Dissolved or Reacted? The Kettle Crust Test

The Crust in the Kettle

1The electric kettle in Dawit Bekele's kitchen has a hard white crust on its bottom. The crust, often called kettle scale, builds up when hard tap water is boiled again and again. Dawit's father removes it by soaking the kettle in white vinegar. Dawit's friend Saoirse Byrne said her grandmother does the same thing, and that the vinegar "just dissolves the crust, like sugar in tea." For the Fairmont Middle School science fair, the two seventh graders asked a question: does vinegar only dissolve kettle scale, or does it react with the scale and form new substances?

2Scientists identify a substance by its characteristic properties, such as its density, what happens when it is heated and its solubility, which tells whether it dissolves in a liquid. These properties stay the same no matter how much of the substance there is. A substance that only dissolves can usually be recovered by letting the water evaporate, and the recovered solid has the same properties as before.

3Dawit's father chipped some scale out of an old kettle. The students set up three clear cups, each with 100 mL of liquid at room temperature. Cup 1 held 3.0 g of scale pieces in tap water. Cup 2 held 3.0 g of scale pieces in white vinegar. Cup 3 held 3.0 g of table salt in tap water, because salt is known to dissolve without forming a new substance. They stirred each cup once an hour for 6 hours. Then they poured the liquid from each cup into a dish and let it evaporate on a sunny windowsill for 3 days. Both students wore safety goggles.

4Cup 2 bubbled as soon as the scale went in, and the pieces got smaller until none were left after about 4 hours. The pieces in Cup 1 did not bubble and looked the same after 6 hours, and the Cup 1 dish dried with almost nothing in it. The salt in Cup 3 disappeared within minutes without any bubbles. The dishes from Cups 2 and 3 each held white crystals.

5Ms. Ocampo, a chemistry teacher at the nearby high school, heated small samples in her lab and collected some of the gas from a repeat of Cup 2. She found that the gas had a density of 1.8 g/L, while room air had a density of 1.2 g/L. The students found the density of each solid from its mass and its volume. Because two of the solids dissolve in water, they measured every volume by how much the sample raised the level of vegetable oil in a small graduated cylinder. Their results are in Table 1.