06 Freezing And Melting

FREEZING AND MELTING POINTS Introduction: A pure substance has definite properties and we use them for identification purposes: water for example, has a melting and a boiling point different from the melting and boiling points of other substances; if we have an unknown sample that boils and freezes at the same temperature that water does, very probably the unknown sample will be water. These properties are also used for assessing purity. Impurities lower the melting point of a substance and change sharp melting point into a broad melting range. The melting point coincides with the freezing point of the corresponding liquid (as you will verify) although instinctively most people think that the freezing point is a little bit lower than the melting point. In fact, the melting point of a substance is the only temperature at which both liquid and solid states of a substance can coexist. Aims: To study the melting and freezing points of different substances. To plot the cooling curves for pure and impure samples. Apparatus You will use beakers, test tubes, stirring rods, capillary tubes, a watch glass, a tripod, a wire gauze, a Bunsen burner and a thermometer Procedure: 1- The melting point of ice Put some crushed ice into a beaker and insert a thermometer. Read the temperature 30 seconds after dipping the thermometer and every minute until about half of the ice has melted. Plot a temperature : time graph (called a cooling curve) and draw your conclusions. 2- The melting point of an ice/salt mixture Put some crushed ice in a beaker and put a thermometer in it. Read the temperature after 30 seconds. Add it with a spoonful of table salt (crystals) and mix stirring with a glass rod. Keep reading the temperature every 30 seconds and plot a temperature : time graph (cooling curve). Compare with the melting plot for pure ice. The system has a lower melting point so ice cannot exist now at 0°C. It has to melt and melting requires energy; ice will take it from its surroundings (the solution that is being formed, the glass of the beaker, the air around it, etc.) cooling it. These mixtures are frequently used to obtain low temperatures in a lab. (Acetone and dry ice will lower the temperature below -40°C).

3- The freezing point of water Put 5 cm3 of water in a test tube and dip a thermometer into it. Read the temperature and take note. Now put the test tube in the ice-salt mixture of point 2 and read the temperature every 15 seconds. Scrape the walls of the tube gently with the thermometer. Keep on reading until the water has frozen and take two further values. Plot a temperature : time graph. Compare with the graph of point 1. 4- The cooling curve of a sample of naphthalene Procedure: 1- Set the apparatus as shown in the figure. Be sure the tube is made of Pyrex glass

2- Put some naphthalene inside the tube 3- Heat the water to the boil and let the naphthalene melt and rise its temperature to 90 – 95°C.

4- As quickly as possible, replace the water bath for an empty beaker and let the sample cool down. (This is done to avoid air droughts) 5- Read the temperature and start a stopwatch (any three-hand-watch will do).

6- Repeat your readings every 30 seconds. Record your results in a two column table. If it is too long cut it in three columns. In case you fail one reading leave an empty space 7- Look at the test tube frequently and mark the temperature at which it starts freezing (solidifying). 8- Keep on reading the temperature and mark the value at which the sample has completely solidified. You should go on recording data for two or three minutes after this happens. 9- Plot your data as a temperature – time graph. 10- The sample has frozen at a relatively sharp temperature or along a wide range of temperatures?

11-Did it start at 79 – 80°C (the freezing/melting point of naphthalene) or at a lower temperature? 12- Do you think your sample is pure naphthalene? Why?