Experiment #5: COLLOIDS MANUNTAG, Monica Earl TUMIMBANG, Glenn Vincent Group 5, Chemistry 14.1, Ms. Fatima Cruz March 27, 2012
I. ABSTRACT A colloid is a type of mixture in which a substance is dispersed evenly over another, basing its definition on the particle size, which is from 1 to 100 nanometers or 10-9 meters. A colloidal system has two phases: the dispersing or continuous phase and the dispersed or internal phase. The former refers to the medium used while the latter are the particles present in the system. Sols are classified under colloids, a type of dispersion to which a solid is dispersed in liquid. Aerosols, a colloidal system in which a liquid is dispersed in either a solid or a gas, is also classified here. And finally, emulsions, a dispersion of liquid in liquid, is also considered as a type of colloid. This system can either exist in a solid, liquid, or gaseous state, and it has two properties: Tyndall effect or the ability of the particles to scatter light and precipitation or the formation of insoluble solid in liquid. Preparation of colloids may be done either through dispersion or condensation. During dispersion, large-sized particles are dispersed to break them down to colloidal size, an example of which is the addition of an emulsifying agent. On the other hand, condensation is a way of uniting moleculesized particles to form colloid-sized particles. The experiment aims to determine the differences between the various kinds of colloids, as well as the properties they exhibit. The experiment also aims to compare the two principal methods in preparing colloidal system, dispersion and condensation. III. KEYWORDS: Colloids, Solution, Suspension, Dispersed phase, Dispersing medium, Sol, Lyophobic, Lyophilic, Emulsion, Tyndall effect, Precipitation
III. INTRODUCTION Colloid is a type of mixture in which a substance dispersed evenly throughout another substance, both of which existing either as a solid, liquid, of gas. These combinations of dispersion give us the different types of colloidal systems which include familiar examples like clouds, fog, mist, smoke, soap, milk, mayonnaise, whipping cream, and even blood. The objective of this experiment is to compare the difference between the two principal methods of preparing colloids: the method of condensation and the method of dispersion. Furthermore, the experiment will be able to determine the different types of colloids, as well as the properties they exhibit. IV. EXPERIMENTAL PROCUDURE For the preparation of colloids, first, fifty mL of water was heated until it reaches its
boiling point then freshly prepared 1 M FeCl3 was added by dropping. Dropping was continued until a change in color can be observed. The solution was allowed to cool for use in the next procedure. A pinch of sulfur was added to 50 mL of water and then stirred. In another set-up, alcohol was heated in a water bath. A pinch of sulfur was added and then the solution was poured into a 50 mL beaker of water. The two set-ups were observed and compared. Next, the proponents placed 10 mL of water in a test tube and added 1 mL of oil while shaking. After observing, it was set aside for 1020 minutes. Then, 5 mL of concentrated soap solution was added and the test tube was shook. Again, changes that can be observed were noted.
The next part of the experiment was the Alexander’s patriotic tube. First, a small amount of agar was dissolved in 15 mL of boiling water. Two drops of 1 M NaOH, a few drops of phenolphthalein to turn it pink, and 1 mL of 1 M Potassium Ferrocyanide (K4[Fe(CN)6]) was added. The solution was immediately pured into a test tube and was set aside for the agar to cool into gel. The test tube was corked and let stand for an hour. The third part of the experiment was the preparation of colloids. For the first part of preparation of colloids, which is the Tyndall effect, Fe(OH)3 sol was placed in a test tube and observed at a right angle against a beam of light form a flashlight. The same was done to sulfur sol, copper sulfate solution, dilute evaporated milk, boiled starch solution, unboiled starch solution and soap solution. The observations were compared to distilled water as negative reference. Next is the experiment on precipitation. Fe(OH)3 sol was placed in three separate test tubes with 2 mL of solution. The first test tube was added with 10 drops of 1 m NaNO3, 10 drops of 1 M Na2SO4 to the second and 10 drops 1 M Na3PO4 to the third. They were then observed for the amount of precipitate present. V. EXPERIMENTAL RESULTS Table 1. Preparation of Colloids COLLOID OBSERVATION The color changed to brownish Fe(OH)3 sol orange. Sulfur did not dissolve in water. Sulfur (in Some particles settled at the water) bottom while others floated. The solution turned cloudy Sulfur sol (in while particles of sulfur settled hot alcohol at the bottom. and water) Two observable layers can be observed with oil on top of Oil (in water) water. A cloudy yellow layer (water and oil) formed beneath the Oil (in soap bubbly and green soap and water) solution. Table 2. Tyndall Effect SYSTEM OBSERVATION Negative; light passed through Fe(OH)3 sol completely without scattering
Sulfur sol
CuSO4 sol Dilute milk Boiled Starch Solution Unboiled Starch Solution Soap Solution
Distilled Water
Positive; light can be seen passing through the cloudy mixture; scattered Negative; light passed through completely without being scattered. Positive; light can be seen passing through the cloudy white mixture; scattered Positive; light can be seen passing through the cloudy white solution; scattered Negative; light passed through completely without being scattered. Positive; light can be seen passing through the light green solution; scattered Negative; light passed through completely without being scattered.
Table 3. Precipitation PRECIPITANT OBSERVATION Less was formed 1 M NaNO3 Least precipitate formed 1 M Na2SO4 Most precipitate formed 1 M Na3PO4 VI. DISCUSSIONS A colloidal dispersion is a type of mixture whose properties are between heterogeneous mixtures and homogenous solutions. The dimensions of its particles range from 1 to 100 nanometers. It has two phases namely, the dispersed phase and the dispersing phase. The dispersed phase are the particles itself while the dispersing phase is the medium or the solvent. Colloids may be prepared through dispersion or condensation. Dispersion is the process of breaking down large particles to colloidal size. Examples of these are grinding and adding an emulsifying agent. On the other hand, condensation is the process of making particles of molecular size unite to form particles of colloidal size. There are several kinds of colloids. We have the sols which is the dispersion of solid in liquid, the dispersion of liquid in liquid which we call emulsions, dispersion of liquid or solid in gases known as aerosols, foams-dispersion of
liquid in solid and gels which is the dispersion of liquid in solid. In the first part of the experiment, the colloid medium or the particles of Fe(OH)3 sol and the particles of the sulfur sol, did not mix well with the dispersing medium or water. This is because according to the solubility rules, Fe(OH)3 and sulfur are insoluble in water. Also, the shape of the sulfur (S8) has a large surface area for the solvent to dissolve. On the other hand, sulfur in hot alcohol and water partially dissolved sulfur since ethanol is less polar and heated as well. The second part of the experiment involved the dispersion of liquid in liquid or emulsion. Oil was mixed with water, and was later added with soap solution. At first, the oil formed scattered droplets in water; but after leaving it for some time, the oil droplets started to gather on top of the water, forming a distinct layer. The soap solution was then added and chemically interacted with both the oil and the water. It acted as an emulsifier, a substance that stabilizes an emulsion, therefore stabilizing the interface between the oil and the water in the suspension. Interestingly, the same principle is used in soap to remove grease in dishwares and other objects for cleaning. The next part of the experiment is the Alexander’s Patriotic Tube. Gel, a type of colloid where liquid is dispersed in solid, was made. Three layers have been formed: pink, whiteyellow and blue. The bottom part was pink caused by phenolphthalein, a base indicator, due to the presence of base. H2O and FeCl3 formed Fe(OH)3. The bluish part called Prussian Blue was made from this reaction FeCl3 + K4[Fe(CN)6]KCl + Fe4[Fe(CN)6]3. The arrangement of layers is made such that way due to the densities of the ions and compounds in the system. The next part was the experiment on the Tyndall effect. The systems that exhibited Tyndall effect were the following: Sulfur sol, dilute milk, and boiled starch solution exhibited Tyndall effect. On the other hand, no scattering
was observed with the Fe(OH)3 sol, copper sulfate solution, soap solution and distilled water. Theoretically, the Fe(OH)3 sol, sulfur sol, dilute milk, boiled starch solution, unboiled starch solution (+/-) and soap solution should have exhibited Tyndall effect while the CuSO4 sol and the distilled water are the only ones that will not exhibit Tyndall effect. The last part involves precipitation. Precipitates of varying quantities formed on each test tube. These are the reactions involved in each test tube: Fe(OH)3 + 3NaNO3 ----> Fe(NO3)3 + 3NaOH 2Fe(OH)3+3Na2SO4 ----> Fe2(SO4)3 +6NaOH Fe(OH)3 + Na3PO4 ----> Fe(PO4) + 3NaOH The quantity of the precipitates formed is based on the charges of the ions that will attach to the Fe3+. Since the PO4 is the most electronegative among the others, it yielded the most prominent precipitates. SO4 came next while NO3 has the least. VII. GUIDE QUESTIONS & ANSWERS 1. Why is sulfur not soluble in water? What kind of colloidal dispersion is a sol? The basic principle in solubility, like dissolves like, enabled the immiscibility of sulfur to water. Since water is polar while sulfur is not, the two did not mix. That is why sulfur particles either settled at the bottom or floated. A sol is a type of colloid of very small solid particles dispersed in a liquid that retains the physical properties of a liquid. It is a lyophobic (solvent-hating) suspension of solid particles (1-1000 nm in size) in a liquid. 2. How would you account for the observation in procedure IA-b2? Sulfur is a nonpolar crystalline structure which makes it harder to dissolve in water under normal conditions. The less
polar ethanol was able to dissolve the sulfur under the presence of heat. A lyophobic colloid was formed when the sulfur-alcohol mixture solution was added to water. 3. What is the role of soap solution in the oil emulsion prepared in procedure IB? The presence of polar and nonpolar ends in a soap solution bridged the immiscibility of oil and water. In the molecular point of view, the soap, having a lipophilic end, covers and stops the oil from separating from the water molecules. This structure is called micelle. On the other hand, the water molecules is attracted to these lipophilic ends and therefore, exhibiting a miscibility with the oil. The soap is called the emulsifying agent. 4. Give an explanation for the results obtained in procedure II. What ions/compounds are responsible for the different colors observed in the Alexander’s patriotic tube? There were four observable layers in the Alexander’s patriotic tube. The bottom gelatinous layer is composed of agar, NaOH, and phenolphthalein, which is seen as pink (due to the presence of a base and a base-indicator). The transparent part above it is caused by the chlorine entering and dispersing to the matrix of the gel. The upper layer, which is blue in color, was formed from the reaction of ferric chloride with potassium ferrocyanide. This layer is Fe4[Fe(CN)6]3 or Prussian blue, which is a highly insoluble substance but tends to form colloids. And finally, the topmost layer, yellow in color, is from the excess ferric chloride.
5. What causes Tyndall effect? Tyndall effect is the effect of light scattering on particles in colloidal systems. It is caused by reflection of light by very small particles in suspension in a transparent medium. This is exhibited when dust particles are seen in the air when sunlight strikes them or when the lights emitted by the headlights from a car on a dusty road can be seen. The Tyndall effect is used to tell the difference between the different types of mixtures, namely solution, colloid, and suspension. VIII. CONCLUSION & RECOMMENDATIONS Colloids are mixtures wherein a substance is dispersed throughout another. Their distinct properties (e.g. Tyndall effect and precipitation) make colloids different from solutions and suspensions. Since these are mixtures, they are physically combined so we may also separate them by physical means like undergoing dialysis. Colloids are everywhere. We may find it from the food we eat, the air we breathe and even in nature. It is important for us to know the properties of colloid that differentiate it from solutions and suspensions. Through testing Tyndall effect and observing precipitation, we may say that a sample is a colloid. Aside from that the nature of substance involved, temperature and polarity may also affect a colloidal system. It is recommended in this experiment to calculate exact amounts of reagents to obtain results close to the theoretical yield. Following the procedures strictly in this experiment can help achieve theoretical results. Below are summaries of the types of colloids and the differences between colloid, solution and suspension:
IX. REFERENCES Brown, T. L., Lemay Jr., H. E., Bursten, B. th E. Chemistry: The Central Science (9 ed.). Singapore: Pearson Education (Asia) Pte Ltd.
Committee on General Chemistry (2006). Laboratory manual in general chemistry (2006 ed.). Manila, Philippines: University of the Philippines Manila Microsoft ® Encarta ® 2007. © 1993-2006 Microsoft Corporation. All rights reserved.
I hereby certify that I have given substantial contribution to this report.
Manuntag, Monica Earl
Tumimbang, Glenn Vincent
