This lab explores the properties of water with salts, oil and detergents and compares those properties with another clear colorless liquid, isopropyl alcohol, commonly found in rubbing alcohol.

There are many way that this is relevant to life Water is a very polar molecule. The oxygen has a partial negative charge and the hydrogens have a partial positive charge. Salts, like monosodium glutamate are comprised of positive ions and negative ions. The negative end of water is attracted to and surround the positive ions and the positive end of water are attracted to and surround the negative ions. Most biologically important small molecules in our body are either polar or salts. Water is the ideal solvent to dissolve them all. We can apply the same argument to oceans and rivers. They also dissolve important nutrients for all the life forms that call these bodies of water home.

Oils and detergents also have important properties with water. Oil is not soluble in water. We observe this when we observe that water and oil do not mix. Water is polar and oil is very non-polar. They avoid each other. Since oil is lighter than water it floats, and if you look at the top of the water, most of the oil will accumulate in one glob, minimizing its surface with the water. When you shake it and force the oil to go into solution, it forms little balls, because the spherical shape has the smallest surface area with water for any given volume. When the balls of water get very small, they can stay suspended in the water for longer, but will eventually float to the surface again and reform the glob.

Detergents are molecules which have polar ends, usually charged, which love water and non-polar ends, which love grease. When we shake the water, oil and detergent, the non-polar end of the detergent becomes embedded in the grease ball (like dissolves like) leaving all the polar ends of the detergent facing the water. Thus the small droplets of oil have become surrounded by the detergent. The surface of this new "fuzzy" grease ball is the polar ends of the detergent, which have an affinity for water and can stay suspended in it longer, and do not stick to the side of the bottle to avoid water, like the grease alone did.

We have molecules like detergents in our blood as well. They serve the same purpose as the detergent. They help to suspend greasy molecules in the blood stream so they don't form globs and clog our veins and arteries.

These detergent-like molecules in our body serve another purpose as well. They can create cell walls which separate the water inside cells from the water outside cells. They do this by forming a bi-layer. The non-polar ends of the detergent-like molecules face one another, leaving the outer surfaces polar.

Water also organizes detergent into soap bubbles. In the soap bubble, the detergent forms a water sandwich, with detergent as an outer and inner layer and water in the middle. Of course the polar ends of the detergent face the inner water layer and the non-polar ends of the detergent are on the outside. Alcohol is not polar enough to do this very well.

Soluble

Insoluble

Polar

Non-polar

The MSG, an amino acid salt, should dissolve in water, but only partially dissolved in the alcohol. If rubbing alcohol is used instead of pure isopropyl alcohol, you may observe a liquid layer separate out with the un-dissolved salt at the bottom of the jar. There is some water in rubbing alcohol. When that water become saturated with the salt it becomes much more dense than the alcohol and it also become less miscible (soluble) with the alcohol, and separates out as a lower layer.

The oil is soluble in neither water nor alcohol. However it floats, like an oil slick on water, but sinks like 10 little marbles in the alcohol (one marble per drop). When you shake each, the oil forms many little balls which either slowly float to the surface or sink depending on the liquid.

Liquid detergent is soluble in both water and alcohol. However when you shake the two, only water can sustain soap bubbles.

When you shake the oil with detergent in alcohol, it doesn't seem to do anything special, but in water the oil turns into tiny spheres, which stay suspended in the water for a long time, and eventually float to the top, but stay in tiny little spheres, which do not stick to the side of the glass like the oil does in the other bottle without detergent. It is now easy to wash the oil out of the jar and get the jar clean.

The students could turn in a completed lab report which would include drawings of their observations as well as the data that they collected.

Wonder Science: Section 4, Water pp 1 13 -145 provides several additional units on the properties of water. Also, "The Water Cycle" and "The Disappearing Puddle!" provide other experiments dealing with water.

SCIENCE CONTENT STANDARD #1: THE NATURE OF SCIENCE

This experiment satisfies benchmark 1 and 3 which state, "Design modify and conduct an investigation through testing, revising, and occasionally discarding ideas, all of which lead to a better understanding of how things work." and "Collect and summarize data from an experiment and interpret the results in terms of the data."

SCIENCE CONTENT STANDARD # 3: LIVING ENVIRONMENT

These experiments help to explain why water is the ideal solvent of life and how it helps to organize structures in a cell.