temperature as pure water.
This keeps part of the mixture unfrozen and contributes to a softer scoopable texture.
Why Too Much Sugar Makes Frozen Desserts Soft
Higher dissolved-solute concentration depresses freezing point more strongly. Less water becomes ice at a given freezer temperature.
Recipe formulation therefore balances sweetness with the amount of ice needed for structure.
Why Sugar Dissolution Matters in Fermentation
Yeast and other microbes access dissolved sugars more easily than large dry crystals because molecules are already dispersed in the liquid environment.
Dissolution therefore precedes many fermentation processes in bread, beverages and cultured foods.
Why Sugar Dissolution Matters in the Mouth
Sugar from food dissolves in saliva, allowing sweet receptors to detect it and oral bacteria to access fermentable carbohydrates.
The same molecular mobility that makes sweetness immediate also helps explain why frequent sugar exposure affects dental health.
Why Rinsing With Water Reduces Sugar Concentration
Adding water dilutes dissolved sugar and physically carries some molecules away when the liquid is swallowed or spat out.
Dilution does not instantly remove every molecule from tooth surfaces, but it changes the local chemical environment.
Why Sugar Dissolution Is Used in Laboratory Experiments
Sucrose solutions are easy to prepare at known concentrations and show clear changes in density, refractive index and osmotic behaviour.
They therefore provide simple models for studying solutions without handling strongly hazardous chemicals.
Why Osmosis Is Related but Different
Dissolving sugar changes the chemical potential of water. If a semipermeable membrane separates solutions, water can move by osmosis toward the side with lower water potential.
Dissolution creates the concentration difference; osmosis describes solvent movement across a selective barrier.
Why Sugar Solutions Create Osmotic Pressure
Dissolved solute lowers the tendency of water to move freely across a semipermeable membrane. Pressure can be required to prevent net water movement.
This colligative behaviour depends largely on the number of dissolved particles rather than their colour or sweetness.
Why Sugar Dissolution Is a Good Scale Lesson
At human scale, a crystal disappears. At molecular scale, nothing mysterious happened: ordered molecules became separated and hydrated.
The lesson is that macroscopic appearance can hide enormous numbers of microscopic interactions.
Why Rate and Solubility Must Be Separated
Powdering, stirring and warming can all make a crystal disappear faster, but only some of those changes alter the final equilibrium concentration.
This distinction prevents a common error: assuming anything that speeds dissolution must also increase how much can dissolve.
What Students Should Be Able to Explain
A strong answer should connect water polarity, sugar hydroxyl groups, hydrogen bonding, hydration and diffusion. It should then distinguish dissolution rate from equilibrium solubility.
If a learner can explain hot water, stirring, saturation and crystallisation using one molecular model, the topic has become transferable understanding rather than a kitchen fact.
The Big Picture
Sugar dissolves because water can stabilise separated sucrose molecules strongly enough to compete with the crystal lattice. Temperature and mixing control how fast the process happens, while thermodynamics controls how far it can go.
The clear solution is therefore not empty water. It is a dense molecular mixture in which every invisible sugar molecule remains present, hydrated and moving.
Why Sugar Water Is a True Solution
In a true solution, solute particles are dispersed at molecular or ionic scale and do not settle out simply because the container is left still. Sugar water meets this definition because individual sucrose molecules remain mixed through constant molecular motion.
This separates sugar water from suspensions such as muddy water, where much larger particles can eventually settle under gravity.
Why Filtering Cannot Remove Dissolved Sugar
Ordinary filter paper traps particles far larger than individual sucrose molecules. Dissolved sugar passes through with the water because the molecules are part of the liquid solution at microscopic scale.
To separate the sugar again, one must remove solvent, change solubility conditions or use a membrane process designed for molecular separation.
Why the Final Answer Is Molecular Competition
The core explanation is a competition between the attractions holding sucrose molecules inside the crystal and the attractions offered by surrounding water. When hydration plus the increase in molecular disorder is favourable enough, the crystal dissolves.
That single model explains why water works, oil works poorly, heat changes solubility, stirring changes rate and cooling can return the dissolved molecules to an ordered crystal.
The familiar glass of sugar water is therefore a complete lesson in solution chemistry: intermolecular forces explain why mixing is possible, kinetics explains how quickly it happens, thermodynamics explains the solubility limit, and crystallisation shows that the dispersed molecules can return to an ordered solid when conditions change.
Nothing has disappeared: the crystal has become a molecularly dispersed solution whose sugar can still be measured, tasted, concentrated and recovered.
The solvent changed the arrangement, not the chemical identity, of the sugar molecules.