1. experimental purpose 1. understand the principle and significance of melting point determination 2. master the operation methods of capillary melting point determination and micro melting point analyzer 2. basic concepts and principles (add figure 1. The curve of vapor pressure changes with temperature, 2. The diagram of phase changes with time. Definition of melting point: The temperature at which the solid-liquid state of a crystal compound is in equilibrium at atmospheric pressure is called the melting point of the compound. Melting range: the range from initial melting to full melting is called melting range. The melting range of pure substances does not exceed .5-1.℃. Substances containing impurities have a lower melting point and a longer melting range than pure substances. Significance: The determination of melting point is of great value for identifying organic compounds and qualitatively judging the purity of solid compounds. Third, the instruments and reagents used in the experiment are urea: cinnamic acid (1:1), urea, cinnamic acid, B-shaped tube, thermometer with notch plug, hob, melting point tube, alcohol lamp, watch glasses, long glass tube and micro melting point instrument. Fourth, the melting point bath of the experimental device: liquid paraffin is used as a hot bath (B-shaped tube). A. The mercury ball should be located between the upper and lower forks of the B-shaped tube; B. The sample part should be located in the middle of the mercury ball; C, adding liquid to the B-shaped pipe to reach the fork pipe; D. heat at the position shown in the figure to make the heating uniform. Experimental content 1. Determination of melting points of urea, cinnamic acid and mixed samples by capillary method: coarse measurement once and fine measurement twice respectively; 2. The melting points of urea and cinnamic acid were determined by a micro melting point tester. V. Operation steps (1) Capillary melting point determination method (1) Preparation of melting point tube: Glass worker's operation: drawing a glass tube with a diameter of 1mm, a length of 7-8cm and a closed end; (2) Loading of samples: the samples should be finely ground, the sample height should be 2-3mm, and the drugs should be loaded evenly and firmly; (3) After the instrument is assembled, heat it with small fire; (4) when the temperature is 1-15 DEG C away from the melting point, the heating speed is controlled to be 1-2 DEG C/min; (5) Write down the thermometer readings when the sample begins to collapse and liquid phase is generated (initial melting) and when the solid completely disappears (total melting), that is, the melting range of the compound. (6) Repeat the above operations and measure separately. (2) Measuring the melting point with a melting point tester (demonstration and explanation) Notes: 1. The samples used must be dried, ground and packed. 2. Observe whether there is shrinkage or softening, gas release and other decomposition phenomena before initial melting. For example, when a substance begins to shrink at 112℃, droplets appear at 113℃ and completely liquefy at 114℃, it should be recorded as follows: melting point is 113-114℃, and it shrinks at 112℃. 3. When measuring the melting point roughly, the heating speed is slightly faster, so as to know the approximate melting point range of the sample. 4. During precise measurement, the heating speed is fast first and then slow, and when it is 1-15℃ away from the melting point, the heating speed is controlled at 1-2℃/min. 5. When measuring another sample, wait until the temperature of the hot bath drops to about 3℃ below its melting point. Thinking and discussion 1. How to determine whether A and B are the same substance by measuring the melting point? 2. When measuring the melting point, what will happen if you meet the following conditions? (1) The melting point tube is unclean. (2) The bottom of the melting point tube is not completely closed, and there is still a pinhole. (3) The melting point tube wall is too thick. (4) The sample is not completely dried or contains impurities. (5) heating too fast. (6) The sample is not finely ground or not tightly packed.