Abstract
The aim of this experiment was to determine the effect temperature on the alum crystal growth by altering the dependant variables ranging from an incubator, a freezer and a cupboard in room temperature for ten days. It was hypothesized that the crystal in the incubator would grow the biggest because the solvent will evaporate faster resulting in the crystals to grow faster. After the experiment was conducted, it was concluded that the crystal in the cupboard grew and weighed the largest. There are certain explanations as to why the crystal in the incubator should have grown the largest and other reasons why the experiment wasn’t conducted successfully; mainly because the crystals were exposed to the allocated environment for a very short period of time.
Introduction
The chemical formula for potassium alum is KAl(SO4)2.12H20 signifying that the reaction is composed of potassium, aluminium, sulphate and water (Barthelmy D, 2005). Potassium alum is the most important and most used alum. It is colourless, has a density of 1.76, a transparent diaphaneity, non fluorescent, vitreous and usually forms a large octahedral formation. Potassium alum has a pH of between 2.3 to 2.8 which makes it an acidic (Microsoft Student, 2007)
Alum is a cost efficient coagulant used in the treatment of water and wastewater treatment for domestic and industrial purposes. It can remove colour from natural organic matter and turbidity from colloidal matter. Alum reacts with and removes heavy metals such as arsenic and lead. Relating to agriculture, when alum is applied to poultry litter, it greatly reduces phosphorus run off from pastures; alum can also reduce ammonia emissions in poultry houses. It is also used in dyeing, tanning, fire proofing textiles, vegetable glues and baking powder (Orica Watercare, 2008).
An ionic lattice is a structure of millions of atomic formations of an ionic substance like buildings into one 3-D formation. The molecules are positioned orderly with a repetitive arrangement. In an ionic crystal lattice structure, the anions are mostly larger than the cations where the anions form crystal array and the cations reside in holds between the anions. Many different kinds of ionic lattices and patterns depend on the complexity of the molecules. Potassium alum crystallises with 12 molecules of water, the crystals belongs to the cubic system and usually forms large colourless octahedron (Western Oregon University, 2009)
Crystal formation is the result of regular ordering of particles in a lattice. During the process, crystalline ionic substances form as ions and arrange accordingly to prevent forces of repulsion between the ions of the same charge. Fast formation tends to form small crystals and slow formation tends to result in larger crystal (eChemistry Textbook, 2007)
There are 8 main factors that affect crystal growth:
- pH
- viscosity of solution
- saturation or supersaturation
- temperature
- cooling rate
- time for growth
- evaporation rate
- the type of solvent
(Anton A, 2005).
A saturated solution contains more solute particles than the conditions and is measured as the maximum mass of solute that can be dissolved in 100 g of solvent in a specific temperature. A saturated solution is when it reaches a point, where no more salt can be dissolved in the solvent. For example: NaCl with water: when adding it to water and stirring it, it quickly dissolves. But when the point of saturation is reached, the salts will not dissolve, but fall on the bottom of the vessel. Solubility depends mostly on the temperature. And the higher the temperature, the more salt can be dissolved in the solvent because the solubility increases too. If a saturated solution is heated up to boiling point and salt is added then it will still dissolve but when the temperature of the solution decreases (f.e. to the room temperature), the excessive salt will crystallize, causing too many molecules of NaCl present and the solution doesn't have enough "place" for it. Some excess solid will crystallize forming crystals at the bottom of the container named a supersaturated fan. Therefore the expression supersaturation is in relation to a solution that consists more of the dissolved material than could be dissolved by the solvent under normal circumstances.
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Peer Reviews
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Quality of writing
The candidate provides proper referencing to a high level of style. The candidate lacks basic commas in places, and sometimes spelling mistakes are present. The style of writing used also makes the text hard to read sometimes. Other than simplistic errors in punctuation, grammar and spelling the majority of the text is fine. The layout of the text is done well but I would have included images from the appendix alongside the results. Referencing is provided as a bibliography at the end of the text in a professional manner.
Level of analysis
The candidate includes an abstract which is normally reserved for professional scientific articles so to see this at gcse level is rare. The introduction shows that the candidate researched the crystals and the aspects of the experiment they are going to create to a high level synthesising a number of sources. The method is presented in continuous prose, and this makes it hard for the reader to interpret each step. The make the method clear it should be presented in a series of bullet points or numbers. The user uses a range of graphs to compare their results which is very good because this means they get accurate interpretations from the data. The candidate explains the trends in the graph well and makes a very good explanation about why their hypothesis may be wrong. The candidate also includes images to document how the experiment was carried out which was also good so the reader can picture how the experiment advanced.
Response to question
The response to the question is done very well. The candidate thoroughly researches and analyses the experiment. Improvements would have been to repeat the experiment or increase the range of experiments that the candidate tested to provide results which had better reliability. This experiment is done to a level I would expect from beyond GCSE level because of the layout of the text, the referencing done and the depth of scientific research.
