Ninette wants to sweeten her iced tea. She looks in the restaurant's sugar dish and sees regular white granulated sugar and a packet of natural sugar. She opens each and sees that the natural sugar has very large crystals, much larger than the granulated sugar. Which sugar would be more likely to dissolve without Ninette having to stir the tea? Explain.
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5 responses so far ↓
1 Chris2457 // Mar, 2008
The granulated sugar would be more likely to dissolve than the natural sugar. This is because the granulated sugar crystal has been cut more than the natural and has more total surface area. With increased surface area, the tea touches more of the crystals, and thus it dissolves them faster. With any solute on Earth: the more surface area you have, the faster the substance dissolves.
2 cattbarf // Mar, 2008
Granulated sugar. The smaller crystals have a larger surface area. The exposure of surface area to water is proportional to the rate of dissolution.
3 SL // Mar, 2008
The granulated sugar. Since the granulated sugar is smaller, it has a larger exposed surface area, which allowed it to be "reacted" to the iced tea. Therefore, it would most likely dissolve without being stirred.
4 kentucky // Mar, 2008
Smaller particles have more surface area than large particles. More surface area equals more contact with the solvent to aid in dissolving, so the granulated sugar will dissolve faster.
5 m B // Mar, 2008
Nice, short answers above, yeah? I think it's even nicer if you can picture it, rather than memorise all the rules.
Let's be ridiculously extreme and say you dropped a single giant crystal of sugar into the tea. Ask yourself, why is everybody talking about surface area? This is because you do not expect a sugar molecule from the middle of the chunk of sugar to tunnel through the solid block of sugar and appear in solution. Only the sugar at the surface of the chuck will "peal off" and go into the solution. Just picture this happening at a certain rate for a single chunk of sugar.
Now picture the chunk being split horizontally in the middle. Two chunks now, same amount (mass) of sugar, but now with two extra surfaces exposed to the tea/water. This is the "more surface area" you get when dividing the sugar. In addition to the sugar pealing off the same surfaces that the single chunk had, there is the sugar pealing off these new surfaces, so the rate at which the sugar is dissolving is higher than for the single chunk.
Divide the two chunks again and you've exposed another two surfaces, and the rate of sugar dissolution goes up more. The size of the individual chunks, you'll notice, becomes smaller with each division. As you divide the sugar again and again, you expose increasingly more sugar surface *for the same mass* of sugar, and each sugar chunk get smaller and smaller. This is how we get the rule "Smaller crystals have relatively larger surface area". The "relative" here is in comparison with a single massive crystal.
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