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Halogens
Halogens
Grade level: 9-12 Subject: Physical Science  Duration: Two class periods

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Objectives
Students will
  • Identify the halogens on the periodic table of the elements and describe their basic characteristics.
  • Describe how the halogens react with hydrogen and how hydrochloric acid is formed.
  • Discuss the positive and negative effects that hydrochloric acid and other acidic materials have on the process of digestion.
Materials
  • Halogensprogram
  • Computer with Internet access
  • Newsprint and markers
  • Copy of the periodic table of the elements
  • Paper and pencils
Procedures
  1. Begin the lesson by asking students to write on a piece of scrap paper the chemical formula for hydrochloric acid. If students do not know the answer, tell them not to worry about it. Then ask them to put the piece of paper away until the end of the lesson.
  2. Direct students' attention to the periodic table of the elements in a book or on a class chart. Point out the halogens. As a class, make a list of the elements in this group, where on the periodic table they are located, and two characteristics of this group. The chart will include the following information.

    Elements in the Halogen Group
    • Fluorine
    • Chlorine
    • Bromine
    • Iodine
    • Astatine

    Group on the Periodic Table
    • Group 7

    Characteristics of the Group
    • Toxic and highly reactive
    • Colored nonmetallic elements
    • Color gets darker as you go down the list
    • Low melting and boiling points
  3. Now that students have a little background about the halogens, ask them to learn more about these elements by watching the segment in the program entitled "Exploring the Halogens." Have students pay close attention to what happens when hydrogen combines with halogens. They will find out that substances called hydrogen halides form. When hydrogen halides dissolve in water, very strong acids form. For example, when hydrogen chloride combines with water, hydrochloric acid forms.
  4. Point out to students that hydrochloric acid plays a role in an important process-digestion. Ask students to research how hydrochloric acid and other acidic gastric juices affect digestion and to write a short paper describing their findings. As students work on this project, they should consider the following questions.
    • How do acids help the digestive process?
    • How can acids negatively affect the digestive process?
    • Why do you think acids have both a positive and negative effect on digestion?
  5. To learn more about this topic, have students watch the segment entitled "Digestion: The Acid Test." The following Web sites also explain the role of acids in digestion.

    Secretion of Bile and the Role of Bile Acids In Digestion
    The Role of Bile Acids as Hormones
    Gastroesophageal Reflux Disease (GERD, Acid Reflux)
    Digestion and Absorption of Food Fats
    Your Digestive System and How It Works
  6. Give students time in class to work on their reports. Encourage them to include illustrations to explain key concepts. If students do not finish their reports during class, ask students to complete the assignment for homework.
  7. During the next class period, ask for volunteers to share their reports. Then discuss why acids have a positive and negative effect on digestion. Make sure that students understand that hydrochloric acid begins the digestive process, stimulating the pancreas and the liver to produce key digestive enzymes. Other acids, called gastric juices, are needed to change food into small molecules that can be absorbed by the body's cells. But because these acids are so strong, they can cause harm if they reach parts of the body where they are not supposed to be. For example, gastro esophageal reflux disease is caused when stomach acids are pushed back up into the esophagus, the tube that connects the stomach to the mouth. Not only is this condition uncomfortable, it also can harm the lining of the esophagus.
  8. Conclude the lesson by asking students to revisit the notes they recorded at the beginning of the lesson. Ask students to correct or write the chemical formula for hydrochloric acid, identify the halogen in this compound, and explain how the acid is formed.(Chemical formula is HCl; chlorine is the halogen in the compound, and the acid is formed when hydrogen chloride is mixed with water, resulting in the formation of hydrochloric acid.)Based on what students have learned about the halogens, can they name common materials that are made up of halogens? What do these substances have in common?(Possible answers include salt, fluoride used to prevent tooth decay, and iodine, an antiseptic used to prevent infection. All of these materials are very strong and even potentially toxic.)

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Evaluation
Use the following three-point rubric to evaluate students' work during this lesson.
  • Three points: Students identified all the halogens on the periodic table of the elements and accurately described several characteristics of these elements; demonstrated a clear understanding of the role acids play in digestion and of why acids can have a positive and negative effect on digestion.
  • Two points: Students identified most of the halogens on the periodic table of the elements and satisfactorily described at least two characteristics of these elements; demonstrated a satisfactory understanding of the role acids play in digestion and of why acids can have a positive and negative effect on digestion.
  • One point: Students had difficulty identifying the halogens on the periodic table of the elements and could not describe any characteristics of these elements; demonstrated a weak understanding of the role acids play in digestion and of why acids can have a positive and negative effect on digestion.

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Vocabulary
digestion
Definition:The process that involves the changing of food into smaller substances that the body's cells can use for nourishment
Context:Without stomach acids, the process of digestion could not take place.

gastric juices
Definition:Acidic materials produced by the stomach to break down food into smaller molecules that can be absorbed by cells throughout the body
Context:Bile is an example of a gastric juice that is produced by the liver and is needed to break down fat.

gastro esophageal reflux disease (GERD)
Definition:A condition that occurs when the valve that is supposed to close between the stomach and the esophagus is not working properly and contents from the stomach come back up into the esophagus, causing an unpleasant burning sensation
Context:In addition to being an uncomfortable condition, if left untreated, gastro esophageal reflux disease, or GERD, can damage the lining of the esophagus, possibly even leading to cancer of the esophagus.

halogen
Definition:The elements in Group 7 of the Periodic Table of Elements that include fluorine, chlorine, bromine, iodine, and astatine
Context:The halogens are all toxic, highly reactive, and important elements in many substances that we use every day.

hydrogen halide
Definition:The gaseous molecule formed when hydrogen reacts with the halogens
Context:When hydrogen combines with chlorine, the hydrogen halide known as hydrogen chloride forms.

hydrochloric acid
Definition:The substance that forms when hydrogen chloride is dissolved in water
Context:The hydrochloric acid in the stomach protects us from many powerful bacteria.

Periodic table of the elements
Definition: An organization of Earth's elements arranged according to atomic number, the number of protons each element's nucleus contains
Context:The halogens are in group 7 of the periodic table of the elements and are all colored nonmetals with low melting and boiling points.

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Academic Standards
Mid-continent Research for Education and Learning (McREL)
McREL's Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education addresses 14 content areas. To view the standards and benchmarks, visithttp://www.mcrel.org/.

This lesson plan addresses the following national standards:
  • Physical Sciences: Understands the structure and property of matter
  • Language Arts-Viewing: Uses viewing skills and strategies to understand and interpret visual media
National Academy of Sciences
The National Academy of Sciences provides guidelines for teaching science in grades K-12 to promote scientific literacy. To view the standards,click hereto visit this Web site.

This lesson plan addresses the following national standards:
  • Physical Science: Chemical reactions
  • Physical Science: Structure and properties of matter

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