How does the UCST of the phenol-NaCl system vary with the molar concentration of NaCl, as determined by mutual solubility curves?

I feel that the most interesting part of studying experimental science is the way it connects real life observations and phenomenon with facts and principles we study in a text book. The genesis of this essay begins from an anecdote along the same idea, which I made much before I was introduced to the term – Extended Essay in the IB Diploma Program. Having passion for cooking, I observed something that really intrigued me; when I took oil in the pan and then added water to it, it did not mix and separated clearly into two distinct layers. But after heating it for some time, it existed as a single layer. In contrast, I have also observed that sometimes if either the amount of oil or water is too less in comparison to the other, they mix with each other at room temperature only. This observation immediately popped certain questions in my mind - Does the miscibility of a pair of liquids depend on the ratio of the mass/volume in which they are mixed or the temperature? What if I add something else in this mixture; will that affect the miscibility in any way?

To satiate the inquirer hidden inside, I explored more. After an effective search with the help of my supervisor, I came across few credible journals and research articles which taught me about something called ‘smart polymers’. The aforementioned are basically preparing polymers that are partially miscible with water and exhibit either LCST or UCST.

These polymers are mainly used in the field of pharmacy and metal extractions, detection of carcinogenic metal ions in foods and so on. What intrigued me the most was the fact that the properties of this partially miscible polymer-water system can be altered by adding various ionic electrolytes or organic compound to make them more effective and useful.

This urged me to explore more on how the mutual solubility of two liquids could be affected by addition of impurities. Further academic reading and consultation with my supervisor introduced me to the system of phenol and water as a common example of partially miscible liquids whose mutual solubility largely depends on temperature as well as the presence of external impurities. To make the discussion more specific and concise, I decided to conduct my Extended Essay in Chemistry based on the following research question - How does the magnitude of upper critical solution temperature (UCST) of phenol(hydroxy benzene)-aqueous NaCl system depend on the molar concentration(moldm^-3) of theaqueous solution of NaCl added to it, determined using mutual solubility curve?

Liquid-liquid system (mixture of two liquids) can be categorized into three types based on their miscibility (solubility of one of the liquid into the other).

Immiscible – If liquid A and liquid B when mixed together at room temperature form two distinct layers, the two liquids are considered to be completely immiscible with each other.

Example - pentane and ethanoic acid.

Miscible – If the two liquids A and B when mixed with each other at room temperature form one single layer, the  liquids are considered to be completely miscible with each other.

Example - water and ethanol.

Partially miscible – There are certain liquid pairs whose mutual solubility depends on the composition of the mixture and the temperature of the mixture. In this case, when the two liquids are mixed, they might form two distinct layers or one single layer depending on the temperature at which they are mixed and the ratio of volume in which they are mixed. Even if they form two distinct layers, the volume of the individual layers after mixing is either greater or smaller than the volume of the individual liquids before mixing. Such kind of liquid pairs are considered to be partially miscible.

A system of phenol and water is one such example of partially miscible liquids.

For any pair of liquids there exists a temperature, above which they are completely miscible with each other/form a clear single layer solution irrespective of the proportion in which they are mixed. This temperature is called Upper critical solution temperature (UCST). The phenol-water system exhibits Upper critical solution temperature.

Refer appendix A.1. for definition of LCST

In order to determine UCST of a partially miscible pair of liquids, I would have to record the temperature points at two instances –

• When the turbidity of the solution disappears i.e. completely mixes into a clear layer due to gradual heating.- t₁
•  When the turbidity re-appears due to cooling - t₂

The average miscibility temperature would be hence given by finding the average of the two temperatures t₁ and t₂. The average miscibility temperature is plotted against mass percentage of any one of the liquid to obtain the mutual solubility curve and the UCST is calculated from the maxima of the curve,

The following is a predicted graph of how the UCST value will be plotted –

Above t₁, the pair of liquids are completely miscible, whereas below t₂, the pair of liquids are completely immiscible irrespective of the proportion or ratio in which they are mixed with each other. Therefore, in between t₁ and t₂ the pair of liquids are partially miscible. The midpoint of this region would indicate the temperature at which the liquids are completely miscible.

The value of UCST showcase the extent of mutual solubility for any pair of liquids. As the UCST increases, it indicates that the mutual solubility for the pair of liquids is decreasing. Hence, as a pair of liquids become more soluble into each other, the value of UCST should decrease.

Definite mass (5.00 ± 0.01g) of phenol was taken in a test tube and the volume of aqueous NaCl  added to it was varied to vary the mass percentage of phenol. Subsequently, different concentration of NaCl solutions were added to the test tube as an impurity. The test tube was then heated in a water bath to record the temperature (t₁) at which the turbidity disappears i.e. a single layer in formed. It is then allowed to cool down to record the temperature(t₂) at which turbidity re-appears (separation into two distinct layers). This method is chosen as it is easy and convenient to perform in a school laboratory.

The UCST of a pair of liquids can also be determined by studying the variation of the morphology of the molecules in the mixture using SEM(Scanning electron microscopy) and dynamic light scattering method. The pair of liquids has to be heated and at various temperature, the morphology of the mixture has to be investigated. At UCST, both the liquids will undergo a significant change in their morphology and behave as an emulsion. This method was not viable for me as it involve the use of high end apparatus which are not available in a school laboratory.

Both phenol and water has inter molecular H bonding in the pure state due to the presence of polar O-H bonds in them. On mixing, these molecules will break the H bonds within their own system and form inter molecular H bonds with each other to become mutually soluble. The formation of inter molecular H bonds between them depends on two factors. - 

• If the amount of one component in the system is significantly lesser than the other, they will form the H bond easily and become soluble with each other even at a low temperature.
•  In case, both the components are present in nearly equal proportion, the temperature of the system has to increase to provide more kinetic energy to the molecules and break the H bond within their own system to form H bond with each other. Thus, as the ratio of the components in the mixture approaches towards 1:1, they become mutually soluble only above a particular temperature which is considered as the UCST value of that system. 

The current investigation deals with a phenol-aqueous NaCl system which is actually a ternary system phenol, water and NaCl but it may be looked upon as a binary system of phenol and water with NaCl as an impurity in it, as the solutions of NaCl used are dilute.

The UCST of a pair of liquids decreases as we add an impurity which is soluble in both the liquids while it increases as we add an impurity, soluble in one of the liquids and insoluble in other. NaCl is an ionic compound which is soluble in water but not in phenol. Hence, presence of NaCl in a phenol-water system would increase the value of UCST. As we add NaCl, more water molecules are engaged in hydrating the Na⁺ and Cl^- ions formed from disassociation of NaCl. Thus, amount of H₂O molecules available to hydrate phenol molecule decreases and hence the mutual solubility of phenol and water decreases, which in turn increases the UCST.

The molar concentration of NaCl (aq) added to phenol is the independent variable. The solutions used are of concentration – 0.0 moldm^-3 (only distilled water; used as control), 0.5 moldm^-3, 0.7 moldm^-3, 0.9 moldm^-3, 1.1 moldm^-3 and 1.3 moldm^-3. All these solutions were prepared by adding requisite mass of NaCl weighed in an electronic mass balance in a 100cm³ volumetric flask. A graduated measuring cylinder was used to measure the volume of water added.

The temperature at which turbidity disappears (formation of one single layer) and reappears (separation into two distinct layers) of the phenol-NaCl(aq) system was noted down by taking the mixture in a test tube, heating it in a water bath and allow it to cool down after that. A stainless steel temperature probe coupled with a Lab Quest was used to record the temperature. The average of these two temperature values was used as average miscibility temperature and plotted against the mass percentage of phenol used to determine the UCST from the mutual solubility curve. In this way, the UCST value of the phenol-NaCl(aq) system was determined as the dependent variable for all values of concentration of NaCl(aq).

• Mass of phenol added – 5.00 ± 0.01 g
• Apparatus used to record temperature-stainless steel temperature probe
• Surface area – same test tube (20 cm³, hard glass) was used in all trials.
• Apparatus used to record mass – electronic mass balance.
• Percentage purity of reagents – both phenol and NaCl used were from the same reagent bottle.

Refer to Appendix A.2.- for the significance and method of controlling these variables.

There is no correlation between the UCST of phenol-aqueous NaCl system and the molar concentration of aqueous NaCl used.

There is a positive correlation between the UCST  of phenol-aqueous NaCl system and the molar concentration of aqueous NaCl used.

Regression analysis will be done to test the hypotheses to be accepted.

Refer to Appendix A.3.

Refer to Appendix A.4.

• Wear protective glasses, gloves and a lab coat at all times in order as phenol is a very corrosive compound and can cause extreme burns and blindness.
• The procedure should be carried out in a well ventilated environment so as to minimize any sort of inhalation caused due to heating of phenol.
• The phenol should be stored in a cool environment and disposal of phenol waste must be carried out judiciously, should be placed in a labeled glass bottle with a tightly secured lid.

Phenol has widespread harmful environmental effects. All phenol waste were diluted and disposed off safely into a waste chemical bin. All glass apparatus used were washed with chromic acid before reuse.

No ethical considerations are involved.

100cm³ of 0.0 %, 0.5%,0.7%,0.9%,1.1%,1.3% of NaCl solution must be prepared in five different 100cm³ volumetric flasks. The following table showcases mass of NaCl needed to make the aforementioned concentrations NaCl solution. NaCl was weighed on a watch glass using a spatula and a digital mass balance, transferred to 100 cm³ volumetric flask using funnel. 100 cm³ of distilled water was added using a graduated measuring cylinder through the same funnel.

Sample calculation

A sample calculation for the mass of NaCl to be added to prepare 0.5% molar concentration of NaCl has been shown below–

Number of moles = concentration × Volume

n = 0.5 × 0.1

(Note the conversion of 100cm³ to 0.1dm³ as the concentration is in dm³)

n = 0.05 moles

Mass = Number of moles × molar mass

m = 0.05  58.44 (Molar mass of sodium chloride = 58.44 grams)

m = 2.92 grams

Therefore, 2.92 grams of NaCl must be added to 100cm³ of water to prepare a 0.5% concentration solution. Similar calculations are to be done for other concentrations as well. Each of these solutions are made and stored in a glass burette for further use.

Average uncertainty in concentration

\(\frac{∆c}{c}=\frac{∆n}{n}+\frac{∆V}{V}=\frac{0.01}{0.05}+\frac{0.05}{100}\) = 0.003

∆c = c X 0.003 = 0.500 X 0.003 = 0.002

The values of average uncertainty in molar concentration differ in the value at the fourth decimal place, for the sake of simplicity it is considered up to three decimal place and same for all values of concentration. Both the electronic mass balance and stainless steel temperature probe was calibrated before use.

• A 250 cm³glass beaker was taken with a 20cm³ hard glass test tube kept inside it. It was placed on the pan of a top pan digital mass balance and tared to zero. 5 ±0.01 g of crystals of phenol was weighed in it transferred using a clean and dry spatula. The test tube was then fitted with the cork.
• A clean and dry burette was taken and filled with 0.000 ± 0.002 moldm^-3 NaCl (aq) up to the zero mark using a funnel. It was also ensured that there was no air bubble formed inside it.
• The cork of the test tube containing phenol was removed and 2 ± 0.1 cm³of 0.000 ± 0.002% moldm^-3 NaCl (aq) was added to it from the burette. The cork was fitted again as soon as the required volume of NaCl was added.

• A 250cm³ glass beaker was taken, filled with tap water and kept on a wire gauge resting on a tripod stand. The water in the beaker was heated using a Bunsen burner. The test tube was kept in the water bath and clamped as shown in Figure-5.
• The Lab Quest was switched on and the timer was set as 0.1 seconds. The temperature at which the two liquids appeared as a single clear layer was noted down.
• The test tube was removed from the water bath and kept in a test tube stand and allowed to cool down.
• The temperature at which the two layers separate again (turbidity reappears) was noted down.
• Steps 3-6 were repeated two more times to collect the reading in triplicates.
• Steps 3-8 were repeated by adding 1 cm³ of water to the same test tube. This was done till a total water volume of 7cm³ was added to the test tube.
• All of the above steps were repeated for other NaCl(aq) – 0.500 ± 0.002 moldm^-3, 0.700 ± 0.002 moldm^-3, 0.900 ± 0.002 moldm^-3, 1.100 ± 0.002 moldm^-3 and 1.300 ± 0.002 moldm^-3.

• The image below displays the appearance of the system when the two liquids were separated into two distinct layers and when they mixed to give one single layer.

(Images were taken by me using i-phone 7 plus during the experiment)

• The color of phenol was white inside the bottle/container and turned whitish brown as soon as it came in contact with air. This was due to the auto oxidation of some amount of phenol to 4 - benzoquinone in presence of air.

• When water was added to the test tube containing phenol, the compound started melting. The bottom of the test tube was observed to be cool due to the fact that the reaction between the two was taking in heat energy showing that it was endothermic in nature.
• Time taken for the turbidity to disappear is longer than the time taken for the turbidity to reappear. In simple words, heating the system to mix the liquids was faster than cooling the system to separate them into distinct layers.

Refer to Appendix A.5. for other raw data tables

*S.D = Standard deviation

Sample calculation for standard deviation

For 2 cm³ of water,

Calculation of standard deviation for temperature at which turbidity disappear(t₁)

Mean value = \(\frac{44.0+43.6+43.5}{3}\) = 43.7

S.D = \(\sqrt{\frac{(44.0-43.7)^2+(43.6-43.7)^2+(43.5-43.7)^2}{3}}\) = 0.26 = 0.3 (rounded of f)

The standard deviation has been rounded off to one decimal place to match the number of decimal place with the trial values.

Determination of mass percentage of phenol

Volume of NaCl(aq) added (V) = 2 ± 0.1 cm³

Density (d) = 1 gcm^-3 [ assuming the density of aqueous NaCl and that of water to be same].

Mass of water (m_w) = V X d = 2 X 1 = 2 g ± 0.1 ; ∆m_w = 0.1

[As density has been considered as a constant value, uncertainty due to density has been ignored and thus uncertainty in mass and volume of solution added are considered to be same.]

Mass of phenol(m_p) = 5 ± 0.01 g ; ∆m_p = 0.01

Total mass (m) = m_w + m_p = 2 + 5 ± (0.1 + 0.01) g = 7 ± 0.11 g ; ∆m = ∆m_p + ∆m_w = 0.1 +0.01 =0.11

Mass percentage of phenol (X_p) = \(\frac{mass\ of\ phenol\ (m_p)}{total\ mass (m)}\) × 100

For 2 cm³ of water,

X_p = \(\frac{5}{7}\) × 100 = 71.42

Determination of average uncertainty in mass percentage of phenol (∆Xp)

 \(\frac{∆X_p}{X_p}=\frac{∆m_p}{m_p}+\frac{∆m}{m}\)

\(\frac{∆X_P}{X_p}=\frac{0.01}{5}+\frac{0.11}{7}\) = 0.02 + 0.016 = 0.018 

∆X_p = 0.018 X X_p = 0.018 × 71.42 = 1.27

Refer appendix A.6. for processed data tables of all other concentrations of NaCl solution added.

Sample calculation

For mass percentage of phenol = 71.43

Average temperature at which turbidity disappear (t_{1 avg}) = \(\frac{(44.0\ +\ 43.6\ +\ 43.5)}{3}\) =  43.7 ± 0.1°C

Average temperature at which turbidity disappear (t_{2 avg}) = \(\frac{(23.6 \ +\ 23.2\ +\ 23.0)}{3}\) = 23.3 ± 0.1 °C

Average miscibility temperature (t) = \(\frac{t_{1avg+}t_{2avg}}{2}=\frac{(43.7+23.3)}{2}\) = 33.4 ± (0.1+0.1) = 33.4 ± (0.1+ 0.1) = 33.4 ± 0.2°C

Percentage uncertainty in average miscibility temperature = \(\frac{∆t}{t}\) x100 =  \(\frac{0.2}{33.4}\) x100 = ± 0.60

The graph above plots the mass percentage of phenol along x axis and the values of average miscibility temperature (t ± 0.2°C) along y axis. The average uncertainty in mass percentage values are not taken into consideration as it is not constant. As the graph indicates, the value of t, increases from 57.1°C to 69.4°C as the mass percentage of phenol increases from 41.6% to 50.0% while it decreases from 61.6C to 33.4°C as the mass percentage of phenol increases from 55.5% to 71.4%. A best fit polynomial line was drawn using MS-Excel. The data value at mass percentage=50.0 was ignored as an anomalous point. It is seen by differentiating the equation of the best fit polynomial line that the curve displays a maxima at x = 51.0 (point C). So, a perpendicular is drawn from C which interescts the trend line at A and another perpendicular is drawn from A which intersects y axes at point B. The value of y axes at point B ( y =  65.00°C) is considered as the UCST value.

The trend line follows the equation

y = -0.078x² + 7.952 x -137.3

\(\frac{dy}{dx}=\frac{d}{dx}\) (- 0.078x² + 7.952 - 137.3) = 2( - 0.078x) + 7.952 = - 0.156x + 7.952

\(\frac{d^2y}{dx^2}\) = 0.156

Since the value of \(\frac{d^2y}{dx^2}\) is negative (<0), the curve exhibits a maxima.

At maxima, \(\frac{dy}{dx}\) = 0

- 0.156x + 7.952 = 0 

x = \(\frac{7.952}{0.156}\) = 50.97 = 51.0 (rounded off *)

It shows that the curve exhibits a maxima (or maximum value of y axes-average miscibility temperature) at a mass percentage of phenol = 51.0

*The value of x represents the mass percentage of phenol and the value which we can plot or determine for x in the graph can have maximum up to one decimal place. So, the value of x as calculated for the maxima of the curve is rounded off up to one decimal place.

Literature value for UCST value of phenol - water system = 66.8°C

Experimental value of UCST of phenol -0.00% NaCl(aq) system = 59.00°C

[ Refer to Graph-A.1 in Appendix –A.7 for this value ]

Percentage error = \(\frac{literature\ value-experimnetal\ value}{literature \ value}\) × 100 = \(\frac{mod(66.8-59.0)}{66.8}\) × 100

=\(\frac{7.8}{66.8}\) × 100 = 11.67%

The independent variable in the investigation – molar concentration of NaCl (aq) (±0.002 moldm^-3) is plotted along the x axes while the dependent variable – UCST (± 0.2°C) of the phenol-aqueous NaCl system is plotted along the y axes.

The data points were plotted in a scattered graph using MS-Excel and a linear trend line was used. The equation of trend line as obtained is –

y = 14.09 x + 58.59

where y = UCST (± 0.2C) of the phenol-aqueous NaCl system

x = molar concentration of NaCl (aq) (±0.002 moldm^-3)

A significant increase in the value of UCST from 59.0 ± 0.2°C  to 65 ± 0.2°C as the molar concentration of NaCl(aq) increases from 0.000 ± 0.002% moldm^-3 to 0.500 ± 0.002% moldm^-3. This clearly indicates that addition of NaCl to phenol water system has a significant positive impact on the magnitude of the UCST value; addition of NaCl to phenol-water system as an impurity increases the UCST value significantly.

A mixture of phenol and 0.000 % NaCl(aq) is a binary system containing only water and phenol. As soon as we add NaCl to it, the system becomes ternary in nature containing phenol, water and NaCl. NaCl is an ionic compound and highly soluble in water while it is insoluble in phenol. Phenol and water can mix with each other at a particular composition or above a particular temperature due to the existence of intermolecular H bond between water molecules and polar OH group in phenol. 

As we add NaCl to the system, NaCl dissociates into Na⁺ and Cl^- ions, these ions are hydrated (both the ions are surrounded by H₂O molecules). Hence on adding NaCl, free water molecules available to make intermolecular H bond with phenol decreases, thus the mutual solubility of phenol and water decreases which is reflected by the increase in the value of UCST. Similar results have been observed on adding water to the binary system of cyclohexane and aminobenzene; water is soluble in amino benzene but not in cyclohexane; on adding to water to cyclohexane-aminobenzene system, the UCST value was increased significantly. 

A gradual increase in the values of UCST has been observed  from 65.0 ± 0.2°C to 77.0 ±0.20°C as the molar concentration of NaCl increases from 0.500 ± 0.002% moldm^-3 to 1.300 ± 0.002% moldm^-3. This indicates a positive correlation between the values of UCST and the molar concentration of NaCl. It shows that as we add more amount of NaCl as an impurity to phenol-water system, the UCST of the system increases. The increase in the values of UCST with the increase in the molar concentration of NaCl is mostly uniform.

During mixing of phenol with aqueous NaCl, enthalpy of mixing (∆H) is positive (endothermic process; Refer to qualitative observations, Page-16, point-3). Entropy of mixing (∆S) is also positive as the disorder of the system increases. Hence, as the temperature of mixing increases, the value of free energy change (∆G) would become more negative and the mixing would become more feasible.

∆G = ∆H - T∆S

As we add more NaCl, the surface tension of junction of two layers increases due to increase in number of particles on the surface which makes the process less spontaneous. So, the mixing occurs at a higher temperature to make the value of ∆G more negative and the process more spontaneous. Thus, as we add more NaCl, the two components –phenol and aqueous NaCl becomes mutually soluble at a higher temperature to make the mixing more spontaneous and hence the magnitude of UCST increases.

The magnitude of coefficient of determination (R²) as 0.993 in the graph between magnitude of UCST in °C and the molar concentration of NaCl(aq) in moldm^-3indicates a strong positive correlation between them.

[Refer to Appendix A.8. for detailed calculation of R²]

The magnitude of constant of determination (R² = 0.963) indicates that the polynomial trend line in the graph between average miscibility temperature and mass percentage of phenol in Graph-1 is appropriate.

As a complementary finding, a graph showcasing the different parabolic shapes of each concentration of NaCl solution was tabulated –

f(x) = 0.5%. g(x) = 0.7%  h(x) = 0.9%  i(x) = 1.1%  k(x) = 1.3%(unit : moldm^-)³

Figure 14 showcases change in parabolic shape from 0.5% to 1.3% NaCl added i.e f(x) to k(x) respectively. Through the graph, the following can be inferred –

• The peak of the each parabola (maxima) increases with the increase in molar concentration of NaCl.
• The value of UCST for all molar concentration of NaCl is obtained at the same value of mass percentage of phenol (51.0°C).

How does the magnitude of upper critical solution temperature (UCST) of phenol(hydroxy benzene)-aqueous NaCl system depends on the molar concentration(moldm^-3) of aqueous solution of NaCl added to it, determined using mutual solubility curve?

• The data collected and processed clearly indicates that the magnitude of UCST  of phenol-aqueous NaCl system increases as we increase the molar concentration of aqueous solution of NaCl added.
• The correlation between the magnitude of UCST (y) and the molar concentration of aqueous NaCl (x) may be quantitatively expressed as – 

y = 14.09 x + 58.59

• The magnitude of R² (constant of determination) as 0.963 indicates a strong positive correlation between UCST and molar concentration of aqueous NaCl. Thus, the null hypotheses is rejected and the alternate hypotheses is accepted.
• A significant difference in the value of UCST (59.0 ± 0.2°C to 65.0 ± 0.2°C) as the concentration of NaCl(aq) added changes from 0.0 moldm^-3 (pure water, used as control) to 0.5 moldm^-3. It makes the fact evident that presence of NaCl as an impurity in a phenol-water system increases the magnitude of UCST of the system significantly.
• As noted in the qualitative observations, phenol added turns brown due to auto oxidation in air and thus contributes towards a major methodological issue.
• As an accessory finding it has been found that, all phenol-aqueous NaCl system exhibits UCST at the same value of mass percentage of phenol (51.0). It means that even if the molar concentration of NaCl(aq) changes, the composition at which the mixture of phenol and aqueous NaCl exhibits UCST does not change.
• As we are adding more NaCl, the mutual solubility of phenol and water is decreasing, thus the temperature at which they can mix increases to make the value of ∆G less negative and the mixing more spontaneous.
• A percentage error of 11.67% has been deduced and that indicates a systematic error of the investigation which may be due to loss of heat from the system.

• Uncertainties in apparatus used like temperature probe, electronic mass balance introduces random error in the investigation. Use of more precise apparatus like using burette to add water instead of a measuring cylinder was done to reduce this. Moreover, the readings have been taken in triplicates and average values have been considered. Both the electronic mass balance and temperature probe was calibrated before use.
• Phenol undergoes oxidation in water via free radical mechanism to form 2-hydroxy phenol and 4-benzoquinone.This is a methodological issue.
• The experimentally obtained value of UCST of phenol and 0% NaCl system (or phenol-water system) is 59.0C while the literature value is 66.8°C. This indicates a negative systematic error and reduces the accuracy of the investigation. It may be due to lose of heat from the test tube. To prevent this, a cork with a hole to insert the temperature probe was used to seal the test tube.
• Although the crystals of phenol in the reagent bottle was colorless but they turned brown when exposed to air while weighing them indicating the auto oxidation of phenol (Refer to qualitative observation, page –16 point - 2).
• The crystals of phenol begun to melt partially while weighing it using a watch glass. Thus, transferring the phenol completely into the test tube from the watch glass using a spatula was difficult. So, a beaker with a test tube inside it was kept on the pan of the digital mass balance (tared to zero) and the phenol was directly weighed in it to ensure that no phenol is lost or left behind while weighing.

• The precision of the temperature readings is definitely characterized as a strength due to the fact that these readings are varying by a very small amount in all of the three trials (the standard deviation is low).
• The accuracy of the experiment is exhibited due to a small percentage error value (11.67%) of the UCST as compared to a literature source.
• Through the experiment, a complementary scientific phenomenon was proven right i.e. the value of mass percentage of phenol at which the UCST value is observed remains same (51.0) for all values of molar concentration of NaCl(aq) solution.
• All secondary sources considered are credible, reliable and appropriate. Use of commercial websites as reference has been avoided.
• Results reported in the investigation are supported by exemplary research findings in the literature (Refer to Page 21)

This experiment has determined the critical solution temperature of phenol water system by varying concentration of ionic compound NaCl solution and thereby recording different miscibility temperatures. It can be furthered extended by monitoring the effect of CST by the addition of other types of ionic impurities like KCl and organic impurities like naphthalene, camphor and comparing its results with that of sodium chloride. Finally, a comprehensive study can be conducted to check exactly which impurity would result in a maximum increase in miscibility of partially miscible liquids like the phenol water system. This can be done to find the best solvent for particular compounds and aids in the ‘salting out’ chemical process.

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The temperature below which a pair of liquids are completely miscible with each other irrespective of percentage composition is called Lower critical solution temperature (LCST).

To find the value of R² (constant of determination), the value of the co-relation constant (R) must be found using the formula –

R² =  \(\frac{n(\sum xy)-( \sum x)(\sum y)}{\sqrt{[n(\sum x^2)}-(\sum x)^2][\sqrt{n(\sum y^2)}-(\sum y)^2]}\)

where,

∑ = summation of

x = molar concentration of NaCl

y = UCST

n = number of trials (5)

Using the table below, to find the value of R for Figure 12

Therefore,

R = \(\frac{6(326.4)-(4.5)(415)}{\sqrt{[6(4.45)-(4.5)^2][6(28919)-(415)^2]}}\) = 0.9967

Hence, R² = 0.993