1. Sugar Does Not Vanish; It Spreads Out as Tiny Separate Pieces
Sugar is a solid made of sucrose molecules. The LibreTexts chemistry textbook describes what happens when it dissolves: the molecules become uniformly distributed among the molecules of water, and they stay that way. Even though sucrose molecules are heavier than water molecules, gravity does not make them settle out over time [1]. So the dissolved sugar stays spread evenly through the water instead of sinking.
Because nothing has left the glass, you can test this at home. Weigh a glass of water and a spoonful of sugar on a kitchen scale, then stir them together and weigh the glass again. The total should not change. If you let a little sugar water sit in a shallow dish for several days, the water slowly evaporates and sugar crystals can reappear. The sugar was in the water all along.
2. Why Water Molecules Can Pull a Solid Apart
A water molecule is not evenly balanced. Its oxygen end is slightly negative and its two hydrogen ends are slightly positive, and because the molecule is bent, those charges do not cancel out [2]. You can think of each water molecule as having a slightly negative side and a slightly positive side.
That uneven charge is what lets water grab other things. When an ionic compound such as table salt enters water, it is surrounded by water molecules, which turn their negative ends toward positive pieces and their positive ends toward negative pieces. This surrounding is called hydration [2]. A second LibreTexts chapter describes the same idea in general terms: when the solvent is water, each particle of the dissolved substance becomes surrounded by water molecules [3]. Sugar is not made of charged pieces, so its story is a little different, but the answer to our question is the same: the sugar molecules end up mixed in among the water molecules, not gone [1].
3. Why Table Salt Splits Into Electrically Charged Pieces in Water
Table salt is sodium chloride. When it dissolves, the crystal comes apart into positively charged sodium ions and negatively charged chloride ions, each one surrounded by water molecules [2]. Sugar behaves differently. Its molecules stay whole when they dissolve [1]. Arrhenius's work on how salt solutions behave, described below, was about exactly these ions.
This idea was once controversial. In the 1880s a Swedish student named Svante Arrhenius studied how salts dissolve in water and conduct electricity. His doctoral work concerned exactly that, and his theory said that salts divide into ions spontaneously when they dissolve [4]. The Nobel Prize site says his doctoral thesis was criticized at Uppsala University, and that he left for Germany with help from the chemist Wilhelm Ostwald [5]. Uppsala University adds that the theory soon won recognition and became a foundation of modern physical chemistry [4]. He received the Nobel Prize in Chemistry in 1903 for this work [5].
4. Why There Is a Limit to How Much Sugar or Salt Water Can Hold
Water can dissolve a lot of sugar, but not an endless amount. Keep adding spoonfuls and eventually some sugar stays at the bottom no matter how long you stir. At that point the water is called saturated. Different substances have very different limits.
Sugar is a high-end example. One reference lists the solubility of sucrose in water at 20°C (68°F) as 2.01 g per mL of water, which is a little over 200 g for every 100 mL [6]. We did not check the original data, so please treat these numbers as rough guides. Table salt reaches its limit much sooner: about 360 g per liter of water at 25°C (77°F), which is roughly 36 g per 100 mL [7]. In other words, in the same amount of water you can dissolve far more sugar than salt. Temperature also matters. For sugar, the same sucrose table lists 259 g per 100 mL at 50°C (122°F) and 369 g per 100 mL at 80°C (176°F) [6]. So at those temperatures, warmer water can dissolve more sugar than cooler water.
One thing to keep apart: stirring or crushing a lump into powder helps sugar dissolve faster, but our sources give no reason to think it raises the limit. That is our own reading of how the two ideas differ, not a claim from the sources.
5. How to Test the Limit of Sugar in Water Yourself
With an adult, measure the same volume of water into two clear glasses. Keep one at room temperature and ask the adult to warm the other with warm tap water, not hot enough to burn. Add one level spoonful of sugar at a time to each, stir, and count how many spoonfuls disappear before some stays at the bottom. Then compare the counts. Use only clean food-grade sugar and clean glasses, and keep warm water away from small children.
If you want to read further, look up how scientists tested and refined Arrhenius's theory of ions after 1903. The sources below are a good place to begin.