Where Did the Sugar Go?
During a matter unit, Devon pours 200 g of water into a clear cup and places the cup on a balance. He adds 10 g of sugar and stirs until no crystals can be seen. The cup is open to the air. The balance reads 210 g before stirring and 210 g after. The sugar is no longer visible, and the water tastes sweet. Devon says, "The sugar disappeared. It was destroyed when it dissolved." His partner Maya says, "It is still in the water, just too small to see." Their teacher gives them a hot plate, a shallow dish, and a safe place to leave the cup overnight. The class must decide whose claim the evidence supports.
1. Whose claim does the balance evidence support? Explain your reasoning.
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The balance supports Maya. The total mass is 210 g both before and after stirring. If the sugar had been destroyed and left the cup as a gas, the mass would have dropped, just as an open cup of water loses mass when it evaporates. The mass did not drop, so the sugar matter is still in the cup. The sweet taste is more evidence that sugar is spread through the water. Devon is confusing 'cannot see it' with 'it is gone.' The sugar particles are too small to see, but they are still there.
2. Describe a test using the hot plate and shallow dish that could recover the sugar, and predict what you would observe and measure.
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Pour the sugar water into the shallow dish and warm it gently on the hot plate so the water evaporates slowly. Do not let it burn. As water leaves as vapor, the sugar should be left behind. You would see white crystals or a sticky film form in the dish. To check the amount, measure the mass of the empty dish first, then measure the dish plus the recovered sugar after it dries. If the sugar was not destroyed, the recovered mass should be close to 10 g, though some sugar may stick to the dish. Recovering sugar is evidence that dissolving does not destroy matter.
3. Why does the whole cup taste sweet, not just the bottom where the sugar was added?
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When the sugar dissolves, its particles separate and spread among the water particles. Stirring moves them around, and particles in a liquid move and mix. Because the sugar particles are spread throughout the water, a sip from the top tastes just as sweet as a sip from the bottom. This also explains why no crystals are visible: the pieces are now individual particles or very small groups, too small to see. The sweetness is evidence that the sugar is still present everywhere in the liquid.