The Boatyard Change Log: Which Rule Spots a Reaction?

Four Changes at Gull Point

1Every summer, the Gull Point Boatyard takes on two teen helpers. This year, seventh graders Marisol Ibarra and Callum Reid helped the yard manager, Mr. Teodor Bratu, keep a change log for the boats in the yard. When a part changes, Mr. Bratu wants to know whether a new substance formed. A chemical reaction on a metal part often means that the metal is being used up, so the part may need to be replaced.

2Scientists identify a pure substance by its characteristic properties, such as its density, its melting point and whether it dissolves in water. These properties stay the same for a substance no matter how big the sample is. The yard has a digital scale, a graduated cylinder for finding the volume of solids that sink in water, and a hot block that Mr. Bratu can use to heat a small sample to 300 °C.

3The students logged four changes. In Change A, Mr. Bratu cut an aluminum bracket in half with a saw. In Change B, he melted a solder joint on a boat's wiring and let it cool into a new joint; the solder melts at about 227 °C. In Change C, the students looked at zinc anodes, blocks of zinc bolted to a boat's hull so that seawater wears away the zinc instead of the propeller. After one season in the water, the old anode was covered with a white crust. The students scraped some zinc from a new anode and some crust from the old one. In Change D, Mr. Bratu heated the end of a copper water pipe with a torch while fixing a leak. The hot end turned black, and after it cooled, the students collected black flakes that came off it.

4Marisol wrote three rules for a quick check of any change. Rule 1: flag a reaction if the mass changed. Rule 2: flag a reaction if the color or shape changed. Rule 3: flag a reaction if the density, the melting behavior or the way the sample dissolves changed, as long as both samples are in the same state.