Water is a molecule composed of two elements of hydrogen covalently bonded with one element of oxygen

Jesina Elliston

AP Biology Essay 1

Water is a molecule composed of two elements of hydrogen covalently bonded with one element of oxygen. Because of the polarity created from the covalent bond these molecules of water are all connected with a hydrogen bond. A hydrogen bond is formed when two molecules are attracted to oppositely charged components of another molecule. A hydrogen bond has about 5% the strength of a covalent bond and is responsible for some of the unique qualities found in water. It takes a large amount of energy to break down the hydrogen bonds in water allowing it to have a high specific heat, giving water its high boiling point and allowing it to maintain a liquid form over a wide range of temperatures. The hydrogen bonds also allow water to have high surface tension. This happens because, although one hydrogen bond is relatively weak, when many of these bonds are formed between two molecules it causes the bonds to be sufficiently strong and quite stable. Hydrogen bonding also influences cohesion, the clinging of a water molecule to itself or adhesion, the clinging of a molecule to another molecule.

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Water is known as the “solvent for life” or the “universal solvent”. Water is a good natural solvent because it exits in the liquid state in its natural habitat. Another one of water’s reasons for being such a good solvent is credited to its feature of having polar molecules. When an ionic or polar substance is added to water it becomes the solute. The high dielectric constant of water reduces the attraction between one atom to the others in a molecule when submerged, making water the excellent solvent it is. Water’s peculiar trait about its order of density is that it is backwards from that which is usually found in other substances. When water takes its solid form it is less dense then in its liquid state and in the liquid form it is less dense then in the gaseous state. This is because as water decreases in temperature it approaches its maximum density. Water is most dense at 4 degrees Celsius. This is reached while water is still in its liquid form, therefore causing ice to float on water.

High specific heat is significant to life on Earth because of the ability to maintain a moderate temperature while that around it is changing. As the sun beats on water during the daytime the temperature of the water slowly rises, and during the night the warm water heats the air above it causing the climate to stay relatively the same. This is why climates near large bodies of water are more moderate. Specific heat also stabilizes the ocean waters making it suitable for marine species to sustain life in their habitats. This characteristic is useful in humans, humans being composed of 65% water, because it helps to maintain a livable body temperature. Water is a great coolant because of this factor as well, for the body, cars, etc. High surface tension is a characteristic that gives greater buoyancy support for greater loads than most liquids. This is very useful for species in their habitats that use water to travel on, without breaking through the surface. Cohesion is used by plants, when water is evaporated from a leaf more water is pulled up from the stem by cohesion. The plant will also use adhesion to repel form the stem walls: this is how the plant works against gravity to transport water throughout the plant.

Water being a universal solvent is very useful for humans. This makes it easier for humans to get the required amounts of water needed to sustain life. Water is the basis of many components in our body and other living organisms, blood, the liquid in cells, the nutrients that is consumed from plants, etc. The abnormal density of water is helpful to keep water reusable. Most fresh water on earth is consumed by glaciers and ice caps. If the solid form of water was not less dense then the liquid state then water would freeze and sink from the surface to the bottom and most likely never be used again. This trait gives water a chance to melt and be reused. Ice floating on top of water protects the liquid below the ice from being exposed to cold air and potential becoming frozen through and through. Water is significant in many ways and has unusual characteristics helping living organisms maintain life on earth.

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groat

8th of Feb 2012

Quality of writing

This essay is structured particularly well. The introduction is clear, and I like how each paragraph has a clear signpost. From doing English Literature at A-Level, this is a great technique when writing essays, but it seems to be neglected in other subjects! Starting each paragraph with a simple sentence will make it clear where the marks should be allocated, and often helps you keep focussed without repeating yourself or going off on a tangent! Spelling and grammar is faultless. This essay is to be admired!

Level of analysis

The explanation of water as a molecule is thorough. I like how this essay is structured, introducing the molecule's structure and then explaining how this structure contributes to the qualities of water. It was nice to see how this essay addressed how each quality is relevant, rather than just explaining it. I particularly liked how they mentioned cohesion enabled transpiration in plants - being able to link topics together will gain credit in exams or coursework as it shows an understanding of the syllabus. The use of scientific terms is strong throughout. This essay correctly explains polarity of water molecules and refers to it as the universal solvent - using terminology such as this is a sign of a strong essay! Whenever talking about bonds in biology, I always draw a diagram. So, an improvement for this essay would be to include diagram showing two water molecules with a hydrogen bond clearly labeled.

Response to question

The analysis in this piece is particularly strong, and goes beyond GCSE level. Although there isn't an explicit question, I imagine this could be a response to an essay question in an exam. They have covered nearly all the marking points relevant to water molecules, and so this is a great example for revision of this topic!

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