Tarnish or Not? Property Clues in a Museum Lab

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Tarnish or Not? Property Clues in a Museum Lab

Calculate densities and compare them with a reference table to decide which changes made new substances.

Measuring the samples

Read the passage and study Table 1. Then answer the questions.

Clues on the Museum Shelf

1Marcus Bell and Yara Saleh spend their Saturdays as student interns in the conservation lab of the Lakeshore History Museum. The museum's conservator, Dr. Lena Hoffmann, cleans and repairs old objects. Before she treats a damaged surface, she wants to know whether it went through a chemical reaction that formed a new substance or only a physical change. The cleaning method depends on the answer.

2This month, four objects needed attention. A silver spoon from the 1890s had a black coating on its surface. A copper weather vane that had stood on the museum roof for 80 years, in sun, wind and rain, was covered with a green crust. A beeswax seal on an old letter had drooped in a hot attic and then hardened again in a new shape. A jar of rock candy in a damp display case had partly turned into sticky syrup, and after the case was dried, new clear crystals grew in the jar.

3Dr. Hoffmann scraped off small samples. Marcus and Yara found each sample's mass on a balance and its volume by how much it raised the liquid level in a small graduated cylinder: water for most samples, but vegetable oil for the candy crystals, which dissolve in water. They pushed the floating wax under with a thin wire. A university lab measured the melting points. A substance's characteristic properties, such as its density, its melting point and whether it dissolves in water, stay the same no matter how big the sample is. Density = mass ÷ volume.

4To identify each sample, the interns compared their results with a reference table of known substances from a chemistry handbook.

Table 1. The Interns' Measurements
Sample	Mass (g)	Volume (cm³)	Melting point (°C)	Dissolves in water?
Silver from an old fork	21	2	962	no
Black flakes from spoon	0.72	0.1	836	no
Copper from inside vane	18	2	1085	no
Green crust from vane	6	1.5	breaks down before melting	no
Wax seal after drooping	1.92	2	63	no
New candy crystals	4.77	3	186	yes

1.

The density of the black flakes is g/cm³, and the density of the green crust is g/cm³.

2.

The density of the new candy crystals is g/cm³.

3.

Marcus says the black flakes are probably still silver, because their mass and volume are much smaller than the fork sample's, and small samples always give smaller numbers. Which response is best?

A. He is wrong, because the flakes and the fork have the same melting point.B. He is right, because the flakes came off a spoon that was made of silver.C. He is wrong, because density does not depend on size, and 7.2 is not 10.5.D. He is right, because a smaller sample of silver has a smaller density.

4.

Why did Dr. Hoffmann measure properties instead of only looking at the color of each coating?

A. Color can change in ways that do not form a new substance, so it is a weak clue alone.B. Black and green look alike under lab lights, so the colors are easy to mix up.C. Black and green coatings are too thin to see well without a microscope.D. Measuring properties removes the coating, which cleans the object at the same time.

Compare with the reference table

Use Table 1, Table 2 and the passage to answer the questions.

Table 2. Reference Properties
Substance	Density (g/cm³)	Melting point (°C)	Dissolves in water?
Silver	10.5	962	no
Silver sulfide	7.2	836	no
Copper	9	1085	no
Copper carbonate	4	breaks down before melting	no
Beeswax	0.96	63	no
Sugar (sucrose)	1.59	186	yes

Silver sulfide is made of silver and sulfur atoms joined together. Green crusts on old copper roofs are often copper carbonate, which is made of copper, carbon, oxygen and hydrogen atoms joined together.

5.

Match each sample from Table 1 to the substance in Table 2 that it most likely is.

6.

What is the best conclusion about the black coating on the spoon?

A. The silver stayed the same substance and only got dirty with black dust.B. Silver atoms joined with sulfur atoms and formed a new substance.C. The silver dissolved in water from the air and dried in a new form.D. The silver melted in a warm room and cooled again as a black solid.

7.

Which two objects changed without forming a new substance? Choose two.

A. The beeswax sealB. The rock candyC. The copper weather vaneD. The silver spoon

8.

A visitor says the rock candy was ruined because it turned into syrup, a new substance. Which evidence best shows the sugar was still there?

A. The new crystals have the density and melting point of sucrose.B. The new crystals dissolve in water, and syrup is wet.C. The candy turned sticky while it sat in the damp case.D. The new crystals grew only after the display case was dried.

9.

Did a chemical reaction happen to the copper weather vane? Write a claim, give at least two pieces of property evidence from Tables 1 and 2, and explain your reasoning. Include where the extra kinds of atoms in the crust most likely came from.