Judith A. Puglisi
Water molecules are naturally attracted to other water molecules as well as to molecules of other substances. The attraction of water molecules to other water molecules is called cohesion. The attraction of water molecules to other materials is called adhesion. These two properties are very important to a plant’s ability to get water.
We can easily see water’s attraction to itself by observing its surface. You can continue to add drops of water to a full glass of water until the water level exceeds the brim of the glass. The surface acts like a stretched elastic membrane or a skin. This property is known as surface tension.
This skin forms on the surface of water due to the force of the molecules in the liquid. Molecules in the middle of the liquid are attracted and pulled on equally by surrounding molecules. Molecules found on the surface of the liquid are pulled in a sideward and downward direction by the other molecules. The upward pull of the surface molecules is produced by air molecules. The air molecules have a much weaker attraction than the water molecules. The stronger pull by water molecules causes the surface molecules to be pulled downward and squeeze together, creating the tight skin that allows insects such as the water strider to walk on water. (8)
Asked students to observe droplets of water on different surfaces such as wax paper or cardboard. Students should have to diagram the different shapes and label them. Ask students how they think they could change the shapes of their water droplets.
Soap and other detergents weaken the surface tension of water. Soap molecules move between water molecules and weaken the cohesion of the water molecules from each other. Substances that reduce surface tension are known as surfactants. Surfactants increase the spreading and wetting properties of a liquid. Give students another opportunity to change the shape of a water droplet, but this time give each student small amounts of various detergent products. The students should observe and diagram the effect each detergent has on the shape and movement water droplets. Ask students to hypothesize what would happen to insects such as the water strider if detergents were added to their aquatic habitat. If the detergent weakens surface tension, could this insect continue to walk on the surface of the water? Challenge students to develop a classroom simulation to test their hypothesis. Allow students to observe insects at a local pond, or create a temporary classroom habitat for your students to observe.
Related Activity
To demonstrate how a surfactant such as soap can break the surface tension of water.
Materials: Two pieces of cardboard each cut into the shape of a boat, one soap chip,
one large tray of water.
Procedure: *In the rear of each boat cut a small triangular notch.
*Place a soap chip in the notch of one boat.
*Place both boats in the tray of water.
*Ask students to explain in writing what they think caused the boat with
the soap to move.
*Break the class up into teams of two students each. Allow each team to
experiment with different boat designs and placement of soap chips, and
soap chip size.(9)
Chemical Elements
Related Vocabulary
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15. synthetic: produced by chemical processes rather than of natural origin.
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16. molecule: a structure made up of two or more atoms.
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17. atomic mass: unit used to measure the masses of subatomic particles; a proton has a mass of 1 amu
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18. chemical bonding: combining of atoms of elements to form new substances.
Everything in the Universe, including plants, animals, humans, and minerals is made up of chemical elements. Scientists have discovered 92 naturally occurring elements and have found ways to produce about a dozen synthetic elements. Each of these elements has their own distinct properties. An element is a substance that contains only one kind of atom. Hydrogen contains only hydrogen atoms, oxygen contains only oxygen atoms. If we mix hydrogen and oxygen atoms in the correct proportion we can create a compound called water. When we link two or more atoms together we produce a molecule. The equation H2O represents one molecule of water.
During the mid 1800’s a Russian chemist named Dmitri Mendeleev studied the physical and chemical properties of elements. As he investigated the elements, it became clear to him that some of the elements were similar to others. Mendeleev believed that there was an order or relationship among all the elements. He decided to develop a pattern or arrangement so that those with similar properties were grouped together. Mendeleev decided to arrange the elements in order of increasing atomic mass. His finished product produced an arrangement where properties of elements recurred at regular intervals. All the elements in a column showed similar physical and chemical properties. The elements to the left of the periodic table are metals. Those to the right are nonmetals. And the elements between the metals and nonmetals are called metalloids, which sometimes act like metals and sometimes act like nonmetals.(10)
After students have investigated the content area of “elements” they will view the video Fun with Science featuring Dr. Earl Danieley of Elon College. Students will need to be adequately prepared before viewing this film. Dr.Danieley speaks rather quickly and can be difficult for a learning disabled student to follow. The teacher should prepare a viewing guide to help the students focus on important points of the video. Dr. Danieley reviews important concepts in an entertaining manner, while emphasizing the importance of chemicals in our daily lives.
Chemical Reactions
A chemical reaction is a process in which the physical and chemical properties of the original substances change as new substances with different physical and chemical properties are formed. Chemical reactions are going on around us all the time. Some reactions involve just two substances, others many more. But whenever a reaction takes place, at least one substance is changed. The rusting of iron and baking of bread are just two examples of chemical reactions.
The substance that enters into the reaction is called the reactant. The substance that is produced by the chemical reaction is called the product. This process of reactants changing into products always involves a change in energy. Energy is always either released or absorbed during a chemical reaction. For example, heat energy is absorbed when sugar changes into caramel, and heat energy is released, when gasoline burns.
During a chemical reaction, atoms can form molecules, molecules can break apart to form atoms, or molecules can react with other molecules. New substances are produced as existing bonds are broken, atoms are rearranged, and new bonds are formed. (11)
Related Activity
To demonstrate that a new substance is formed during a chemical reaction.
Materials: The following materials will be needed for each team of two students.
One empty soda bottle, one-half teaspoon of baking soda, one-quarter
cup of vinegar, and one large balloon.
Procedures: *Place one-half teaspoon of baking soda in the empty soda bottle.
*Add one-quarter cup of vinegar to the soda bottle. Quickly place a
balloon on the neck of the bottle.
*Carefully shake the bottle to mix the vinegar and the baking soda.
*Set the bottle on the table and make observations.
Observation: Students will answer the following questions after observing reaction.
*Do you see bubbles when you add the vinegar?
*Do the bubbles rise in the bottle?
*How long does it take the bubbles to stop foaming?
*What happened to the balloon when you put it on the bottle?
*Does shaking the bottle cause more bubbles to form?
*Does the balloon get larger when you shake the bottle?
*After reading the above definition of a chemical reaction, summarize
what you think happened during this demonstration.(12)
The Atom
Related Vocabulary
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19. indivisible: cannot be divided any further.
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20. compound: substance made up of molecules that contain more than one kind of atom; two or more elements chemically combined.
Teaching Strategy
It is important to explain to your students that the models they will see and build to represent abstract concepts are meant to be representations and should not be mistaken as “ the real thing”. Recent studies indicate that the majority of high school students may still be concrete operational learners. Concrete learners identify explicitly with models as “the real thing” and do not extend their ideas beyond them until the transition to abstract thinking.(13) The majority of the student’s time should still be spent observing and experimenting how their own actions affect the chemical world around them. Time spent on such abstract concepts such as atomic structure will serve as information for answers to the questions commonly asked by my students. It should not be the main focus of the unit nor would I expect a student to memorize and be tested on it.
Having students compare their models to historical models can help the student see that even real scientists can begin with flawed ideas before they refine them to represent proven and accepted theory.
The concept that everything in the Universe was made up of tiny particles called atoms was debated for more than two thousand years. It was not until the 1800’s when chemist John Dalton performed a series of experiments that the idea of the atom was finally accepted. The basic ideas of Dalton’s atomic theory are as follows:
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*All elements are composed of atoms. Atoms are indivisible and indestructible particles.
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*Atoms of the same element are exactly alike.
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*Atoms of different atoms are different.
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*Compounds are formed by the joining of atoms of two or more elements.
This theory of matter became one of the foundations of modern chemistry.
Have students write Dalton’s theory in their science journal. Show students visual representations of atoms, elements, and compounds. Explain to students that there are many different ways to represent atoms and compounds. Have students sort the pictures into three groups, single atoms, elements, and compounds.
