Experiments involving chemistry investigations of solutes, solvents, and solutions provide students the opportunity to inquire into the nature of density. This specifically occurs during science activities involving sinking and floating liquids. These types of activities cause a discrepancy in what students expect to happen and what actually happens. As students use their critical thinking skills in these types of science activities they develop a better understanding of science.
Understanding Density of Objects
Why do objects the same size sometimes have different masses or weights? The answer has to do with density. Density is determined by comparing mass to volume. When a granite rock and a cork the same size are compared (equal volume), which is heavier? The granite rock is, because it has more mass. This greater density is due to the atomic structure of the elements, molecules, and compounds that make it up the rock as compared to the cork.
This application of density is applied in dissimilar chemistry experiments that involve different types of liquids placed in the same size container (equal volume); these different liquids have different densities. For example an equal volume of vegetable oil is less dense than the same volume water.
When the vegetable oil is mixed with water, the oil separates and floats on top the water. This concept applies to all oils, because any oil mixed with water will eventually float on top the water.
Solution and Solute
A solution is a mixture of matter, one of which is usually a fluid. A fluid is matter that flows, such as a liquid. The fluid of a solution is called the solvent. The material combined with a solvent to make a solution is called the solute. This evident when sugar (solute) is combined with a solvent (water) to make a sugar and water solution.
Another similar science experiment is when sea salt is added to water, it dissolves into a solution. When a 3.5 percent solute of sea salt is added to 500 milli-Liters (mL) of water, the result is sea water. However, if too much salt is added to the water it becomes a saturated solution.
When a solution reaches the point of saturation, the water will no longer dissolve the salt solute. This saturation point is evident when salt crystals begin to settle on the bottom of a beaker and do not dissolve.
Investigation of Sinking and Floating
This chemistry experiment involves the investigation of sinking and floating liquids as sea salt is added to water. Students are also engaged in the use of science process skills as they conduct the science activities.
Materials per Group:
- 1– Large Beaker or Similar Sized Glass
- 200 mL of Vegetable Oil
- 200 mL of Water
- Sea Salt
- Spoon
Procedures:
- Make a prediction of what happens when the vegetable oil is combined with the water.
- Add the 200 mL of water to the beaker.
- Add the 200 mL of vegetable oil to the same beaker.
- Observe what happens and compare with prediction.
- Make a prediction of what happens when salt is combined with the vegetable oil and water.
- Using a spoon, slowly sprinkle some salt crystals into the beaker.
- Observe what happens and compare with prediction.
Typical Observations:
Since the sea salt is denser than both the vegetable oil and water, it sinks to the bottom of the beaker. Because of the molecular structure of sea salt, vegetable oil droplets attach to the sea salt crystals as they sink. As the salt crystals near the bottom of the beaker they dissolve creating salt water. When this occurs, the vegetable oil droplets are released and float back to the top.
Students can repeat this process several times until the water becomes saturated with salt and the salt no longer dissolves in the water. At the point of saturation the sea salt crystals will remain attached to droplets of vegetable oil on the bottom of the beaker.
Science Questions for Students:
- Which is denser, water or vegetable oil?
- Was a solution created when the vegetable oil and water were combined?
- What happened when the sea salt was poured into the vegetable oil and water mixture?
- What is the solvent in this experiment?
- What happens if too much salt is added to the mixture?
Scientific Method Process
Students follow the process and steps of the scientific method and problem based learning as they investigate this science experiment. During these science activities students make observations and communicate their findings. Students are engaged in the process as they use critical thinking skills to draw conclusions.
Making Connections with Density and Liquids
Through the science questions for students and the use of their critical thinking skills, students develop a better understanding of the density of objects. This experiment and additional chemistry experiments also cause students to increase their reasoning and logic skills as they complete science activities.
David R. Wetzel - Dr. David Wetzel's experience includes more than 25 years in continuing, adult, and teacher education.
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