Hi, Maddi and Matt - First we need a mental picture for the water particles (molecules) ...
Imagine all 30 pupils in class 6C are in a street dance workshop at school! They are like the molecules of water. They are moving in 3 kinds of ways:
- They are vibrating from side to side...
- They are rotating (spinning) as well (perhaps on their head!) this takes more energy...
- And they are also moving their feet to cross the floor (translating) ... this takes the most energy!
Can you imagine all 30 pupils doing all three in the gym?! In the same way, the molecules of all liquids such as water undergo madly hectic, non-stop movement.
This is quite a good way to think about how the molecules of a liquid behave all the time!
Now for a mental picture of the sugar particles (molecules)
What do you think would happen if you introduce,say, half a dozen new pupils to the group? With all the vibrating, rotating and translating, the newcomers would be mixed into the group quite quickly without any effort! If you watched, you would see them being buffeted around from one place to another by the twirling pupils (water molecules). This random, zigzag movement is called Brownian motion.
In the same way, sugar molecules show Brownian motion as they are knocked around by the ever-moving water molecules until they spread out (diffuse) throughout the container of water. Once spread out (diffusion), they then fit into the spaces between the water molecules rather than take up their own space - which is why the sugar seems to disappear.
And finally - why does sugar dissolve faster in hot water than cold water?
Even ice-cold water has a temperature (*why? see note below) - this means the water molecules have energy (heat energy). They use this energy to vibrate, rotate and translate.
The hotter the temperature, the more heat energy the molecules have.
Therefore, the water molecules will be vibrating, rotating and translating with more energy - and this speeds up the spread of the sugar molecules among the water molecules.
This is why sugar appears to dissolve faster in hot water. Perhaps you would like to check with the street-dancing model - pump up the volume, speed up the track and watch the energy of the dancers rise and the new pupils spread out or diffuse more quickly!
* Note: Why? Because, if the temperature drops down to 273.15 degrees C below 0 degrees C, then you would notice the translating stop first, then the rotating and finally even the spinning would stop.