CR yr12 Biology 01/11/2010
SOME OBSERAVATION ON WATER
Experiment # 1: Place a drop of water on a smooth plastic sheet or on the bench. Look at it closely from the side. Draw the outline of the drop. How are the molecules held in their place?
- The drop of water and the drop of detergent have a different thickness, because the molecules are held differently. The drop of water, in fact, has a higher thickness. The molecules are closed to each other and it's possible to notice the surface that behaves as an elastic membrane that surrounds and compresses the underlying liquid. There is a force of cohesion that determines the surface tension.
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Experiment #2: Fill a clean 250 ml. beaker with water to about 1 cm. below the top. Carefully float a small filter paper on the surface. Carefully drop a needle, exactly horizontal, on the paper. Wait until the paper becomes soaked and sinks. Observe the needle carefully. After you have observed it, add one drop of detergent with a glass or plastic rod.
- the piece of paper floats on the water and then it sinks because it get too wet. the needle continues to float, although its specific gravity is higher than the water's one, maybe because the water surface forms a kind of membrane ( see: ex.#1), impenetrable by small objects (as the needle). Adding the detergent, the needle has sunk because the detergent breaks the bonds between water molecules that allowed to the needle to float. Very carefully, it's possible to notice that the water surface gets curved under him ( as when we break a membrane, exactly).
Experiment #3: Place a 250 ml. baker on a quite bench. Carefully, fill it into the brim (the top). Now carefully add water drop by drop until it begins to overflow. Now carefully add small amounts of Ammonium Chloride to the beaker using a spatula. How much can you add before the water overflows?
- Although we put lots of ammonium chloride, the water didn't overflow. That's really strange, because another little drop of water would make it overflow! This happened because the ions of the ammonium chloride attract the water molecules, so they are closer together and therefore they take less space.
Experiment #4: Fill a baker to halfway. Scatter chalk dust over the surface. Now add one drop of detergent with a glass or plastic rod. Observe and explain.
- The Lycopodium moved immediately when it enters in contact with the drop of detergent. That's maybe because the detergent is totally not attracted by the lycopodium. [ ... ]
Experiment #5: Fill a clean 250 ml. beaker to about 1 cm. below the top. Place two glass rods in the beaker, side by side. Where is the water between them? If it's not easy to see, add some drops of a dye to make water more visible.
- Water is just attached to the glass, and it is possible to find it also outside the beaker, in the back of the two glass rods (as shown in the figure). That's probably because if the space is smaller, the pull is stronger. Glass molecules are bigger than water molecules, so they use to attract them.
Here's what a star student thought of this essay
Quality of writing
Minor grammatical errors present in places, also some careless spelling mistakes present. Other than this most of the document is fine in terms of punctuation, grammar, spelling and layout
Level of analysis
The experiments are very cleverly done and an attempt is made at an explanation. However, the first explanation of cohesion is not very well done. Bonds between water are explained at a base level. The diagrams help in the explanation and the methods of each experiment are stated clearly. One or two hypothesis seem to be guesses but most of the candidates work is correct even though it lacks deeper scientific detail.
Response to question
Overall a cleverly carried out and explained experiment. It provokes thought from the reader and the candidate shows well thought out reasons as to why each experiment proceeds as it does. To improve their mark, the candidate should include a conclusion on the properties of water tying all the different experiments together.