Broken up, not vanished
Where Does Sugar Go in Water?
Dissolving pulls a solid apart into individual molecules spread through the liquid.
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Step 1 of 5
The sugar is still present.
Weigh the water before and after and it is heavier by exactly the sugar's weight. Nothing was destroyed — you can taste it.
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Step 2 of 5
Water molecules pull the crystal apart.
Water surrounds the crystal's surface and tugs individual molecules away. Each one drifts off separately into the liquid.
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Step 3 of 5
Individual molecules are invisible.
A sugar molecule is far smaller than a wavelength of light, so it cannot scatter light or be seen. Spread out, it disappears from view.
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Step 4 of 5
Stirring and heat make it faster.
Stirring brings fresh water to the surface; heat makes molecules move harder. Neither changes what happens, only how quickly.
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Step 5 of 5
There is a limit.
Once the water holds as much as it can, extra sugar just sits at the bottom. Hot water holds more, which is why it drops out as it cools.
Not everything dissolves in water. Oil molecules are not attracted to water at all, so water cannot pull them apart.
The short version
The sugar is all still there — the water weighs exactly that much more. It vanishes because a single molecule is smaller than a wavelength of light. Stirring only changes the speed, and cooling makes the extra drop back out.
Try it yourself
Weigh a glass of water, stir in a spoon of sugar, and weigh again. The extra weight is exactly the sugar.