Acknowledgment I am are greatly thankful for the cooperation and help from my Chemistry teacher, Mrs.Neena Singh and the Chemistry lab staff for the successful completion of this investigatory project in the CSC Chemistry lab. Without their help and proper guidance my project might not have completed. IndexINTRODUCTION When a liquid is placed in an open vessel, it slowly escapes into gas phase, eventually leaving the vessel empty. This phenomenon is known as evaporation. Evaporation of liquids can be explained in terms of kineticmolecular model. Although there are strong inter-molecular attractive forces which hold molecules of a liquid together, the molecules having sufficient kinetic energy can escape into gas phase if such molecules happen to come near the surface. In a sample of liquid all the molecules do not have same kinetic energy. There is a small fraction of molecules which have enough kinetic energy to overcome the attractive forces and escape into gas phase. Evaporation causes cooling. This is due to the reason that the molecules, which undergo evaporation, are highenergy molecules; therefore the kineticenergy of molecules which are left behind is less. Since the remaining molecules have lower average kinetic energy therefore, temperature must be lower. If the temperature is kept constant the remaining liquid will have the same distribution of molecular kinetic energies and the high-energy molecule will keep on escaping from the liquid into the gas phase. If the liquid is taken in an open vessel, evaporation will continue until whole of the liquid evaporates. 1. Introduction 2. Theory 3. Objectives of project 4. Experiment No.1 INTRODUCTION When a liquid is placed in an open vessel, it slowly escapes into gas phase, eventually leaving the vessel empty. This phenomenon is known as evaporation. Evaporation of liquids can be explained in terms of kineticmolecular model. Although there are strong inter-molecular attractive forces which hold molecules of a liquid together, the molecules having sufficient kinetic energy can escape into gas phase if such molecules happen to come near the surface. In a sample of liquid all the molecules do not have same kinetic energy. There is a small fraction of molecules which have enough kinetic energy to overcome the attractive forces and escape into gas phase. Evaporation causes cooling. This is due to the reason that the molecules, which undergo evaporation, are highenergy molecules; therefore the kineticenergy of molecules which are left behind is less. Sinc e the remaining molecules have lower average kinetic energy therefore, temperature must be lower. If the temperature is kept constant the remaining liquid will have the same distribution of molecular kinetic energies and the high-energy molecule will keep on escaping from the liquid into the gas phase. If the liquid is taken in an open vessel, evaporation will continue until whole of the liquid evaporates. Theory 6. Higher temperatures also increase the rate of evaporation. it can have a lower capa city for the substance evaporating. 1. thus encouraging faster evaporation. The stronger the forces keeping the molecules together in the liquid or solid state the more energy that must be input in order to evaporate them. Inter-molecular forces. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant. At higher temperatures. Concentration of other substances in the air. 1. Concentration of the substance evaporating in the air. If the air is already saturated with other substances. the escaping molecules accumulate as a vapor above the liquid. If fresh air is moving over the substance all the time. The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate. the remaining molecules have lower average kinetic energy. its kinetic energy must exceed the work function of cohesion at the surface. the same amount of water will evaporate faster if spilled on a table than if it is left in a cup. molecules in motion have more energy than those at rest. If the substance is hotter. as the faster-moving molecules escape.Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. Also. 5. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. with returning molecules becoming more frequent as the density and pressure of the vapor increases. a larger surface area allows more molecules or atoms to leave the liquid. Temperature of the substance. Factors influencing rate of evaporation:1. This is in part related to the concentration points above. Surface area and temperature: Because molecules or atoms evaporate from a liquid¶S surface. 2. Many of the molecules return to the liquid. and . molecules or atoms have a higher average speed. 5. If the evaporation takes place in a closed vessel. the rate of evaporation is limited. and evaporation occurs more quickly. 3. then the concentration of the substance in the air is less likely to go up with time. 4. and so the stronger the flow of air. In addition. the vapor is said to be ³saturated.´ and no further change in either vapor pressure and density or liquid temperature will occur. the greater the evaporating power of the air molecules. that is. For example. If the air already has a high concentration of the substance evaporating. 2. When the process of escape and return reaches equilibrium. and the temperature of the liquid thus decreases. then evaporation will be faster. 4. then the given substance will evaporate more slowly. 3. Flow rate of air. we shall investigate various factors that have already been discussed such as nature of liquid . Intermolecular forces: ± Most liquids are made up of molecules. Objective of project In this project. and other regions that carry a slight positive charge. acetone and diethyl ether. 3. Beaker.more particles are able to break free of the liquid¶S surface. 2. As a result. For example. Acetone. Watch PROCEDURE: 1. Attractions between molecules arise because molecules typically have regions that carry a slight negative charge. Diethyl ether. gasoline evaporates more quickly than water. Water molecules share a mutual attraction ²positively charged hydrogen atoms in one water molecule attract negatively charged oxygen atoms in nearby water molecules.1 Aim: To compare the rate of evaporation of water. and the levels of mutual attraction among different molecules help explain why some liquids evaporate faster than others. Pipette. These regions of electric charge are created because some atoms in the molecule are often more electronegative (electron-attracting) than others. The oxygen atom in a water (H2O) molecule is more electronegative than the hydrogen atoms. Dish A-Acetone Dish B-Water Dish C-Diethyl ether . because water molecules have stronger mutual attractions than gasoline molecules (the electric charges are more evenly distributed in gasoline molecules).surface of liquid and temperature and find their correlation with the rate of evaporation of different liquids. Distilled water. As a result. Intermolecular attractions affect the rate of evaporation of a liquid because strong intermolecular attractions hold the molecules in a liquid together more tightly. for example. For example. Pipette out 10 ml of each sample. a wet street will dry faster in the hot sun than in the shade. the oxygen atom in the water molecule carries a partial negative charge. Experiment no. Take three china dishes. Weighing balance Measuring flask. liquids with strong intermolecular attractions evaporate more slowly than liquids with weak intermolecular attractions. Materials required: China dish. enabling the oxygen atom to pull electrons away from both hydrogen atoms. while the hydrogen atoms carry a partial positive charge. 33 2.8 0.67 50 60 59. Leave the three dishes undisturbed for ½ an hr and patiently. 5. OBSERVATION: wait Water (gm) Weight of dish Weight of (dish + substance) before evaporation Weight of (dish + substance) after evaporation Weight of substance evaporated Inference and conclusion: Diethyl Ether>Acetone>Water Reason: - Acetone (gm) 50 57. . Record the weights of the samples after the given time.2 The rate of evaporation of the given three liquids is in order : - Water has extensive hydrogen bonding in between oxygen atom of one molecule and hydrogen atom of another molecule.30 Diethyl Ether (gm) 50 57 54.85 55.4.55 2. Compare the prior and present observations. But this is absent in the case of acetone. Record the weights before beginning the experiment. 7. 6.