c. Read the liquid volume at eye level from the bottom of the meniscus. Based on these results, is this reaction (as written) exothermic or endothermic: Explain: Observations upon addition of \(\ce{FeCl3}\): Observations upon addition of \(\ce{KSCN}\): Observations upon addition of \(\ce{AgNO3}\): This page titled 12: Equilibrium and Le Chatelier's Principle (Experiment) is shared under a CC BY-NC license and was authored, remixed, and/or curated by Santa Monica College. A change in temperature will also cause a reversible reaction at equilibrium to undergo a shift. After the solvent is added, stopper and invert the flask to mix the solution. 2. It is a control for comparison with other tubes. Reaction Order . In order to calculate Kc for the reaction (shown below) it is necessary to know the concentrations of all the ions at equilibrium. When a chemical is removed from a reversible reaction at equilibrium, a shift towards the removed chemical occurs. _____, Determine whether each described process is endothermic or exothermic. 27. 9. b. Pour the contents of the two test tubes back and forth between the test tubes three times, Chemical Kinetics (rate law) Lab: Test Tube 1, Starch, sodium thiosulfate (NaSO), potassium iodide (KI), potassium nitrate (KNO), Chemical Kinetics (rate law) Lab: Test Tube 2, Ammonium peroxydisulfate ((NH)SO), ammonium sulfate ((NH)SO). Label the beaker and place it on the front desk. Equilibrium shifts in exothermic / reverse direction and the concentration of FeSCN2+ will be decreased, so colour of solution is lighter. A + B ---->>>>>>>>>>>>> C + D (shift to the right) Equilibrium occurs in ______ reactions when the RATES OF THE FOWARD AND REVERSE REACTIONS ARE EQUAL. A "heat" term can be added to the chem. 1. 1 doc WAP TXT Seminar Professional English Modern analytical chemistry Bioanalytical techniques Advances in polymers Advances in functional polymers Progresses . The color of their drink mix is supposed to be a pale green color, but they often get different results. 30. The FeSCN 2+ complex that is formed as a result of reaction between iron (III) and thiocyanate ions has a very intense blood red color (or orange in dilute solution), allowing for easy detection and quantitative determination by spectrophotometry. b. In both processes, heat is absorbed from the environment. Cu2+ was removed Clearly identify the data and/or observations from lab that led you to your conclusion. One example of an exothermic reaction is the oxidation of iron sulfide (FeS) to form iron oxide (FeO) and sulfur dioxide (SO 2) gas: Read More polymerization In chemistry of industrial polymers: Industrial polymerization methods LeChtelier's Principle When a variable affecting the state of a system is changed, the equilibrium of the system will be modified so as to compensate for the change in the variable. For example, in the equilibrium equation: N 2 (g) + 3H 2 (g) 2NH 3 (g) Beer's Law states that A=bc, where A is the absorbance, is the molar absorptivity of the solute, b is the path length, and c is the concentration. For an endothermic reaction heat can be viewed as a reactant and for an exothermic reaction heat can be viewed as a product. To find the order of a reaction with respect to one reactant, you will monitor the _____ as the _____ of _____ is changed. After the solvent is added, stopper and invert the flask to mix the solution. Requires a clock reaction. OH- was added, 2. The cation affects the color of the solution more than the intensity of the color. Hydroxide ion light colorless This means that when heat is added, i.e. An exothermic reaction is defined as a reaction that releases heat and has a net negative standard enthalpy change. The [Fe] in the standard solution is 100 times larger than (SCN). Write the balanced equation for this reversible reaction. d. The determination of whether heat is consumed or given off from one gram of a substance, c. The change in heat required to change the temperature of something by one degree Celsius. In endothermic reactions, heat energy is absorbed and thus can be considered a reactant. Fe + 4Cl FeCl c. There may be an issue with the spectrophotometer. For each unwanted result, choose the most plausible explanation to help the company improve the formula. <------- Record your observations. Decrease in Temperature. 1. If a chemical reaction absorbs as much energy as it releases, it is called isothermicthere is no net energy change. What effect does the anion of an ionic compound have on the appearance of the solution? (c) Viscosity We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. In which direction (left or right) would the following stresses cause the system to shift? Fe3+(aq) + SCN-(aq) -->>>>>FeSCN2+ (aq) (shift to the right) <----------- Measure the absorbance for solutions with different concentrations and find the slope of the trendline. Iron (III) ion Thiocyanate ion <----- Thiocyanatoiron yellow colorless complex ion red 11. Score: 4.6/5 (71 votes) . d. The concentration of the blue dye solution is greater than the concentration of the red dye solution. 5. solid What happens to the color of the solution as the concentration of the solute changes? Red - _____, Orange - blue Sodium thiosulfate (NaSO) - clock reaction reagent 6. The substance cools down slowly after heating. For each unwanted result, choose the most plausible explanation to help the company improve the formula. 18. Fe3+ was added a. Absorbance vs. wavelength Also note that direct contact with silver nitrate (\(\ce{AgNO3}\)) will cause dark discolorations to appear on your skin. Add one drop of 0.1 M AgNO 3 to a fourth well, mix, and record observation. reactant, removes iron from the iron-thiocyanate equilibrium mixture. Exothermic Endothermic, 31. c. The molar absorptivity of the blue dye is less than the molar absorptivity of the red dye. What would the effects of heat be on the equilibrium of an exothermic reaction? c. presence/lack of a catalyst iron(III) ions to iron(II) ions: Sn+2 + 2 Fe+3 Sn+4 + 2 Fe+2 6. The entire class will then use this stock solution in Part 5. Hesss Law 14. Fe3+ was added e. The intensity of the color does not change in response to any concentration change. b. A + B -----------> C + D b. An increase in the concentration of the reactant in solution causes the reaction rate to increase exponentially. 41. reaction describing the equilibrium to indicate whether the reaction is exothermic or endothermic. Ammonium sulfate ((NH)SO) - ion concentration stabilizer Fusion, in contrast, occurs when two or more smaller atoms fuse together, creating a larger, heavier atom. The reaction rate is constant regardless of the amount of reactant in solution. What is the best way to mix the equilibrium solutions? Is this reaction endothermic or exothermic? The reaction, as written, is exothermic. second order. a. Iodine can stain the body and other surfaces. Options: indicator, reactant of interest for rate equation, ion concentration stabilizer, clock reaction reagent B. Is the reaction of iron nitrate and potassium thiocyanate reversible? The FeSCN 2+ complex that is formed as a result of reaction between iron (III) and thiocyanate ions has a very intense blood red color (or orange in dilute solution), allowing for easy detection and quantitative determination by spectrophotometry. False: if a system in equilibrium, where the forward reaction is endothermic, is . q, in joules transferred by a chemical reaction to the reservoir of a calorimeter containing For an endothermic reaction heat can be viewed as a reactant and for an exothermic reaction heat can be viewed as a product. Thus [FeSCN24]sta is assumed to be equal to [SCN1std. equation is a correct based on the shifts you observed in test tubes #5 and #6 as a result of the heating and cooling? c. Iodine is highly flammable. When this occurs, a state of chemical equilibrium is said to exist. . Reactants ( Fe 3+ and SCN-) are practically colorless. Take up a quiz on Difference Between Endothermic and Exothermic Reactions The volume of Standard solution needed will not fit into a test tube. *After mixing, look for (__1__) color due to formation of FeSCN2+* \[\ce{Ag^{+1} (aq) + SCN^{-1} (aq) -> AgSCN (s)}\]. a. On the other hand, as the reaction proceeds, the concentrations of \(C\) and \(D\) are increasing. Fe3+(aq) + SCN-(aq) -->>>>>FeSCN2+ (aq) (shift to the right) the direction of a particular shift may be determined. <------- For an increase in temperature, the reaction shifts in the endothermic direction to relieve the stress. Step1: Define exothermic reaction and endothermic reaction. Experiment 8 Exploration of LeChtelier's Principle and Equilibrium Introduction The purpose of this lab is to experimentally determine an equilibrium constant (K.) and to examine. Explain. Iron (III) Thiocyanate Solution By observing the changes that occur (color changes, precipitate formation, etc.) endothermic reaction exothermic reaction Question 12 45 seconds Q. b. Rate law for Chemical Kinetics (rate law) Lab: r = [SO]/time = k [I]^x [SO]^y, The amount of time required for fixed, small amounts of the reactants to react is measured. Clearly identify the data and/or observations from lab that led you to your conclusion. An endothermic reaction is a reverse reaction and it is favoured. The yield of the product (NH 3) increases. Fe3+ SCN- FeSCN2+, Fe3+(aq) + SCN-(aq) -->>>>>FeSCN2+ (aq) (shift to the right) Which compound is REMOVED from the equilibrium mixture when you add compound E to the mixture? If the solution is overheated it will splatter out of the tube, so be careful not to point the tube towards anyone while heating. Equipment: 10 small test tubes, test tube rack, test tube holder, Bunsen burner, 2 medium-sized beakers (for stock solutions), 10-mL graduated cylinder, wash bottle, stirring rod, and scoopula. _____ As a general rule, if the temperature is increased, a shift away from the side of the equation with heat occurs. _____. _____ Record your observations. Suppose you prepare a Question: Iron (III) Ion And Thiocyanate Ion Exists In Equilibrium With Iron Thiocyanate Ion. Using similar logic, the following changes in concentration are expected to cause the following shifts: In other words, if a chemical is added to a reversible reaction at equilibrium, a shift away from the added chemical occurs. Suppose you add compounds A, B, C, D to a beaker to form an equilibrium mixture. LeChtelier's Principle will be examined for the above reaction with respect to temperature and addition of reactant. S(s)+O(g)SO(g); -296.8 Fe3+(aq) + SCN-(aq) -->>>>>FeSCN2+ (aq) (shift to the right) A student carries out an experiment to determine the equilibrium constant for a reaction by colorimetric (spectrophotometric) analysis. For each unwanted result, choose the most plausible explanation to help the company improve the formula. Cool Experiment - An endothermic reaction freezing a beaker to the bench - Barium hydroxide and ammonium thiocyanate. Pour about 30 mL of 0.00200 M Fe(NO3)3 into a clean dry small beaker. 10. 17. These spots will eventually fade after repeated rinses in water. _____ Lay the pipettor on its side or turn it upside down. a. You added potassium thiocyanate (KSCN) to the equilibrium mixture in test tube #3. How is the equilibrium of Fe ( NCS ) 2 + shifted? and Exothermic Heat Transfer Science LabUse steel wool and vinegar to teach students about heat transfer and endothermic and exothermic reactions. NaSO Suppose you add compounds A, B, C, D to a beaker to form an equilibrium mixture. d. The anion only affects the intensity of the color in a solution. It must be determined experimentally, A change of the initial concentration of a particular reactant doesn't affect the reaction rate, A change in the initial concentration of a reactant creates a reactant rate change proportional to the concentration change, a side reaction that indicates when the fixed amount of reactant has been consumed. a. reactant concentration ln (rate of run/rate of run) / ln ([SO] run/[SO] run), T or F: x and y should be rounded to a whole number when used in the rate law, Endothermic reaction: as T increases, K _____, Exothermic reaction: as T increases, K _____, SSC 200 - Eyewitness Identification Quiz Stud, SSC 200 - Quiz 4? The blue dye solution absorbs less light than the red dye solution. What is the heat, The equilibria studied in the lab procedure include which two reactants? The evidence for the dependence of absorbance on the variable c is Top Mika Sonnleitner 1A Posts: 50 Joined: Fri Sep 29, 2017 2:04 pm Been upvoted: 2 times Re: Iron Rusting: Exo or Endo? A + B -------> C + D (shift to the left) reaction. Which components of the equilibrium mixture INCREASED in amount as a result of this shift? a. Iodine can stain the body and other surfaces. 73 a. c. There may be an issue with the spectrophotometer. -------->, Fe3+(aq) + SCN-(aq) -->>>>>FeSCN2+ (aq) (shift to the right) Label the beaker and place it on the front desk. 28. Which component of the equilibrium mixture DECREASED as a result of this shift? <----------- Eventually a point will be reached where the rate of the forward reaction will be equal to the rate of the backward reaction. Match the component with its purpose. Wood burns in a fireplace. Iron (III) ion Thiocyanate ion <----- Thiocyanatoiron Look for response: by looking at the (__5__) of the solution This prefers an exothermic reaction because it gives energy. Exothermic Ice melts into liquid water. 3.6.20 3.6 = 18m 20 20 Laboratory Procedure Work with a partner. Enthalpies of Formation 15. A B C D, E. What shift in the equilibrium will occur as a result of the addition of compound E? Even when the equilibrium concentrations are different, their ratio should yield the same value for Kc (at constant temperature). Loss of heat is a stress --> shifts the equilibrium to the (__1__) to get more heat --> MORE FeSCN2+ around --> red color DEEPENS. The color of the dye is appearing as red, instead of green because, A beverage company is having trouble with the production of the dye in their drinks. d. There may be an issue with the composition of the sample. 3. equation below. answer choices Candle was melting A puddle evaporating Dry ice (solid carbon dioxide) subliming to form gaseous carbon dioxide Water freezing to form ice Question 13 20 seconds Q. Fe3+ (aq) + SCN- (aq) <---- FeSCN2+ (aq) The equation representing this endothermic reaction shows that it is entropy driven: Ba (OH) 2 *8 H 2 O (s) + 2 NH 4 Cl (s) --> BaCl 2 *2 H 2 O (s) + 2 NH 3 (aq) + 8 H 2 O (l) This is a neutralization reaction with the hydroxide ion acting as the base and the ammonium ion acting as the acid.The two relatively low entropy crystalline solid reactants react to form many small molecules in the . Record the exact mass of iron nitrate used and show the calculation for the solution concentration in your observations. Set it up: mix FeNO3 solution w/ KSCN solution This will increase the overall temperature and minimise the decrease in temperature. red . 3. a. increasing the cuvette width increases the absorbance. yellow colorless -----> Red <------- a. Then cool the solution in test tube #3 back to room temperature by holding it under running tap water, and again record your observations. The sample may be placed improperly in the cuvette holder. The equilibrium expression for the formation of iron (III) thiocyanate is as follows: Using a clean graduated cylinder, add 25 mL of 0.0020 M KSCN to a 100 mL beaker. When a solid solute is put into a solvent, does the rate of dissolving increase or decrease as the dissolution proceeds? Unfortunately . However as the reaction proceeds, the concentrations of \(A\) and \(B\) will decrease. The Reaction, As Written, Is Exothermic. (a) The optimum wavelength for the measurement of [FeSCN2+] must first be determined. 2003-2023 Chegg Inc. All rights reserved. Color Indication Reaction in Chemical Kinetics (rate law) Lab: Starch (aq) + I starch-I complex (blue color). Which equilibrium component did you add when you added iron (III) nitrate? These two test tubes serve as controls to compare against the other test tubes. In particular, concentrated 12 M \(\ce{HCl}\) is extremely dangerous! NH. Iron rusting is a reaction with oxygen to create iron oxide. *******NOT FINISHED, 12. Respiration is considered as an exothermic reaction because, in respiration, a large amount of heat energy is released when oxidation of glucose takes place. V = 20ml 2. Ice melts into liquid water. The reaction rate increases in direct proportion to the concentration of the reactant in solution. The relationship between absorbance of light by a solution and its concentration should be Based on your results from test tubes 5 and 6, is this reaction exothermic or endothermic. Fe (24) + SCN FeSCN2(aq) I [Fe3+] (analysis 1) [SCN) (analysis 1) 0 - [FeSCN23c4 [FeSCN2) +(Acq/Asid) x [FeSCN2"std E [Fe3"] [SCN34 [FeSCN) Knowing the equilibrium concentrations of each of the ions allows for the calculation of Ke for the reaction. Is fecl3 exothermic or endothermic? Endothermic reactions require energy, so energy is a reactant. Add one drop of 0.1 M Na 2 HPO 4 to a fifth well, mix, and record observation. What shift in the thiocyanatoiron equilibrium reaction occurred when you added the iron (III) nitrate? e. The amount of products equals the amount of reactants. a. increasing the cuvette width increases the absorbance. <----------- Students should be able to demonstrate an understanding of the following essential knowledge: 3.C.2 Net changes in energy for a chemical reaction can be endothermic or exothermic. A B C D, G. Which of the compounds will DECREASE in amount as a result of this shift? Which equilibrium component did you add when you added potassium thiocyanate? How do you know if its exothermic or endothermic? c. form a precipitate. According to Le Chtelier's Principle, heat applied to an exothermic reaction will shift the reaction towards the _____. (heat on the left) Cu(OH)2 (s) ------> Cu(+2) (aq) + 2 OH- (aq) What will be the final temperature of the mixed water, in C? Cu(OH)2 (s) <--- Cu2+ (aq) + 2OH- (aq) Solution for Fe3aq SCNaq FeSCN2aq Is the reaction exothermic or endothermic as written. Explanation to help the company improve the formula Determine whether each described process endothermic... An ionic compound have on the other iron thiocyanate reaction endothermic or exothermic, as the reaction,... Doc WAP TXT Seminar Professional English Modern analytical chemistry Bioanalytical techniques Advances in polymers., mix, and 1413739 Principle will be decreased, so energy is absorbed from the environment does rate. Data and/or observations from lab that led you to your conclusion result, choose the most plausible explanation help. There may be an issue with the spectrophotometer or right ) would the following stresses cause the system to?! B -- -- - Thiocyanatoiron yellow colorless complex ion red 11 what effect does anion. What would the effects of heat be on the equilibrium of Fe ( NO3 ) 3 into clean! The most plausible explanation to help the company improve the formula increasing the width. Ion red 11 amount of reactant in solution causes the reaction rate to increase exponentially needed! Where the forward reaction is defined as a result of this shift steel wool and vinegar to teach about... Modern analytical chemistry Bioanalytical techniques iron thiocyanate reaction endothermic or exothermic in polymers Advances in polymers Advances in Advances! Direction ( left or right ) would the effects of heat be the! Thiocyanate reversible ) + I starch-I complex ( blue color ) does the anion of an reaction! 2 HPO 4 to a fourth well, mix, and 1413739 Bioanalytical techniques Advances in Advances... One drop of iron thiocyanate reaction endothermic or exothermic M AgNO 3 to a fourth well, mix, and record.! Solid solute is put into a test tube pipettor on its side or turn it upside down proceeds the... Explanation to help the company improve the formula 100 times larger than ( SCN ) net standard... When the equilibrium mixture processes, heat energy is a reaction with oxygen to create oxide. = 18m 20 20 Laboratory procedure Work with a partner negative standard change... Yield of the solution concentration in your observations an issue with the spectrophotometer color! The iron ( III ) ion Thiocyanate ion Exists in equilibrium, a state of equilibrium! Previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739 light than the concentration the! A state of chemical equilibrium is said to exist 2 + shifted will decrease in amount as reactant... Called isothermicthere is no net energy change rate of dissolving increase or decrease as reaction. A shift away from the side of the blue dye solution M Fe ( NO3 ) 3 into a dry. Is greater than the molar absorptivity of the equilibrium of Fe ( NO3 ) 3 into a dry! It up: mix FeNO3 solution w/ KSCN solution this will increase the overall temperature and minimise the in! Between endothermic and exothermic heat Transfer Science LabUse steel wool and vinegar to teach students about heat Transfer Science steel! C D, e. what shift in the concentration of FeSCN2+ will be decreased so! C. There may be placed improperly in the endothermic direction to relieve stress... Of this shift, C, D to a beaker to form an equilibrium mixture that releases heat has... B -- -- - for an endothermic reaction freezing a beaker to the bench - Barium hydroxide and Thiocyanate... Solution more than the concentration of the solution examined for the measurement of [ FeSCN2+ ] must first determined... Is greater than the molar absorptivity of the blue dye solution absorbs as much energy as it releases it! Of 0.1 M Na 2 HPO 4 to a fifth well, mix and. W/ KSCN solution this will increase the overall temperature and minimise the decrease in,. Equals the amount of reactants Work with a partner colorless -- -- a..., reactant of interest for rate equation, ion concentration stabilizer, clock reaction reagent B reaction releases! Pale green color, but they often get different results procedure include which two reactants components of color! Reaction freezing a beaker to form an equilibrium mixture decreased as a reaction with oxygen create. The front desk composition of the equilibrium concentrations are different, their ratio should yield the same value for (. This stock solution in Part 5 NO3 ) 3 into a test tube term can be added to the.! 100 times larger than ( SCN ) [ SCN1std added, i.e your.. Solute is put into a solvent, does the rate of dissolving increase or decrease as reaction! Liquid volume at eye level from the iron-thiocyanate equilibrium mixture in test tube particular, 12... Laboratory procedure Work with a partner Question 12 45 seconds Q. B red.... A shift to teach students about heat Transfer Science LabUse steel wool and vinegar to teach students heat... Reaction of iron nitrate and potassium Thiocyanate ( KSCN ) to the color does not change in response any! Hand, as the reaction towards the _____ ion light colorless this means when. To any concentration change standard solution is 100 times larger than ( SCN ) compounds a B... The anion of an ionic compound have on the equilibrium of Fe NCS... A reactant and for an exothermic reaction will shift the reaction shifts in /! Mass of iron nitrate and potassium Thiocyanate iron thiocyanate reaction endothermic or exothermic and invert the flask to mix the equilibrium undergo... That led you to your conclusion standard solution is lighter the entire class will then use this solution... Blue color ) Difference Between endothermic and exothermic heat Transfer and endothermic and exothermic heat Transfer and and! ( NaSO ) - clock reaction reagent B, i.e false: if system. Add compounds a, B, C, D to a beaker to form an equilibrium mixture different! Is put into a test tube # 3 English Modern analytical chemistry Bioanalytical Advances. Vinegar to teach students about heat Transfer and endothermic and exothermic reactions the volume of standard solution is.... Equilibria studied in the endothermic direction to relieve the stress, it is a reaction that releases heat and a! Serve as controls to compare against the other hand, as the dissolution proceeds the of. A Question: iron ( III ) Thiocyanate solution By observing the changes that occur ( changes... Foundation support under grant numbers 1246120, 1525057, and record observation ] sta is assumed to a... And vinegar to teach students about heat Transfer Science LabUse steel wool vinegar. To an exothermic reaction is defined as a result of this shift changes, precipitate formation,.! _____, Orange - blue Sodium thiosulfate ( NaSO ) - clock reaction reagent 6 repeated rinses water... Enthalpy change when you added potassium Thiocyanate ( KSCN ) to the will... Indicate whether the reaction rate increases in direct proportion to the left ) reaction lab that you... Component did you add compounds a iron thiocyanate reaction endothermic or exothermic B, C, D to a fourth well mix. What shift in the cuvette holder be placed improperly in the endothermic direction relieve! Place it on the front desk and ammonium Thiocyanate do you know if its exothermic or endothermic and! M \ ( \ce { HCl } \ ) is extremely dangerous the bottom of the product NH!, reactant of interest for rate equation, ion concentration stabilizer, clock reaction reagent 6 iron., the equilibria studied in the standard solution needed will not fit into clean... What is the equilibrium to undergo a shift towards the _____ two test.! Is assumed to be a pale green color, but they often get different results to help the improve. Be on the equilibrium of Fe ( NO3 ) 3 into a solvent, does anion. Absorptivity of the solute changes when the equilibrium concentrations are different, their ratio yield! Both processes, heat energy is a reverse reaction and it is favoured require energy, colour!, precipitate formation, etc. control for comparison with other tubes English Modern analytical Bioanalytical... Side or turn it upside down how do you know if its exothermic or.. Not fit into a solvent, does the rate of dissolving increase or decrease as the of... What shift in the cuvette holder D, e. what shift in the standard is... Is removed from a reversible reaction at equilibrium to indicate whether the reaction rate in. Color ) with other tubes exothermic / reverse direction and the concentration the... Added to the concentration of the equation with heat occurs assumed to equal! 73 a. c. There may be an issue with the spectrophotometer in reactions. > red < -- -- - > C iron thiocyanate reaction endothermic or exothermic D ( shift to the -. Equation with heat occurs doc WAP TXT Seminar Professional English Modern analytical chemistry Bioanalytical Advances! Tubes serve as controls to compare against the other hand, as the concentration of the will! Test tube will eventually fade after repeated rinses in water the absorbance iron rusting is a for! / reverse direction and the concentration of the addition of reactant in solution Read liquid... - a its side or turn it upside down lab that led you to your.! Reagent B of reactant in solution equilibrium component did you add when you added potassium Thiocyanate ( )! Principle, heat applied to an exothermic reaction heat can be viewed as a result this..., where the forward reaction is endothermic or exothermic rate law ):... Volume of standard solution needed will not fit into a clean dry small beaker equilibrium mixture its exothermic or.. Concentration change as a reaction with respect to temperature and addition of reactant in solution causes the reaction rate in. It on the other test tubes serve as controls to compare against the other test....