Super critical carbon dioxide - its characteristics and use in making decaffinated coffee.
Green Chemistry
Super critical carbon dioxide
By Gemma Taylor
Part 1:
When there is a relatively high temperature and pressure the line between whether a material is in its liquid or gas state begins to blur. The material has properties similar to a gas and a liquid. It is like a gas as it expands to fill any available space and it is like a liquid as it is able to be used as a solvent. When the materials are in this state they are called a superficial fluid. Carbon Dioxide forms a superficial fluid when there is a pressure of around 73 atmospheres and when the temperature is about 31 degrees. As this is a low critical temperature it makes critical carbon dioxide very easy to work with. Another useful feature of critical carbon dioxide is that when it is in its solvent form it is able to be altered by making slight adjustments to temperature and pressure. The phase diagram below of carbon dioxide shows us that its triple point ( where all states of carbon dioxide exist together) is found at the temperature -56.4 degrees and a pressure of 5.11 atm. The liquid state of carbon dioxide does not exist in the normal atmosphere pressure (1 atm). The solid carbon dioxide changes directly into its gaseous state. It is because of this property that solid carbon dioxide (also called dry ice) is a very useful refrigerant. At higher temperatures then the critical carbon dioxide state the gaseous carbon dioxide will not liquefy.
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Peer Reviews
Here's what a star student thought of this essay
Quality of writing
Spelling, grammar and punctuation are all to a very high level with no errors. Rather than using the titles 'part 1 and part 2' I would have used clear subheadings which allowed the reader to clearly ascertain the subject of that body of text. In addition, points could be split into further sub-paragraphs to make the different points easier to follow rather than being two large blocks of text.
Level of analysis
The candidate explains the concept of supercritical carbon dioxide very well in this essay. At first the candidate explains the concept of what 'supercritical' actually means and how this relates to carbon dioxide and possible uses. The candidate synthesises a number of sources to explain this concept clearly with a diagram that supports the text. Using a number of texts means the reader understands the concept more, and is able to communicate the concepts in an easy to understand way. The text is split into two parts which helps the reader understand the concept of 'supercritical' before the reader explains the use and method of this in making decaffinated coffee and shows good research depth by comparing it to prior methods that used to be used. Rather than the references being listed at the bottom, I would have used a better referencing system which outlines where certain points in the text came from as this is the correct way for professional scientific articles.
Response to question
The response to the topic is very well researched and described. The candidate uses terms and communicates in a way which make it very easy for the reader to understand the concept of 'supercritical' by first explaining what this means in the terms of gas and liquids and then relating this to carbon dioxide, and the uses of this. The candidate starts simple and builds on the different concepts. The candidate should have included a clear introduction which set out the concept of supercritical, and then related this to carbon dioxide in further text rather than setting this out in one block of text as this reads a lot better.
