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Unknown Metals Chemical and Physical Properties

Identification of metals Abstract
This experiment will investigate chemical and physical properties of three unknown metals in order to identify them from nine different metals Mg, Al, Fe, Zn, Ni, Pt, Ag, Sn, Pb. Chemical and physical properties were tested through density, Sodium Hydroxide, displacement and Hydrochloric acid tests. The density and sodium hydroxide tests were acceptable while displacement and hydrochloric acid tests were inconsistent; hence, the four results contracted to each other. However, the analysis proved that metal A was aluminum, metal B was zinc, and metal C was Iron. Since the results were inconsistent, they became unreliable, for the final identification of the metals may be incorrect.
Aim: To test the chemical and physical properties of three unknown metals by using four methods, density, sodium hydroxide, displacement and hydrochloric acid, in order to identify the specific metals out of nine different choices – Mg, Al, Fe, Zn, Ni, Pt, Ag, Sn, Pb.
Introduction – 1 This EEI investigates students’ understanding of the unique properties of metals which they are required to design and perform four different tests. The experiments examine both physical and chemical properties of the three unknown metals in order to identify them.
All substances have chemical and physical properties that we use to distinguish or compare from a substance from another. In addition, chemical and physical properties can help us model the substance; thus, understanding how a substance will behave under various conditions. “The chemical property of an element describes its potential to undergo some chemical change or reaction by virtue of its composition to produce a new compound. On the other hand, physical properties are those which a substance can show without losing its identity and can be used to describe their behaviour (Ophardt, C., 2003)”
Furthermore, the structure of a metal explains the chemical and physical properties. For instance, metal is a lattice of positive ions in a sea of delocalised electrons (Spence, R. Gemellaro, T. Bramley, L. Wilson, D.
In the experiment, four tests were chosen to identify the metals which were density, displacement, hydrochloric acid and sodium hydroxide.
1.1. Density
First, density, which is defined as the measure of the relative mass of objects with a constant volume, is a physical property, as each element and compound has a unique density associated with it. Density test is relatively accurate and can be used to identify the metals since they all have their own unique density.
Formula to find density: (Ophardt, C., 2003)
Density = mass/volume = g/cm3
The densities of silvery metals: (Density of metals, n.d.)
Substance
Density (g cm-^3)
Substance
Density (g/cm-^3)
Aluminium
2.70
Lead
11.34
Nickel
8.912
Magnesium
1.74
Zinc
7.14
Tin
7.31
Iron
7.86
Silver
10.49
Table 1
Platinum
21.45
1.2. Sodium Hydroxide
Second, the sodium hydroxide test mixes two solutions and an insoluble substance is formed (France, C. n.d.).
For example:
Copper sulfate Sodium hydroxide à Copper hydroxide solid(precipitate) Sodium sulfate
CuSO4(aq) 2NaOH(aq) à Cu(OH)2(s) Na2SO4(aq)
Precipitation of silvery metals: (Laboratory tests, n.d.)

1.3. Displacement reaction
Third, the displacement reaction test measures chemical properties of metals where a metal displaces a less reactive metal in a metal salt solution (AUS-e-TUTE, n.d.)
For example:
Fe(s) CuSO4(aq) à FeSO4(aq) Cu(s)
Note that copper (II) sulphate is blue, and iron sulphate is colourless. During the reaction, the blue solution loses its colour, and the iron turned into pink-brown as the displaced copper becomes deposited on it. If a less reactive metal is added to a metal salt solution there will be no reaction; therefore, nothing will happen. For instance, Iron is less reactive than magnesium.
The orders of the metals in the reactivity series can be worked out by using displacement (see figure ). Moreover, the reactivity of a metal is affected by the number of valence electrons and the energy levels (TutorVista, n.d.).For instance, the greater the number of shells and lesser the number of valence electrons, the greater the reactivity of the metal is.
The order of reactivity series or silvery metals: (TutorVista, n.d.)
Most reactive to less reactive
Number of energy levels and valencies.
Mg
2,8,2
Al
2,8,3
Zn
2,8,18,2
Fe
2,8,14,2
Ni
2,8,16,2
Sn
Pb
Ag
Pt
Table 3
1.4. Hydrochloric acid
Fourth, the hydrochloric acid test is similar to the displacement test which determines the reactivity of metals. The higher the reactivity of the metal is the louder the “pop” sound will occur.
Example:
Zn 2HCl à ZnCl2 H2
Displacement and Hydrochloric acid tests were chosen for the experiment because these two experiments identifies the metals by testing the reactivity of each metal, and, from the order of reactivity series (see figure x) the metal can be identified.
2. Materials and methods 2.1. Density
Materials:
Electronic scale
Micrometre screw-gauge
Vernier caliper
Method:
All materials and apparatus was attained
The three unknown metals were weighed on the electronic scale to figure out their weight
The density of the three unknown metals was worked out through the measurement of their length, width and height and then multiplied to find the volume; the mass was then divided by the volume to figure out its density
2.2. Sodium Hydroxide
Materials
Spotter tile
2M Sodium Hydroxide
Method:
The three unknown metal nitrates were placed into 3 different spots on the spotter tile
The sodium hydroxide was then added to each one and observations were recorded
The metals were then worked out by comparison with the table below and recorded
2.3. Displacement
Materials:
Tin-snips
Magnesium Nitrate
Aluminium Nitrate
Iron Nitrate
Zinc Nitrate
Nickel Nitrate
Silver Nitrate
Tin Nitrate
Lead Nitrate
Piece of A4 paper
9 test tubes
Test tube rack (must have more than 9 test tube holes)
Method:
The Chemical terms for all 9 nitrates were written on a piece of paper and then slipped under the test tube rack to make sure the solution in each test tube was correct
9 test tubes were placed into the test tube rack with each being filled with the specified solution
Each unknown metal were cut up into 9 small strips
The first unknown metal strips were placed into each test tube
The degree of reactivity was recorded into the table below
Steps 2 – 4 were repeated with cleaned test tubes, new solutions and the other unknown metals until all 3 were tested
2.4. Hydrochloric Acid
Materials:
Matches
6M Hydrochloric acid
3 test tubes
Test tube rack (3 test tube holes minimum)
Tin snips
Stop watch
Method:
6M Hydrochloric acid (HCl) was obtained and 3 test tubes were placed into a test tube rack
Metal A, B and C were cut up and each one was placed separately into each of the test tubes
Hydrochloric acid was added and the test tube was then covered with a thumb and timed for 1 minute using a stop watch
After 55 seconds a match was lit up and ready to be placed into the mouth of the test tube as soon as the thumb was taken off
The thumb was taken off, the match was placed in and the intensity of the reaction was recorded

3. Analysis By testing and analysing the chemical and physical properties of each give metal, we eliminate the possibilities and identify the specific metal.
3. 1. Density
First, the experimental results for density were compared to the theoretical values (see table 1) to identify the metals. For instance, metal A had a density of 1.15g/cm3 which was close to magnesium’s theoretical value of 1.74 g/cm3 or aluminium’s density of 2.70 g/cm3. Metal B had density of 7.28 g/cm3 which was extremely close to tin’s theoretical value of 7.31 g/cm3 and zinc’s density of 7.13 g/cm3 and iron’s density of 7.86 g/cm3. Finally, metal C’s density was 7.16 g/cm3 which was close to density of zinc, 7.13 g/cm3 and tin, 7.31 g/cm3 and iron, 7.86 g/cm3. The density test indicated that metal A is magnesium or aluminium and metal B and C are iron or zinc or even tin.

3.2. Sodium hydroxide
Second, the sodium hydroxide test examined the precipitation of each given metal and compared them with theoretical results (See table 2 and 5). Metal A and B showed faint white precipitation while metal C produced hard white precipitation. These results were used to further analyse the possibility for metal A, B and C. For instance, metal A, which could be magnesium or aluminium, precipitated white. According to the table (?), metal A was still unknown since aluminium and magnesium both had white precipitation. Metal B, which could be zinc or iron or tin, precipitated white. However, the theoretical results showed that iron and tin do not precipitate white; therefore, metal B can be identified as zinc. Similarly, metal C showed almost same result as metal B; therefore, it could be identified as zinc. However, as metal B and C cannot be both zinc, further analysis will clarify the contradictory and anomalous results

3.3. Displacement
Third, the displacement test examined the reactivity. The displacement test places the metals in a more accurate spot on the reactivity series (See table 9). It showed the reactivity level of each metal by observing the metal salt solutions they displaced. However, the displacement test should have provided more precise indication to identify each metal, but they became unreliable as the results were inaccurate. According to the result, metal A displaced nickel, tin and silver, but didn’t displaced lead and any higher reactivity metals above nickel, which indicated that metal A was more reactive than nickel, yet less than zinc, for metal A was an iron. However, the results and the analysis of density and sodium hydroxide are contradicting the displacement results; hence, further analysis through of hydrochloric acid test will identify metal A. On the other hand, metal B displaced all the metals below zinc, except iron. This proved that metal B cannot be zinc; in fact, it was magnesium or aluminium. Once again, the results were contradicting the density and sodium hydroxide results. Furthermore, metal C displaced all of the metals except magnesium, zinc and lead. According to the density and sodium hydroxide analysis, metal C is zinc or iron; therefore, it cannot be more reactive than aluminium. However, the result suggested that metal C was magnesium as it did react with aluminium. To sum up, metal A had could be identified as iron, aluminium and magnesium. However, since the density of iron is 7.86 g/cm3, and the metal A’s density is only 1.15g/cm3, which is close to aluminium or magnesium, iron cannot be metal A. Also, metal B’s density is close to zinc and iron, but is more reactive than iron; hence, metal B may be identified as zinc. Since the same element cannot react each other, this can be seen as an anomalous result. The analysis of metal C suggested that it cannot be zinc nor can it be iron. Metal C cannot be magnesium or aluminium since there is huge difference in density. However, further analysis of hydrochloric acid test will reveal the specific metals.

3.4. Hydrochloric acid
Finally, the hydrochloric acid test was similar to the displacement test as both determined the reactivity of given metals. However, the hydrochloric acid test reacts with hydrogen. In this experiment, metal A and B displaced hydrogen gas from the hydrochloric acid, while metal C did not react (See table 7). Metal A had a small reaction when the flame went out instantly without making a pop sound. However, metal B was extremely reactive, making the loud pop sound. However, the solution reacted with the metal only for one minute; however, some metals take more than one minute to react. For instance, metal A only had a small reaction under one minute, but reacted aggressively after two minutes, which indicates that metal A is aluminium. On the other hand, the results revealed that metal B is zinc as it was very reactive with hydrochloric acid, suggesting that it is the most reactive metal from the possibility of zinc and iron. Furthermore, metal C did not react with hydrochloric acid at all which was evident from the flame as it didn’t go out. Analysis of the previous tests showed that metal C had possibility zinc or iron. Since the reaction of metal C is smaller than metal B; metal C can be identified as iron.
In short, the analysis of the four tests for chemical and physical properties revealed that metal A is aluminium, metal B is zinc and metal C is iron.

5. Discussion The three given metals were identified by using four tests: density, sodium hydroxide, displacement and hydrochloric acid. By carrying out these experiments, the chemical and physical properties were tested to identify the metals. The experiment identified metal A as aluminium, metal B as zinc and metal C as iron. However, even though the tests identified the metals, many anomalous results were found.
Firstly, the density test was measured by the micrometre screw gauge, the vernier calliper and electronic scale. The anomalous results of this experiment were that the density of metal A, B and C did not match the theoretical values. For instance, metal A, which was identified as aluminium, has a density of 1.15; therefore, does not match with theoretical value of 2.70. In fact, the density of metal A is more close to magnesium. Same anomalous were observed the metal B and C. To modify and improve the density experiment, one has to measure the width, breath and length correctly. In addition, putting the metals into water and see how much they dispersed is another way to improve the test.
Secondly, the sodium hydroxide test had consistent results. All of the metals had a white precipitation which was expected.
Thirdly, the displacement tests had major inconsistent results; hence, unreliable. For instance, metal B cannot be zinc when it reacted with zinc. This can be seen as an anomalous. This peculiar reaction is thought to be a repercussion of using unclean equipment, or the accidental mixing of solutions, for the test tubes were not cleaned before use. The same anomalous results occurred when testing metal B and C. For example, metal C reacted with aluminium but did not react with zinc, lead and tin. This is a false result since aluminium is more reactive than zinc or lead or tin. This experiment can be improved by having cleaned test tubes and the test tubes to avoid mix ups.
Finally, the hydrochloric acid tested the reactivity of metal A, B and C. In this experiment, a stop watch was used to measure how fast the metal reacted with hydrochloric acid in one minute, and matches were used to test the reactivity of the metals and hydrogen. However, this test also had numerous anomalous results because metals did not react in just one minute. For instance, if metal C which had no reaction and thought to be iron was left three to five minutes longer, there might have been a reaction. In fact, after recording and disregarding the test tube for three minutes, it started to react despite no reaction in one minute. The same anomalous results were recorded for metal A and B. To modify and improve the experiment, the time for the reaction need to be extended to three minutes or five minutes. This will give time for metals to react; therefore, trap more hydrogen, making the results more accurate. Moreover, the time taken to insert the flame into the test tube was approximately 3 seconds. If the time was decreased to one second, there might have been a reaction for metal C.

Effect of Humidity on Respiration Rate of Porcellio Scaber

Introduction Porcellio scaber are small, land dwelling crustaceans more commonly known as slaters or woodlice. They play an important role in the community in which they live as they feed on decaying matter such as dead leaves and rotting bark, allowing essential nutrients to re enter the food chain faster by depositing them in the soil through their faeces.
P. scaber are most likely to prosper in a habitat that is damp and dark, and are therefore commonly found under bark, fallen logs and leaf litter, although in domesticated areas e.g. gardens, they can also be found underneath pots and firewood stacks. Apart from their thick exoskeleton, which forms armoured plates on their backs, P. scaber has little in the way of defence against predators. Thus they display photo kinesis, an orientation response where the stimulus, light, causes the slater to begin running in a random pattern, ceasing only when it has found a place that is dark. Kinesis is a response where the stimulus determines the rate of movement, but not the direction thus the slater will continue making random turns as it runs, to increase the chance of finding a more suitable environment. In this fashion P. scaber is adapted to avoid predators, as when it is in a dark place, for example the underside of a log, it cannot be seen by birds or small mammals, and is therefore safer than if it was exposed. Because of its preference for low light levels P. scaber is most active at night, when birds and many other predators are inactive, increasing its chances of survival.
P. scaber has also developed a second orientation response, hygro kinesis. With this adaptation P. scaber is better able to find an environment with a favourable humidity. This is important as P. scaber, like most slaters, respires through “gills”, more accurately called pseudo tracheae, which require a certain level of moisture for respiration to occur; otherwise the individual risks desiccation, and resultantly death. Slaters tend to lose moisture very quickly through their cuticle and their pseudotrachea, and as such depend even more highly on the humidity of their environment to keep them damp than other species better adapted to dry environments. Due to the hygro kinesis it is expected that P. scaber will have a higher activity rate in areas with lower % humidity, as they are moving to find more favourable conditions, and thus the respiration rate of individuals in these drier conditions is higher compared to those in damper conditions. This is because cellular respiration is the method whereby energy is produced for functions within the cells, and thus an increased activity rate will require more energy and result in a greater respiration rate, as the individual needs more oxygen for cellular respiration to occur.
Aim: To determine the effect of different % humidity on the respiration rate of Porcellio scaber.
Hypothesis: Due to the positive hygro kinesis of Porcellio scaber, the higher the % humidity, the lower the respiration rate of the individual.
Null Hypothesis: The % humidity of the environment will not affect the respiration rate of Porcellio scaber.
Method: Take the boiling tube and add 1ml of magnesium chloride in the bottom. Ensure no magnesium chloride touches the sides of the boiling tube as this may poison or otherwise affect the Porcellio scaber in the experiment. The magnesium chloride will create a 33% humidity above its surface, thus when the boiling tube is stoppered a uniform % humidity will be created.
Place enough gauze in the bottom of the boiling tube that the magnesium chloride is blocked in the bottom. This also prevents the Porcellio scaber in the experiment from being poisoned. It is helpful to leave a tail of gauze up to the mouth of the boiling tube to make it easier to remove and give the slaters something to climb over and hide beneath.
Take 0.2g of soda lime and wrap it in gauze. This ensures the slaters will not come into contact with the soda lime, but the wrapping is permeable enough to allow gases to enter the packet. The soda lime will absorb the carbon dioxide produced by the respiration of the slaters, ensuring that the change in volume of air is representative of the oxygen used in respiration.
Place 4 adult Porcellio scaber in the boiling tube. These need to be of a similar size so that their respiration rates are similar.
Place the bung with a hole in the centre into the mouth of the boiling tube tightly so that there is no chance of water leaking into the boiling tube and drowning the slaters. Thread a graduated pipette into the hole. This creates an enclosed environment, preventing the slaters from escaping and ensuring the % humidity is kept constant. The pipette is kept steady by the bung, preventing fluctuations in the volume of water in the pipette.
Place the boiling tube sideways in a bath of water, ensuring the tip of the pipette is completely submerged. As the oxygen in the boiling tube is used by the slaters for respiration, and the carbon dioxide produced is absorbed by the soda lime, the volume of water in the pipette will increase as the volume of air decreases, allowing a change in volume to be calculated.
Record the initial volume of water in the pipette. This is most easily achieved by measuring the amount of water in centimetres, and then converting the measurement into millilitres.
Cover the experiment so that the level of light is 0.17 lux or less. This ensures that the activity of the slaters that will determine the rate of respiration will be due only to their hygro kinesis, not both the hygro and photo kinesis combined.
Leave the experiment for 7 hours to allow enough time for the volume of water in the pipette to change due to the respiration of P. scaber.
Record the new volume of water in the pipette. As the water in the pipette is replacing the oxygen, we can conclude that the volume of water is also the volume of oxygen used in the respiration of P. scaber. Place results in a table and calculate the change in volume, and thus the respiration rate using the following formulas
To find the volume of water: (cm÷1.8)x0.1
As 1.8cm is equal to 0.1mL, in the graduated pipettes used in this experiment.
To find the respiration rate: (change in volume of experiment- change in volume of control) ÷ 28
As the control is an experiment without P. scaber in it to give the value of water moving into the pipette without respiration. Dividing by 7 and 4, for the number of hours and slaters to give the respiration rate in mL per hour for 1 slater.
Repeat steps 1-10 twice more with magnesium chloride, and then once more omitting step 4, so that there are 3 experiment set ups with slaters and 1 control with no slaters.
Repeat steps 1-11, changing the magnesium chloride for magnesium nitrate (52.9% humidity), sodium chloride (76% humidity), potassium chloride (85% humidity), and potassium nitrate (93.5% humidity).
Repeat steps 1-12 at least 3 times to create a fair test environment, meaning the sample size is large enough to be a fair representation of the population involved, and enough results can be obtained to provide accurate conclusions.
Conclusion: From the results it can be concluded that as the % humidity increases, the respiration rate of P. scaber decreases. For example the highest humidity, 93.5% produced an average respiration rate of 0.003017 mL per hour, while the lowest humidity, 33%, produced an average respiration rate of 0.007804mL per hour, which is considerably larger. Thus we can see that the respiration rate of P. scaber is affected by the % humidity of its habitat.
Discussion: Porcellio scaber is a species that is remarkably well adapted to living in dark and damp environments, where other organisms may struggle to survive. Because of these adaptations, its ecological niche is very specific, and thus slaters have developed orientation responses (photo and hygro kinesis) that enable them to detect when the habitat has become less than ideal and discover a new environment more suited to them. However as kineses are non directional responses, the slater will continue to run until it has found a more favourable environment. It was this behaviour that was the basis for my experiment.
In animals, energy is produced by cellular respiration, a process where by ATP (adenosine triphosphate), the universal energy carrier in cells, is “charged” by breaking down chemicals such as proteins, lipids and carbohydrates from food. The energy is then released by breaking one of the bonds within the ATP molecule, forming ADP (adenosine diphosphate), and providing the energy required for all functions in the cells. For cellular respiration to occur however, a gas exchange system is required, as aerobic respiration needs oxygen to occur and releases carbon dioxide. In Porcellio scaber this respiratory structure takes the form of pseudotrachea, which are “Respiratory structure developed in pleopods of some Isopoda for air- breathing; they consist of small ramified tubules inside limb opening outward in slit like apertures and filled with air”1.Gas exchange occurs when air moves through the pseudotrachea through a system of ever smaller tubules that end either adjacent or near to each cell in the body. The ends of these tubules are filled with fluid. This fluid moistens the semi permeable cell membrane, allowing the diffusion of oxygen into the cell from the area of high concentration in the tubules to the area of low concentration in the cells. It also allows the waste product of respiration, carbon dioxide, to diffuse out of the cell into the tubules and move out of the pseudotrachea. Because slaters are unable to close off their respiratory structures as they are formed from a hard substance called chitin, it is very easy for them to lose water during respiration, and if there is no fluid in the ends of the tubules then the semi permeable membrane will not be moist. This means that the gaseous oxygen and carbon dioxide cannot dissolve in to a liquid state, and are thusly unable to diffuse into and out of the cell, preventing cellular respiration from occurring. This means that in order for slaters to survive in their habitat, there needs to be a fairly high % humidity, to ensure that the slaters don’t desiccate and die.
The respiration rate of an organism is directly related to its activity rate. If an individual is more active, then it requires a greater amount of energy, and thus the respiration rate increases. Due to the hygro kinesis of P. scaber, an orientation response in reaction to humidity, the activity of the slaters increased in lower % humidity, as they attempted to move into a more suitable humidity. Thus the lowest % humidity, 33%, produced an average respiration rate of 0.007804 ml of oxygen per hour, the highest respiration rate in the results. Then came 52.9% humidity, producing 0.0071 ml per hour, 76% humidity, producing 0.006507 ml per hour, and 85% humidity, producing 0.00543 ml per hour. Finally 93.5 % humidity, the highest sample, produced the lowest average respiration rate of the experiment, 0.003017 ml per hour. When these results are graphed it becomes apparent that the effect of % humidity on the respiration rate of the slaters forms a non-linear regression trend, showing a direct cause and effect relationship between the two variables, humidity being the cause, while respiration rate shows the effect. Thus we can see that the lower humidities produce respiration rates that are similar in size, which then get progressively smaller and less similar as the humidity increases. From this we can see that the change in respiration rate is not regular, but instead increases readily as the humidity decreases, before slowing and evening out. I suspect that an even lower humidity than 33% would reveal that the rate of respiration eventually reaches a point where it stays the at the same level despite the continued decrease in % humidity, as the slater would reach its maximum respiration rate and thus be unable to perform a greater amount of gas exchange than it already is. Thus the amount of energy produced from cellular respiration would remain at the same rate and so would the activity rate of the individual.
Therefore it can be proven that the rate of respiration of P. scaber decreases as the % humidity of its environment increases.
1. Moore R.C. and McCormick L., 1969, General Features of Crustacea, as cited in Definitions- Pseudotrachea, http://crustacea.nhm.org/glossary/define.html?term=Pseudotrachea
Evaluation: There were several controlled variables used in this test to ensure that the results obtained were valid and could be both analysed and discussed. Firstly, in terms of equipment, all the experiments were done using the same sized boiling tubes, bungs and graduated pipettes. This ensured that one sample of slaters did not have a larger volume of air in their set up than another sample, and thus all slaters had the same amount of oxygen available to them to perform respiration. It also meant that no discrepancies in measuring the change in volume occurred due to some pipettes having a different volume to others, as all pipettes had a ratio of 1.8cm to 0.1ml. The pipettes were fitted tightly into the bungs so that they were held steady, preventing the pipette from moving which would have caused fluctuations in the level of water in the pipette, which would have affected the validity of the results. All set ups were also left for the same amount of time before measuring the change in volume, thus when the change in volume in the experiment was measured the variation in results was due only to the respiration of the slaters and not to some experiments being left running longer than others. The amount of all saturated solutions (magnesium chloride, magnesium nitrate, sodium chloride, potassium chloride, potassium nitrate) used in the experiments was 1ml. This ensures that there were enough of the solutions to produce the desired % humidity, but not enough that there is excess chemicals in the boiling tube to poison or otherwise adversely affect the slaters in the experiment. It also means that no set up had more of the chemical than another, thus all had the same environment during the experiment except for the differences in % humidity. All set ups also had the same type and amount of gauze. This meant that the wrapping around the soda lime was the same thickness for all set ups, and thus the same amount of carbon dioxide is able to move through the gauze to be absorbed by the soda lime in all experiments, so that each experiments change in volume was not affected by less carbon dioxide being absorbed by the soda lime than in other set ups. Using the same amount of gauze also ensures that the saturated solution’s % humidity is able to filter through to the rest of the boiling tube, as too much gauze would form a barrier that would keep the air in the area containing slaters at a different humidity to what was required for the experiment, and too little would allow the chemicals to leak through and harm the slaters. Thus having all experiments with the same amount of gauze means that the experiment is consistent across all % humidities. The amount of soda lime in each set up was 0.2g. The consistent level of soda lime ensures that the amount was not large enough in any of the boiling tubes to absorb enough moisture from the air to change the % humidity thus ensuring that the % humidity was consistent across all set ups. The slaters that were used in the experiment were selected randomly to ensure that the samples were an accurate representation of the population. They were all adults, and of a similar size, because juveniles are much smaller than adults and as such would require less oxygen for respiration, thus to make the results valid I made sure that all slaters used in the experiment were close to each other in terms of size, and thus age, so that their respiration rates would be similar. The water bath that the experiment set up was put in was always at room temperature. This was because colder water would cause the air molecules to lose energy and the air to become denser, thus the volume of water in the pipette would increase, compromising the validity of the results by making it seem like more oxygen had been used for respiration than actually was. The opposite effect would occur in warmer water, as the air would expand, giving a reading that was less than it should have been, thus keeping the water at room temperature helps ensure the validity of the results. In each experiment there are 3 set ups, enabling an average to be found. The experiment is repeated 3 times after that to give a final average for all % humidities. This creates a fair test environment and provides a large enough sample range of slaters to be considered representational of the population.
Before doing the experiment I measured the % humidity and lux levels of the slaters ecological niche. This meant that I was able to firstly determine at what level of light the slaters did not respond with photo kinesis. This turned out to be 0.17 lux, and I was therefore able to incorporate this into my method by covering the experiment. Thus I ensured that the activity that would result in an increase in respiration rate was due only to the slater hygro kinesis, without photo kinesis being a factor. I found that the % humidity in their natural habitat was an average of 87.3%, and thus when choosing my range of values for % humidity in my experiment I decided on 33%, 52.9%, 76%, 85%, and 93.5%. I decided not to use a saturated solution of lithium chloride, as I decided the % humidity it caused, 12.5 %, was too low for long term exposure to slaters as the difference from their habitat was large enough that it would almost certainly case desiccation and death.
When I did statistical analysis of my results I first put the final averages into a scatter graph as I was trying to determine if there was a relationship between % humidity and the respiration rates. I then did the regression test, beginning by adding a trend line. However a linear trend line produced an R2 value of 0.6349, which was very low. However after looking at my points again I attempted a non-linear regression. This fitted my points much better, giving an R2 value of 0.9162. The R2 value is an indication of how well the pints on the graph fit the line, in the case of my second graph, as shown earlier in the report; the points fit the line quite well, as the line accounts for 91.62% of the spread of the data. Non-linear regression means that the change in the independent variable (% humidity) causes a change in the dependant variable (respiration rate). However the size of the changes are not corresponding, in this case: in the higher humidities the differences between respiration rates was greater than in the lower % humidities. Thus the statistical analysis shows that my results were not likely to be due to chance, and therefore shows that the respiration rate of Porcellio scaber is affected by the % humidity of the environment.
Thus my method and results were both valid, due to my controlled variables, investigation into the ecological niche and adaptations of the slaters, the number of repeats of the experiment done and the statistical analysis if the results.
References: Natural History Museum of Los Angeles County, Definitions- Pseudotrachea, 11/4/10, found at http://crustacea.nhm.org/glossary/define.html?term=Pseudotrachea

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