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)
Sample	State at 20 °C	Density (g/cm³)	Warmed to 120 °C	Mixes into acetone?
Glue stick, before	Solid	0.95	Melts into a thick liquid	—
Glue on a strap pad, after	Solid	0.95	Melts into a thick liquid	—
Resin Part A, before	Liquid	1.18	Stays a liquid	Yes
Resin Part B, before	Liquid	1.04	Stays a liquid	Yes
Visor clip, after	Solid	?	Stays solid; does not melt	No

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.

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.

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?

A. He is right, because the clip is clear like both liquids, so it is made of the same substances.
B. He is right, because a density between 1.04 and 1.18 g/cm³ shows the clip is still Parts A and B.
C. He is wrong, because a clip that was a simple mix of the parts would be denser than both of them.
D. He is wrong, because the clip stays solid at 120 °C and does not mix into acetone, unlike both parts.

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?

A. She is right, because the glue on the pad has a lower density than the glue stick had.
B. She is right, because the glue on the pad stays solid at 120 °C, just like the visor clip.
C. She is wrong, because the glue on the pad has the stick's density and melts at 120 °C like the stick.
D. She is wrong, because the glue on the pad is a liquid at 20 °C, the same as the stick.

Underline the two observations that are evidence, on their own, that a new substance formed when the resin hardened.

The clip is clear, and so are Parts A and B.The clip stays solid at 120 °C, but Parts A and B are liquids at that temperature.The clip took the shape of its mold.The clip does not mix into acetone, but both parts do.The clip's density is between the densities of Parts A and B.

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)
Sample	Mixed with water	Mass lost at 200 °C
Fresh plaster powder from the bag	Warms and hardens in about 20 minutes	About 6% of its mass
Ground chip from the dry test head	Stays a soft paste; does not warm	About 21% of its mass

Use paragraph 3.

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

Tavita dried the head until its mass stopped changing, yet it still had more mass than the powder. Which explanation fits paragraph 3 best?

A. The rubber mold left a thick layer of rubber stuck to the outside of the head when it came out.
B. Some of the water joined with the plaster and became part of a new solid, so it did not dry out.
C. The head was still soaking wet inside, and that water was slowly dripping out during the week.
D. The sun added heat to the head, and that heat has mass, so the head became heavier as it dried.

Which conclusion about the test head do the results in Table 2 support best?

A. A new substance formed: the ground chip will not harden with water and loses 21% at 200 °C.
B. The plaster just dried: the ground chip warmed and hardened with water, just like fresh powder.
C. A new substance formed: the ground chip lost far less mass at 200 °C than fresh powder did.
D. The plaster just dried: both powders lost about the same share of their mass at 200 °C.

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?

A. Ground set plaster, so it will not harden in the head mold
B. Fresh plaster powder, so it will harden in the head mold
C. Fresh plaster powder that lost mass because the oven destroyed part of it
D. Ground set plaster, so it will harden again if extra water is added

10.

Use both tables and the passage. Sort each change from the camp.

Glue sticks melting in the glue gun and cooling on the padsParts A and B hardening into visor clipsExtra water evaporating from the hard test head in the sunPlaster powder and water hardening into the test headA dry chip of the test head being ground into powder

A new substance formed	No new substance formed

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.