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Glue, Resin and Plaster: Property Clues From a Helmet Design Camp

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Glue, Resin and Plaster: Property Clues From a Helmet Design Camp

Compare properties before and after each change to decide whether a new substance formed.

The glue and the resin

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

Building the Test Gear

1Every summer, the city's bike-safety program runs a design camp for middle schoolers at the community college. This year, seventh graders Tavita Leota and Kenji Moreau joined a team building a helmet that fits younger riders better. Before the team could test a single helmet, it needed three things: soft pads glued to the chin strap, small clips to hold the visor and a solid test head to hold each helmet. The camp leader, Ms. Rosalind Vanterpool, is a materials engineer. She handled every powder, hot tool and liquid resin herself, wearing gloves and goggles in a well-ventilated room, while the students measured and recorded.

2Kenji's notebook describes three changes. First, Ms. Vanterpool loaded solid sticks of glue into a hot-glue gun. The glue flowed out as a thick, sticky liquid and turned solid again within a minute as it cooled on the strap pads. Second, she mixed equal volumes of two clear liquids from a casting-resin kit, called Part A and Part B, and poured the mix into small clip molds. The cup grew warm, and after about 30 minutes each clip was a hard, clear solid.

3Third, she stirred 1,500 g of white plaster powder into 1,000 g of water and poured the creamy mix into a head-shaped rubber mold. Within 20 minutes the mold felt warm, and the plaster was hard. She took the head out of the mold and set it in the sun to dry. Tavita weighed it every day for a week, until its mass stopped changing. The dry test head had a mass of 1,780 g.

4Scientists identify a substance by its characteristic properties, such as its density, the temperature at which it melts and how it behaves in water or another liquid. These properties stay the same no matter how much of the substance there is. The team tested samples from before and after each change. Ms. Vanterpool did the heating and the acetone tests in a vented hood, and the students recorded the results in Tables 1 and 2.

5Kenji was still not sure about the test head. "It's like mud," he said. "Mud is wet, then the water dries out and leaves hard dirt." To check, Ms. Vanterpool ground a chip from the dry head into a fine powder. Then she ran the same two tests on the ground chip and on fresh powder from the plaster bag.

Table 1. Glue and Resin Samples (— = not tested)
SampleState at 20 °CDensity (g/cm³)Warmed to 120 °CMixes into acetone?
Glue stick, beforeSolid0.95Melts into a thick liquid—
Glue on a strap pad, afterSolid0.95Melts into a thick liquid—
Resin Part A, beforeLiquid1.18Stays a liquidYes
Resin Part B, beforeLiquid1.04Stays a liquidYes
Visor clip, afterSolid?Stays solid; does not meltNo
1.
Kenji lowers one visor clip into a measuring cylinder of water. The water level rises by 5.0 mL, so the clip's volume is 5.0 cm³. The clip has a mass of 5.7 g. Density = mass ÷ volume.

The density of the visor clip is g/cm³.

2.
Suppose Parts A and B had only mixed, with no new substance. Then a mix of equal volumes would have a density equal to the average of the two densities in Table 1.

That average density is g/cm³.

3.
Kenji says, "The clip's density is close to the average from question 2, so the resin just mixed and then dried hard." Which reply fits Table 1 best?
Question 3 options
4.
Tavita says the glue on the strap pad is a new substance, because it went from a solid stick to a runny liquid and back to a solid. Which reply fits Table 1 best?
Question 4 options
5.
Underline the two observations that are evidence, on their own, that a new substance formed when the resin hardened.

The plaster test head

Use paragraphs 3 and 5 and Table 2 to answer the questions.

Table 2. Plaster Tests (heating done in an open dish in an oven)
SampleMixed with waterMass lost at 200 °C
Fresh plaster powder from the bagWarms and hardens in about 20 minutesAbout 6% of its mass
Ground chip from the dry test headStays a soft paste; does not warmAbout 21% of its mass
6.
Use paragraph 3.

The dry test head has g more mass than the plaster powder used to make it.

7.
Tavita dried the head until its mass stopped changing, yet it still had more mass than the powder. Which explanation fits paragraph 3 best?
Question 7 options
8.
Which conclusion about the test head do the results in Table 2 support best?
Question 8 options
9.
The next summer, the camp finds an unlabeled tub of white powder. Ms. Vanterpool tests a spoonful. Mixed with water, it stays a soft paste and does not warm. Heated at 200 °C, it loses about 21% of its mass. What is the powder most likely, and will it work for casting a new test head?
Question 9 options
10.
Use both tables and the passage. Sort each change from the camp.
11.
Kenji says, "The plaster test head only dried out, the same way wet mud dries into hard dirt." Do you agree? Write a claim, give at least two pieces of evidence from Table 2 and paragraph 3, and explain your reasoning.