Experiment 37 Testing liquids for polarity Aim To test nine liquids in order to find out if their molecules are polar. Theory In this experiment, the effect of a charged rod on a stream of a liquid from a burette. A deflection of the stream indicates that the liquid consists of polar molecules. On the contrary, non-polar liquid show no deflection. This is because when the molecule is polar, one end of the molecule will be attracted by the oppositely charged rod. This attraction will cause the molecules as well as the running liquid to deflect towards the charged rod. Structural formulae of some of the organic liquid used in this experiment are shown as follows: Procedure 1 The polythene rod was rubbed with fur. A negative charge was given to the rod. 2 A beaker was positioned beneath the jet of one of the burette. The cork was removed and a stream of the liquid was allowed to run from the burette with the tap fully open. 3 The charged rod was brought close to the stream of liquid and any deflection that occurred was noted. The extent of deflection was also noted on an arbitrary scale from 0 to 3. (0 = no deflection; 3 = greatest deflection.) The results were recorded. 4 The liquid was poured from the beaker back into the labeled burette. The above procedure was repeated for the other organic liquids. Questions 1 Water is a polar molecule and thus showing a deflection under the influence of charged rod. Polar molecules have permanent dipole moments, when a charged rod is put near the stream of water, the water molecules are attracted towards the charged rod and the stream is deflected. 2 The polar liquid will be deflected towards the charged rod. The deflection is independent of the charges carried by the rod. This is because when a positively charged rod is used, the negative ends of the dipole in the polar molecule are attracted towards the rod and vice versa. Therefore, the jet of polar liquid will always be attracted towards the charge rod. Discussion 1 Safety precautions a) Toxic: Trichloromethane and tetrachloromethane b) Flammable: Cyclohexane, Cyclohexene, Hexane, Hexan-1-ol, Methyl ethanoate and Propanone Precautions: a) Perform the experiment in a fume cupboard b) Keep the stoppers on the bottle as much as possible c) Wear gloves and safety spectacles 2 Sources of errors a) The charged rod Error #1: The distance between the charged rod and the jet of liquid may not be constant for different trials, causing variation in the electrostatic field. Improvement #1: The distance between the charged rod and the jet of liquid should be precisely measured so that it is the same for all the trials. It is better to clamp the rod into position to prevent non-uniform electrostatic field due to shaking hand. Error #2: The density of the electric charge may decrease due to ionization of air molecule from time to time. Improvement #2: Rub the rod with fur repeatedly before a new trial. The times of rubbing should be constant and be great enough compare to the size of rod so that the charge is “saturated” with charges. Improvement #3: The rod can be made sharp-end and point it towards the jet of liquid so to increase the charge density as well as the sensitivity of the experiment. b) The burette and the containers Error: The burette and the container may have electrostatic charge induced in them (e.g. by rubbing). Thus it may cause the liquid to be charged and deflect towards the charged rod even when it is not polar. Improvement: “Earth” the burette and containers with copper wire at once before starting each trial. Conclusion ______________________________________________________________ _________________________________________consist of polar molecules; while ___________________________________________________ do not.