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Growing Crystals for the Gem Show

Grade 7 physical science: use crystal growth data and atom models to compare a salt crystal's joined pattern with a sugar crystal of separate molecules.

Grade 7ScienceInteractive · Printable

Aligned toMS-PS1-1

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
10 questions
Classroom-ready

Details

Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students use models to compare an extended structure with a crystal made of separate molecules.
Students use growth data and atom counts to explain how crystals grow by adding particles.

How to use

Read the passage aloud and sketch both models on the board before students start; let pairs argue question 10 before writing.

Curriculum

NGSS standard covered

MS-PS1-1

Model the atoms in simple molecules and in extended structures such as crystals.

Preview

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Name Date

Growing Crystals for the Gem Show

Model the particles inside salt and sugar crystals.

The crystal jars

Read the passage and Table 1, then answer the questions.

Two Jars on the Club Shelf

1The Riverbend Mineral Club runs a table at the county gem and mineral show every spring. This year, club members Tavita Faleolo and Benedikt Novak planned a display called "Crystals You Can Grow at Home." Their adviser, Ms. Odalys Fuentes, dissolved table salt in one jar of hot water and table sugar in another. Once the jars had cooled, the students set them on a shelf, where the water slowly evaporated. Every two days, they measured the largest crystal in each jar with a ruler and a magnifier.

2The salt grew into small cubes with all their edges the same length. The sugar grew into long crystals with slanted ends. The students recorded their measurements in Table 1.

3To explain the shapes, Ms. Fuentes showed the students computer models of both substances. Table salt is sodium chloride. In the salt model, sodium atoms and chlorine atoms take turns along every row, in every direction, like a checkerboard that is stacked in layers. The atoms are not grouped into separate molecules: the joined pattern simply goes on to the edges of the crystal. Because there are no separate groups, the formula NaCl gives the ratio of atoms in the pattern, one sodium atom for every chlorine atom. The crystal grows when more sodium and chlorine atoms from the solution join the pattern at its surfaces.

4Table sugar is sucrose, C₁₂H₂₂O₁₁. In the sugar model, each sucrose molecule is a separate group of joined atoms: 12 carbon, 22 hydrogen and 11 oxygen atoms. In a sugar crystal, whole molecules sit side by side in a repeating pattern, but the molecules are not joined to one another by bonds. The crystal grows when whole molecules from the solution settle onto its surfaces and fit into the pattern.

5Ms. Fuentes also showed the club a reference book. It lists the melting point of salt as 801 °C. Sugar melts at about 186 °C and quickly turns brown. One more fact surprised Benedikt: a salt cube that is 1 mm on each edge has about 3.5 million atoms lined up along each edge.