What Makes Water Special?
Let's Talk About It
This activity affords students an opportunity to learn about liquids, in general, and to begin to build understanding of some basic physical science concepts, such as polarity of molecules and cohesion (forces of attraction among molecules of a given substance). In addition, the activity provides practice in making and recording observations using drawings and words. After students have had an opportunity to complete their observations, review each question on the “Do Your Liquids Behave? student sheet, and discuss students’ responses as a class. When conducting a class discussion, expect a variety of answers and observations, and use questions to encourage students’ thinking.
Through observation and discussion, students will be able to uncover these differences for themselves. Students also might observe that both liquids have many similar characteristics. Possible student answers for each question are provided below.
What did the drops look like from the top?
- Liquid 1 - Circular, round, shiny, clear, transparent, convex, colorless, transparent, able to flow, etc.
- Liquid 2 - Circular, round, concave, reflective, colorless, transparent, able to flow, etc.
What did the drops look like from the side?
- Liquid 1 - Round, high, domed, convex, colorless, transparent, able to flow, etc.
- Liquid 2 - Flat, spread out, caved-in, transparent, able to flow, etc.
What are some words you used to describe the drops?
- Liquid 1 - Clear, domed, plump, shiny, half a hemisphere, colorless, transparent, able to flow, etc.
- Liquid 2 - Clear, flat, thick, transparent, colorless, able to flow, etc.
In general, students will discover that drops of Liquid 1 (water) are rounded and dome-shaped (hemispherical), and that drops of Liquid 2 (oil) can have various shapes and are a flattened when viewed from the side. Keep in mind that the two liquids used in this activity represent two different types of molecules.
Water molecules are polar, which means that one end of the molecule has a partial negative charge and the other end has a partial positive charge. This makes water molecules act like minuscule magnets that want to stick together. Oil molecules, on the other hand, are essentially neutral and are attracted less strongly to one another than what happens with water molecules. As a result of these fundamental differences, drops of the two liquids will behave differently.
Would the drops split?
- Liquid 1 - It was difficult to split but once it did; it was very distinct. The smaller drops are shaped liked the original larger drop.
- Liquid 2 - It smeared and did not form two different drops.
When students try to split the drops, they will find that it is difficult to force water drops to divide into smaller drops, but that once the separation is accomplished, several smaller, identically shaped droplets will form. The oil drops will not separate into coherent droplets.
Would the drops join together?
- Liquid 1 - It reformed easily. It has the same shape it had to begin with.
- Liquid 2 - No, the liquid smeared.
Individual drops of Liquid 1 (water) will be attracted to one another and will coalesce to form larger drops—all with similar, rounded shapes. This force is called cohesion and creates surface tension at the boundary of the drop or on the surface of a larger body of water. Drops of Liquid 2 (oil) will not join together in this fashion. Instead, oil drops will form larger smears of oil on the wax paper.
Ask students why this happened. They should understand that the attraction among molecules of the same kind is called cohesion. Without this property, liquids do not hold their shape.
What happened when color was aded to each?
- Liquid 1 - The color dissolved into the drop evenly.
- Liquid 2 - The colored drop floated on top of Liquid 2. The colored liquid remained separate from the liquid in the drop.
When added to the drops, food coloring, which usually is water-based, will disperse readily throughout drops of Liquid 1 (water), but will not mix with Liquid 2 (oil). This happens because the water molecules and water soluble dye in the food coloring are attracted strongly to one another. When combined with oil, the polar molecules are attracted more strongly to one another than they are to the oil molecules, which essentially are crowded out.
Compare and Contrast
Students might observe that both liquids have many similar characteristics such as being colorless, transparent, are able to flow, etc. But Liquid 1 tends to maintain a domed shape on the wax paper, while Liquid 2 is tends to spread out and take on a much flatter shape. The colored water dissolved in Liquid 1 but remained separated in Liquid 2. Ask students if they discovered the identity of each liquid as they worked through the investigation, and why they identified the liquids as they did. Explain that Liquid 1 is water and Liquid 2 is oil. Water molecules are attracted to each other like tiny magnets. This attraction causes the water drops to be cohesive or tightly stuck together and form droplets. Forces of attraction among molecules in most liquids, like Liquid 2 (oil) are not as strong as in water molecules. Help students understand that liquid water is an excellent solvent, which is valuable to organisms for transport of nutrients and waste products in cells and tissue, But this characteristic also allows water to become polluted easily. Many other liquids, like oil, do not act as solvents as readily as water.
As an extension, discuss other types of similar-looking or acting liquids.
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Funded by the following grant(s)
The Environment as a Context for Opportunities in Schools
Grant Numbers: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education