Drops of Liquid on a Penny- Gloria Hernandez Scipio
Title: Drops of Liquid on a Penny
Principle(s) Investigated:
- Surface tension
- intermolecular forces (or cohesive forces)
- Hydrogen Bonding
- Polarity
- Micelle
- Scientific Method
- Common Core State Standards for Mathematics: Math
- 4. Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) [2010] ...show
- a. Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin. (Grade 7) [2010] ...show
- Summarize, represent, and interpret data on a single count or measurement variable (Grades 9 - 12) [2010] ...show
- International Technology and Engineering Educators Association: Technology
- I. Interpret and evaluate the accuracy of the information obtained and determine if it is useful. (Grades 6 - 8) [2000] ...show
- Next Generation Science Standards: Science
- Make observations and measurements to identify materials based on their properties. (Grade 5) [2013] ...show
Materials:
- 3 small beakers (50-100 ml) or plastic cups (about 4 to 8 ounces) per group
- 1 disposable pipette per student, plus a few extras
- 1 penny per student
- rubbing (isopropyl) alcohol, two 16-ounce bottles
- vegetable oil, one 32-ounce bottle
- paper towels, several per student
- water
Procedure:
- Get in groups of 3
- Make a hypothesis on the form: Hypothesis
- Put drops on a penny
- Insert into the following form:Results
- Do 3-4 trials. Fill out new form each time.
- Click here for Results
Student prior knowledge: What prior concepts do students need to understand this activity?
- Students should be able to calculate the average of four numbers less than 50.
Explanation:
Reactions and Solution to Problem
This experiment demonstrates the intermolecular forces (or cohesive forces) between molecules of a substance. These forces are responsible for the observed surface tension in liquids.
Surface tension is the phenomenon where strong forces between molecules cause the surface of a liquid to contract. That contraction of the surface molecules of a liquid results in the drops of liquid being formed into a spherical shape. Substances where the surface tension is strong, such as water, will tend to bead up on a smooth surface. This is the reason for water to form “beads” of liquid on a waxed surface. Substances where the surface tension is weak will tend to spread out on a smooth surface such as an oil spreading over a surface.
The forces of attraction between two molecules is due to the types of atoms bonded together in the compound and the shape of the molecule.
A chemical bond is formed between two atoms when they share electrons between them. In the ideal case, both atoms attract the electrons equally, like a tug-of-war between two equal teams. When two different atoms are bonded together, each attracts the electrons differently like a tug-of-war between two unequal teams. That unequal attraction of the electrons produces a polarity in the molecule, making one part of the molecule positive and one part of the molecule negative. (This is similar to the polarity that occurs in a magnet.) The positive part of one molecule with be attracted to the negative part of another molecule.
The shape of a molecule also contributes to the polarity. Molecules containing three or more atoms may be linear in shape or bent. Depending on how symmetrical or how bent the molecule is, the polarity of the molecules will differ. For example, a water molecule, which consists of two hydrogen atoms attached to a single oxygen atom, is bent in an angle of 104°, which is slightly more than a right angle (90o). The unequal attractions of the hydrogen atoms and the oxygen atom produce a polarity in the individual bonds (see diagram below) and the bend in the water molecule increases the overall polarity making the total polarity of the molecule stronger than that of the bond by itself.
How many drops of each liquid can be placed on a penny?
Explain the reasons for any differences you observe for the four liquids.
- (negative end) -O–H+ O
(negative end) (positive end) / \ HH
A oxygen-hydrogen bond + (positive end) A water molecule
The polarity of the water molecules is quite strong and results in a strong attraction between two adjacent water molecules (and, to some degree, between all surrounding water molecules). Since this type of bonding, where a hydrogen-oxygen group occurs in a molecule, is quite common in a number of substances, it is given the special name, hydrogen bonding.
Water has the strongest hydrogen bonding and the highest surface tension. Because of the strong attraction between water molecules, the water forms a large “sphere” of water on coin. The number of drops that the students can put on the penny will vary from about 30 to 50 depending on size of drops. (NOTE: The actual number of drops of water is not important to the solution of the problem. The relative number of drops of water, as compared to the other liquids is important.)
A detergent reduces the surface tension of the water. Detergent molecules are polar on one end and non- polar on the other end. The polar end is attracted to water molecules and the non-polar ends are hydrophobic, that is, they will not mix with water molecules (like oil separates from water). In water, the detergent molecules will form a cluster and will orient themselves with their polar ends toward the water molecules and their non-polar ends oriented toward the center of the cluster. The resulting cluster, called a micelle, is essentially a large polar molecule. The presence of the clusters (micelles) reduces forces of attraction between the water molecules and the overall surface tension of the detergent solution. When washing clothes, dirty hands, etc., the detergent molecules will surround dirt like football players surrounding the ball carrier on the opposite team and hold the dirt in solution. About 20 to 30 drops of the detergent solution can be placed on the coin depending on drop size. (Note: students may not be familiar with detergent structure and the name “micelle”)
O⎯⎯⎯⎯⎯⎯ Polar non-polar
End end
A detergent molecule (schematic diagram)
A micelle
(cluster of detergent molecules)
In a compound such as ethane, (commonly referred to as a hydrocarbon molecule since it is composed of carbon and hydrogen atoms) the small polarities in the bonds between the carbon atoms and the hydrogen atoms are canceled out due to the symmetry of the molecule, making the molecule non-polar. (Note: The actual structure of an ethane molecule is 3-dimensional, but it is symmetrical.)
HH || H—C—C—H || HH
An ethane molecule
As a result, an ethane molecule has little or no attraction for other ethane molecules.
Ethyl alcohol has a structure that is similar to both ethane and water. The O-H group on one end of the molecule is bent, similar to the O-H in a water molecule, but the other side of the O-H bond is a C-O bond
that is less polar than the O-H bond. This configuration in the molecule produces some hydrogen bonding which is weaker than that in water molecules. As a result, the ethyl alcohol has less surface tension than water. About 20 to 30 drops of ethyl alcohol can be placed on a coin depending on drop size.
HH || H—C—C—O
||| HHH
An ethyl alcohol molecule
Mineral spirits is a mixture of molecules similar to the ethane, diagrammed above, but larger in size. (In reality, ethane is a gas at room temperature. Liquid hydrocarbon compounds must contain a central carbon skeleton of 5 or more atoms.) Mineral spirits is a mixture of non-polar hydrocarbon compounds, similar to the ethane molecule shown above (but larger in size). Only 2 to 5 drops of mineral spirits will stay on a coin before overflowing the edges.
Students should be able explain polarity of liquids. They should recognize the following points:
Hydrogen bonding is strongest in water and weaker in the alcohol. Detergent reduces the hydrogen bonding of the water.
There is no hydrogen bonding in the mineral spirits
Questions & Answers:
1.) Does adding soap to a penny reduce the surface tension of the water being added?
-With soap on penny, fewer drops of water can be added.
2.) How does adding soap to a penny affect how many drops can fit on it?
Because soap reduces cohesion of water surface tension decreases and water molecules break apart and can't stay on the penny.
3.) Why don't the other liquids hold as many drops?
Because they don't have hydrogen bonding. There are other molecules that prevent the cohesiveness of the hydrogen bonds.
Applications to Everyday Life:
Raindrops fall and the molecules cohere giving it a generally spherical shape only distorted by wind resistance. That a drop of water balls up as a sphere on wax paper, or on a waxy leaf surface is cohesion. This is the attraction of matching substances. Solids display the strongest attractions between molecules in the material. Iron rods have great cohesion. Below certain temperatures the attraction which existed between the particles holds them so their random motions are reduced and the material becomes solid.
As the rain sinks into the soil it is not pulled deep into the ground by gravity because it adheres to the soil crumbs. Water climbs up the capillary sized vesicles in plants by adhesion to the xylem.
Adhesion is the attraction between different substances.
Two solids with matching surfaces can adhere when pressed together. Webber gage blocks can stack and adhere so specific and accurate guages can be assembled from smaller units.
Photographs:
- Put drops on a penny
- Insert into the following form:Results
- Do 3-4 trials. Fill out new form each time.
