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Chalk From Two Clear Liquids: Following the Atoms

Grade 7 physical science: use mass data and an atom model from making chalk in an open cup to show that atoms are rearranged and mass is conserved.

Grade 7ScienceInteractive · Printable

Aligned toMS-PS1-5

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What’s included

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

Details

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

About this worksheet

What students practice

Students use mass data to show that the total mass stays the same when a solid forms in an open cup.
Students count atoms in a model to explain why one reactant can be left over.
Students explain why an open cup loses mass only when a product is a gas.

How to use

Read the passage together and have students trace each kind of atom from the two cups into the chalk or the salt water before they start the vinegar test questions.

Curriculum

NGSS standard covered

MS-PS1-5

Model how atoms are rearranged but not created or destroyed in a chemical reaction, so total mass is conserved.

Preview

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Name Date

Chalk From Two Clear Liquids: Following the Atoms

Make chalk for a bike safety course and track every atom.

Making the chalk

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

Chalk for Bike to School Day

1Every spring, Maplecrest Middle School holds a Bike to School Day. The day before, students draw a practice course on the blacktop with sidewalk chalk: stop lines, bike lanes, turn arrows and a crosswalk. This year, seventh graders Keanu Akana and Sofía Marchetti asked whether their science class could make the chalk instead of buying it. Their teacher, Ms. Odalys Ferrer, showed them how to make calcium carbonate, the main substance in many kinds of chalk, from two clear liquids.

2Ms. Ferrer wore goggles and gloves and handled both substances herself. She dissolved 11.1 g of dry calcium chloride, a salt sold for melting ice on sidewalks, in 100 g of water in one cup. She dissolved 10.6 g of sodium carbonate, also called washing soda, in 100 g of water in a second cup. Both liquids looked like plain water. She set both cups side by side on a digital scale that reads to 0.1 g, and Keanu recorded the total mass. Then she poured one liquid into the other and set the empty cup back on the scale. A cloudy white solid formed at once and slowly settled to the bottom. The cup stayed open the whole time, and Keanu read the scale again after 1 minute and after 10 minutes.

3Ms. Ferrer explained the change with a model. Calcium chloride is made of groups of atoms, each with 1 calcium (Ca) atom and 2 chlorine (Cl) atoms. Each group in sodium carbonate has 2 sodium (Na) atoms, 1 carbon (C) atom and 3 oxygen (O) atoms. When the liquids mix, the atoms are rearranged. The new white solid is calcium carbonate (CaCO₃). The other new substance, sodium chloride (NaCl), is table salt, and it stays dissolved in the water. The water itself does not take part. The model is CaCl₂ + Na₂CO₃ → CaCO₃ + 2 NaCl.

4Next, the class poured the cloudy mixture through filter paper, rinsed the solid with a little water and left it to dry on a windowsill for a week. The dried chalk powder had a mass of 10.0 g. Ms. Ferrer let the filtered liquid evaporate in a shallow dish over two weeks, and 11.7 g of salt was left behind. The students mixed their chalk powder with a little water and white glue, pressed it into molds and let the sticks dry for the big day.