Academic journal article The Science Teacher

Fats, Oils, and Colors of a Nanoscale Material

Academic journal article The Science Teacher

Fats, Oils, and Colors of a Nanoscale Material

Article excerpt

Byline: George C. Lisensky, Dana Horoszewski, Kenneth L. Gentry, Greta M. Zenner, and Wendy C. Crone

Phase changes and intermolecular forces are important physical science concepts but are not always easy to present in an active learning format. This article presents several interactive activities in which students plot the melting points of some fatty acids and explore the effect that the nanoscale size and shape of molecules have on the material's macroscopic phase properties.

In this article, two sets of suggested active learning questions and activities are posed, followed by answers and concepts that are related to the questions and activities. In the first set of activities, students investigate the connection between molecular shape and the melting point of fats and oils. In the second set of activities, the same shape consideration is then applied to a series of liquid crystals-a key nanoscale material used as sensors (see "Activity: Liquid crystal sensors," p. 34). Liquid crystals with varying composition can illustrate how intermolecular forces affect phase transition temperatures. Although the visible color changes that occur with liquid crystals are not at the phase transition (which is higher), the color change is a measure of changes that are occurring with intermolecular forces as the temperature changes.

Basic phases of matter

Questions for students: How do solids, liquids, and gases differ at the macroscopic level? Consider shape and volume of the materials. How might you distinguish between a solid, a liquid, and a gas at the nanoscopic level?

Answer/Concept: Solids, liquids, and gases are the traditional phases of matter taught in physics and chemistry classes. Whether matter is a solid, a liquid, or a gas depends on its temperature. The form of H[sub]2[/sub]O as ice, water, and steam is a familiar example. Molecules in a solid have a fixed orientation and position, so the bulk solid has a volume and a shape. When the temperature of a solid is sufficiently increased, the solid melts to become a liquid. Molecules in a liquid are more weakly attracted to each other, so liquids have a volume but flow to take the shape of their container. When the temperature of a liquid is sufficiently increased, the liquid boils to become a gas. Molecules in an ideal gas act independently, so gases expand to fill the entire volume of their container.

Fats and oils

Most students are familiar with fats and oils. Fatty acids are components of the fats and oils we eat and cook with. Fats and oils, also called triglycerides, are made of three fatty acids plus a sugar alcohol called glycerol.

The triglyceride's melting point is usually very similar to that of the fatty acid of which it is composed (Figure 1).

Triglycerides that are liquids at room temperature are called oils. Examples are corn, safflower, and olive oil. Triglycerides that are solids at room temperature are called fats. Examples are butter and lard. In general, animal sources are high in saturated fat and plant sources are high in cis-monounsaturated oils. An unsaturated molecule contains carbon-carbon double bonds, and therefore contains less hydrogen than a saturated molecule. Thus, a

Figure 2. Examples of the chemical structures of trans- monounsaturated and cis- monounsaturated fatty acids.

saturated fat is "saturated" with hydrogen atoms. Hydrogenation of the double bond can convert an oil to a fat; food labels often indicate "partially hydrogenated vegetable oils," or trans-unsaturated fats, that have been chemically modified to increase their melting point (Figure 2). Trans-unsaturated fats rarely occur in plants and animals and mainly result from an isomerization during the hydrogenation reaction. Food labeling regulations now require inclusion of the trans-fat content in foods because both trans-fat and saturated fat have been linked to an elevated risk of coronary heart disease. …

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