Introduce your students to chemistry without tackling the whole periodic table at once.

Recognizing basic elements as building blocks is essential to the study of science. And looking closer at one element in particular—chlorine—can help ignite students' interest in chemistry.

The following two-day study of building block chemistry uses basic concepts and easy-to-find materials.

Objectives

The students will:

  • Understand that chlorine is one of several important single elements or "building blocks" of matter which can be combined to produce a variety of different compounds.
  • Observe and record different chemical and physical properties of selected chlorine compounds.
  • Understand that chlorine compounds are used to make products that play important roles in homes, schools, and industry.

Safety Notes

Instruct students not to bring the chemicals into contact with their eyes, skin, nose or mouth. Remind students to wash their hands after the activity. Follow your school policy regarding the use of student safety eyewear. Safety eyewear is strongly recommended.

Disposal of Solutions

These solutions can be disposed of in the sink or collected for later disposal, according to existing local regulations. Always check for local restrictions on disposal.

Teaching Strategies

Day One—Building Awareness

(Before class, fill two clear plastic 8-ounce cups half full of warm water. Add one drop of red food coloring to one cup. Add 3-4 droppers full of full strength household bleach to the other. If larger containers are used, adjust the proportions accordingly.)

Ask students to observe very closely, because you are about to perform some chemical "magic." Display the two cups. Ask the students to describe what they see. Most will say that one is a solution of red food coloring and the other contains water. Pour the red solution into the "water" and set aside. In 30-60 seconds (exact time varies) the solution will mysteriously clear!

Ask students to explain what they have observed. Explain, if needed, that the second cup contained bleach, or sodium hypochlorite. (You may wish to contrast observation and inference at this time.) Tell students that chlorine is present in many other compounds that are a common part of our daily lives. Distribute student sheet 1, "Chlorine in our lives," and ask students to mark off those items they use or see in school.

Discuss their responses. Explain that chlorine likes to combine with other elements and compounds, and scientists have found ways for chlorine to help build or improve things. Define "element" and "compound" as needed. Ask them to take the student sheet 1 home to check for household items containing chlorine or requiring chlorine during the manufacturing process. Tell them they may want to ask their parents for help.

Introduce chlorine as an important "building block," one of a handful of single elements that combine to form most of the matter on the earth. Write the names of the following elements on the chalkboard or overhead projector: oxygen (0), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), hydrogen (H), phosphorus (P), chlorine (Cl), and carbon (C). Use chemical abbreviations if desired. Tell students that these elements are sometimes called "building blocks" because they make up over 99% of the earth's crust, atmosphere, and oceans, by weight.

Tell the students that tomorrow they will explore a variety of chlorine compounds.

Day Two—The Science

Recall the list of chlorine compounds the students took home yesterday. Discuss their findings. Review the "building block" concept with the students, noting that chlorine combines with many other elements to form different compounds, each with different chemical and physical properties.

Tell them they will get a chance to work with several different chlorine compounds. Distribute student sheet 2, "Exploring chlorine compounds." Discuss the procedure with students. Highlight the difference between physical properties (appearance, color, odor, etc.) and chemical properties, which are determined by the interaction of the chlorine compound with other substances.

Make materials available and allow students to complete the activity, assisting as needed. Be sure that students follow recommended safety precautions when using these materials. The chloride salts can be obtained easily and inexpensively from almost any vendor of school science supplies, or contact local high school chemistry teachers. Litmus or other indicators can be used instead of pH paper. Sample student data tables and answers are listed below.

Discuss student results and summarize the main points of the activity. If students had trouble observing the temperature change with calcium chloride, tell them to try again with more solid. Hydrogen gas is given off when copper chloride reacts with aluminum foil. Iron chloride reacts with aluminum foil, but much more slowly, taking several minutes to produce observable results.

Answers to Questions:

  1. Which of the chlorides seem to have the most similar physical properties and chemical properties? Sodium and calcium chloride seem to have the most similar properties. Although dissimilar in appearance and in their reaction with water, they are both white, form neutral solutions and do not react with aluminum foil or ammonia. Cupric and iron chloride both form acid solutions in water and react with ammonia and aluminum foil. The least similar physical properties and chemical properties? Answers vary. Cupric chloride has no common properties with either ammonium chloride or calcium chloride. Others may have some common properties.

  2. How do you explain the differences in the chemical and physical properties of these chlorides? Student answers should indicate that all the compounds contain chlorine. Therefore any differences in properties must he due to the other chemicals present in the compounds.

  3. How do the observations you made in this activity support the idea of chemicals as "building blocks?" (Note that chlorine is a gas at room temperature and has a strong odor.) Each chloride is a solid at room temperature and has no detectable odor. Each chloride has chemical and physical properties that differ from chlorine gas—and each other chloride. These changes result from the different elements or "building blocks" that combine with chlorine to form each compound.

Sample Student Data Table

compound

appearance

temperature change in water

acid base, or neutral

aluminum foil reaction

color change with amonia

iron chloride

yellow-brown chunks

no change

acid

bubbles, gets warm, brown solid

red-brown

calcium chloride

large white irregular chunks

gets warmer

neutral

no reaction

no change

sodium chloride

white crystals

no change

neutral

no reaction

no change

cupric chloride

light blue chunks

no change

slightly acid

bubbles, gets warm, brown solid

dark blue solid forms

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