A controlled variable is a variable that stays the same throughout an experiment such as: adding a specific amount of water to test tubes filled with different amounts of ammonium chloride. An independent variable is the variable that is changed during an experiment, e.g. different amounts of ammonium chloride added to separate test tubes in an experiment. A dependant variable is a variable that changes because of the independent variable, e.g. the difference of temperatures when the first crystals begin forming in the separate test tubes filled with different amounts of ammonium chloride.
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In this experiment the controlled variable will be 10mL of water that is added to each test tube filled with ammonium chloride. The independent variable will be the different amounts of ammonium chloride put into each test tube. The dependant variable will be the temperature of the first crystals appearing in the test tubes filled with different amounts of ammonium chloride that are dissolved in water.
Ammonium Chloride
Ammonium chloride is a substance which has the molecular formula NH‚„Cl and is highly soluble in water. It was first manufactured during the 13th century in Egypt and Europe and was originally known as sal ammoniac. It is used for a variety of purposes. In medicine it is used as an expectorant, which clears the phlegm from the bronchi, lungs and trachea area. It is also used as a component in dry cell batteries, an ingredient in cough medicine and as a dietary supplement to maintain pH levels. (See http://www.wisegeek.com/what-is-ammonium-chloride.htm . Last Updated March 13th 2013.)
Solubility
Solubility is when a substance is dissolved in a solvent such as water. This is then measured in g/100mL to calculate the solubility of that substance in the solvent, e.g. The solubility of 50g of Ammonium chloride would be shown as 50g/100mL of water. Solubility is a method used to calculate how much of a substance can be dissolved in a solvent. This method can be explained in three different ways: a saturated solution, an unsaturated solution and a supersaturated solution. (See also: Study on Chemistry 1, page 295-96.)
A saturated solution is where no more solute can dissolve in the solvent at a specific temperature. An unsaturated solution is where more solute can be added to dissolve in the solvent at a specific temperature. However, a supersaturated solution is where an unexpected amount of solute can still be dissolved in a solvent at a specific temperature. A supersaturated solution can only be achieved (with difficulty) by changing the conditions of the saturated solution.
In this experiment, the substance is Ammonium chloride and the solvent is water and when mixed together, the Ammonium chloride dissolves. Repeating this with different amounts of ammonium chloride gives us the solubility of Ammonium chloride in g/100mL of water. When adding more ammonium chloride, the max amount of the substance that can dissolve in the solvent is discovered, this is called a saturated solution. If a specific amount of ammonium chloride that always dissolves in g/100mL of water is used, it is called an unsaturated solution because more of the substance can be added to the solvent, to dissolve. If the saturated solution of the substance is reached but more of the substance is added and surprisingly dissolves in the solvent, it is called a supersaturated solution. This experiment can consist of a saturated or an unsaturated solution because the solubility of Ammonium chloride in water is unknown. (See also: URL’s displayed in bibliography with a * next to it)
Solubility Curve
Solubility curve is a graph of solubility vs temperature. Solubility curves are used to predict the maximum amount of a substance that can be dissolved in a solvent at a specific temperature.
Solvents
A solvent is a substance that is either a liquid, solid or gas that dissolves a solute to create a solution. The maximum amount of a substance that any solvent can dissolve depends on the temperature of the solvent, e.g. If water can dissolve a maximum of 30g of ammonium chloride at 50°C, but the temperature was increased to 60°C it could dissolve more of the ammonium chloride. There are two categories of solvents: polar and non-polar solvents. A polar molecule has two sides; one is positive and the other negative, also known as a dipolar molecule. Polar molecules have polar bonds, though some can have polar bonds but are non-polar molecules. This is because the polar bonds are arranged in a way that they cancel each other out. The overall polarity of a molecule depends on the direction of the bond dipoles in a molecule which is determined by the shape of the molecule.
Polar solvents can have a small electrical charge because of the shape of the compound. A compound such as water has the hydrogen atoms at opposite angles of the oxygen atom. The hydrogen atoms can create a small electrical charge because of the direction of the bond dipoles, which is determined by the shape of the hydrogen atom. A molecule may mix in a polar solvent if it has a polarity of its own. Salt and sugar both dissolve in water easily because their molecules are attracted to the small electrical charges of water. Non-polar solvents don’t have an electrical charge and cannot mix with a polar solvent.
Polar and non-polar solvents use a dielectric constant to provide a rough measure of the solvents polarity. Dielectric constants are the electrical properties of a solvent using a capacitor, in which electrical currents pass through. Non-polar solvents are considered to have a dielectric constant of less than 15. The polarity index measures the ability of a solvent to dissolve different polar materials. The results of both these tests are used in a table of common solvents and in future can be used for identifying solvents in chemical processes. (See also: http://www.wisegeek.com/what-is-a-polar-solvent.htm : Last Updated: 14th March 2013.)
(See also: http://www.erowid.org/archive/rhodium/pdf/solvent.miscibility.pdf : Last Updated 14th March 2013.)
Polar and non – polar solvents are related to this experiment because water is a polar solvent and ammonium chloride is a polar solute. This means that the two can mix together. If either one was a non-polar solvent or solute, they wouldn’t mix because they don’t have a positive or negative pole that binds them together.
Endothermic Reactions
An endothermic reaction is where a product absorbs energy from its surroundings, causing its surroundings to drop in temperature. In an exothermic chemical reaction the reactants have more energy than the products. However, in an endothermic chemical reaction the products have more energy than the reactants because it absorbs the energy from the reactants and the environment.
If ammonium chloride is added in a beaker filled with water and dissolved, the beaker would become cold. This is because it is an endothermic reaction, where the ammonium chloride absorbs the energy from the solvent (water) and its surroundings. This is why we heat ammonium chloride, so that more of it can be dissolved in a solvent (water) and the temperature will not drop rapidly. (See also http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemreac/energychangesrev1.shtml : Last Updated 14th March 2013.)
What is being investigated?
The idea of this investigation is to observe what happens when ammonium chloride is added in a test tube filled with a specific amount of water and heated. The next part of the investigation is to observe what happens when the product is allowed to cool.
How is it being investigated?
This experiment is being investigated by using a range of equipment. An electronic balance is being used to weigh the ammonium chloride and get an accurate result, and a burette to measure an accurate, 10mL of water which is added into a test tube with the ammonium chloride. An electric hot plate is then used to heat up this mixture, and dissolve it while in the process of heating. Next, a retort stand is used to cool down the mixture, so that you don’t have to hold the test tube with your hand since it would be really, really hot! Finally, a digital thermometer is used to get a result of the temperature of when something happens in the test tube, when it is cooling down. These results of the experiment are written down and put into tables and graphs so that others may understand how the results were gathered using these equipment.
How will the results be analysed?
The results will be analysed by finding trends in the statistics that have been written down. These results will then be put into a table and graph. The table will be analysed to investigate if there are any results that don’t fit in and if there is a reason why this result occurred during the experiment. The graph will be analysed by finding if there is a trend between the results on the graph (e.g. a constant) and how they match up. Next, the graph is analysed by checking if there is any inconsistencies or results that seem out of place. Finally, the graph is analysed by testing or checking the results to see if they are correct or incorrect.
Materials
solid ammonium chloride
2 retort stands
500 ml beaker
Hand lens
250 mL distilled water
burette
burette holder
large clean test tube
electric hot plate
spatula
stirring rod
digital thermometer
2 retort stand clamps
heat mat
electronic balance
black card
safety glasses
Method
4gms of solid ammonium chloride was measured and the actual mass was recorded.
The solid ammonium chloride was put into the large clean test tube.
The burette was filled with distilled water till the level reached 10mL.
This 10mL of water was added to the large clean test tube with the solid ammonium chloride.
300mL of hot water was added to the 500mL beaker.
The beaker was placed on the electric hot plate.
The beaker was heated up to boiling temperature and the large test tube was slowly placed inside with the water level in the beaker 3cm higher than the water level inside the test tube using the retort stand and clamp.
The mixture in the test tube was gently stirred using the glass stirring rod until dissolved.
The large test tube was removed from the beaker after the solid ammonium chloride dissolved and allowed to cool.
The large test tube was stirred and watched using the hand lens and the black card to observe the first crystals form.
The digital thermometer was used to measure the temperature of the first crystals forming.
The mixture was then heated up again and using steps 7 -11 the process was repeated two more times to get the most accurate results of the crystals forming with 4gms of solid Ammonium Chloride.
Steps 1 – 13 was repeated again using 5g, 6g and 7g of solid ammonium chloride.
Discussion
Trends
The results in Table 1, 2 and Graph 1 all have one trend in common; as the temperature increases, the solubility of ammonium chloride increases. This trend shows that the results are reliable but not valid because graph 2, which is the accepted results, shows that the solubility curve of the two graphs do not match i.e. the values do not match but they follow the same pattern. Another trend in the results is the temperature, which shows that for the solubility of ammonium chloride, each gram that was tested roughly differs by 7°C. This trend is neither reliable nor valid because they do not match graph 2 and the patterns in the graph are different.
Saturated, Unsaturated or Supersaturated
These results also indicate that this could be a saturated solution, unsaturated or a supersaturated solution because 4g of ammonium chloride dissolved in water at a temperature of 73.43°C, though the 7g of ammonium chloride dissolved at a higher temperature of 94.56°C meaning that it took longer to dissolve 7g than 4g. This, in turn, shows that if more ammonium chloride was added to the 7g of ammonium chloride it would have taken longer to dissolve and once it passed boiling temperature water would have evaporated till there was no solvent left and there would still be ammonium chloride. This would mean that 4, 5, 6 and 7g of ammonium chloride was an unsaturated solution. This is an error because it is completely different when compared to the result of graph 2. This error means that something occurred during the experiment which made the ammonium chloride at a higher temperature or simply a random error where the water was heated before the ammonium chloride was put into the beaker. This error has significantly changed the result as it has increased the temperature of dissolution when it should be a lot lower.
Comparison (validity and reliability)
Table 1 shows the 4, 5, 6 and 7 grams of ammonium chloride was tested three times to get the most accurate result. These three results were then divided to give an average. The average of the 6 and 7 grams of ammonium chloride in comparison to graph 2 is reliable because the solubility of ammonium chloride increases with the temperature. The average of the 4 and 5 grams of ammonium chloride in comparison to graph 2 could be reliable but the percentage of error is too high for this to be acceptable meaning that there was an error in the consistency of the experiment.
Graph 1: ‘Solubility of Ammonium Chloride’ shows the four averages of each test of ammonium chloride, with a trend-line that predicts the solubility of ammonium chloride between 0°C and 100°C. In addition the graph shows that when you keep adding more and more ammonium chloride the temperature for it to dissolve will increase, but only by a few degrees each time. Graph 2: ‘Accepted Values – Solubility of Ammonium Chloride’ are the accepted values of the solubility of ammonium chloride. In comparison to graph 1, both graphs have a curve, though graph 1 has more curve than graph 2, where the curve is very slight. Graph 1 and 2 do not match at all, meaning that there are errors and/or inconsistences in this experiment.
Table 1, 2 and Graph ‘Solubility of Ammonium Chloride’ all fit the theory that ammonium chloride dissolves in water. When we match up all the results in this experiment, they clearly do not match the accepted values. The accepted values are vastly different as graph 1 shows that 39.99 grams of ammonium chloride dissolves at 73.43°C and graph 2 shows that 39.99 grams of ammonium chloride dissolves at roughly 28.12°C. The results fit the theory because all four tests of ammonium chloride dissolved in water. The only factors that could have changed this outcome could have been the amount of substance in a solvent which is heated to a specific temperature. In the end graph 1 explains as the temperature increases, so will the amount of ammonium chloride that dissolves.
Errors
Possible errors that could have occurred during the experiment are: the solid didn’t dissolve properly, temperature could have been wrong during the cooling down process, the recrystallization of the ammonium chloride in water was difficult to judge and malfunction of equipment could have caused errors. The reason why the solid might not have dissolved is because the temperature might not have been high enough or I didn’t stir it enough. In the case of a random error the solid might not have dissolved but the eye might not have been able to see a very small particle even with a small magnifying glass. The reason the temperature could have been wrong is because when I stirred the mixture, the heat from the bottom of the test tube would have mixed with the cool temperature at the top of the test tube, meaning that when you saw the first crystals appearing the temperature could have still been changing. During the experiment the ammonium chloride solution from the first trials was recycled to be used for further trials. This could have changed the purity of the ammonium chloride because it looked very different to the normal ammonium chloride. This could mean that while the weight did not present a problem, it may have not been completely dry which could affect the results. The malfunction of equipment could have occurred without my knowledge, such as the balance scale being inaccurate (e.g. the small empty cupcake cups on the scale did not balance at 0). Another malfunction could have occurred with the thermometer not working properly.
The loss of ammonium chloride not dissolving could have lowered the solubility of the substance. The likelihood of the scale not working is very low and could be counted as a random error, but if it wasn’t working the solubility of the substance would be lower than expected if there was less ammonium chloride and higher if there was more ammonium chloride. The recrystallization could have been the exact same as normal solid ammonium chloride or it could have weighted more or less. This would make the answer higher if the ammonium chloride weighed more than it should. Overall I believe the results and answers I recorded might have been a bit too high or even low, but I definitely know that they aren’t exact. One way to possibly improve the experiment is to use the exact amount of water and ammonium chloride to get a measurement of solubility (g/100mL). This would prove to be more accurate and would be better suited in a beaker, on top of the electric hot plate as the heat would be dispersed around the entire beaker. Further investigations that could be included into this experiment could be finding out the saturated solution of ammonium chloride at a specific temperature. This would be good because it is very likely that every group would get different results and have to explain more in their discussion about errors and if they think that their answer is valid or not.
Improvements
One way to possibly improve the experiment is to use the exact amount of water to get a measurement of solubility (100mL). This would prove to be more accurate and would be better suited in a beaker placed on an electric hot plate as the heat would be dispersed around the entire beaker. Further investigations that could be included could be finding out the saturated solution of ammonium chloride at a specific temperature. This would be good because it is very likely that every group would get different results and have to explain more in their discussion about errors and if they think that their answer is accurate or not.
Conclusion
In conclusion I discovered that 4, 5, 6 and 7 grams of solid ammonium chloride was soluble in water at different temperatures. The hypothesis that different amounts of ammonium chloride will dissolve in water at varying temperatures was answered. The results that I recorded weren’t valid because they did not match the accepted values, though the results were reliable in some cases as they did have a pattern and this pattern matched the accepted values.
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