07 Sublimation

SUBLIMATION Introduction

Some solids can easily change into vapours ( volatilisation); these vapours (instead of condensing into a liquid and the liquid freezing back to the solid) can sublime, that is, the vapours form a solid without going through the liquid state. Even ice can be volatilised and sublimed (needs to be at reduced pressure, 1/100 th of the usual atmospheric pressure). Volatilisation / sublimation is useful to purify some solids as menthol, naphthalene, iodine, etc. Heat sensitive medicines and enzymes are dehydrated by means of cooling them down until water freezes and ice formed can be sublimed. Aims

To purify different substances by means of sublimation. Apparatus

Tripod, wire gauze, Bunsen burner, beaker, watch glass, spatula, microscope, slides, and coverlids, capillary tubes, thermometer, ice, water and samples. Procedure

1- Put some iodine mixed with sand (sample 1) in a beaker and heat it gently until violet vapours appear inside.

2- Stop heating and cover the beaker with a watch glass put some ice on the

watch glass to refrigerate and allow the vapours to deposit (sublime); wait for some time and observe the crystals formed.

3- Gather them and keep them in a polythene bag. Observe them through a microscope and draw what you see.

4- Do you think that iodine has been purified? How would you know? 5-

Repeat all items from (1) to (3) using impure benzoic acid instead of iodine. Be sure that the beaker is absolutely dry or humidity will condense on the watch glass and the crystals will be spoiled.

6- Compare both the impure and the sublimed benzoic acid. 7- Now carry out a third sublimation but using your raw naphthalene sample from last week\u2019s experiment (Freezing and Melting). 8- After looking at the naphthalene crystals through the microscope, collect as much sublimed naphthalene as you can in a clean watch glass. 9- Seal on end of a capillary tube but putting its tip on aflame. It gets red hot and after some 3 \u2013 4 seconds should be sealed. Show your teacher before you follow on.

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10- Fill the capillary tube as described below 11- Pack the capillary tube by pressing the open end gently into a sample of the compound to be analyzed. Crystals will stick in the open end of the tube. 12- The solid should fill the tube to a depth of 2-3 mm. Tap the bottom of the capillary on a hard surface so that the crystals pack down into the bottom of the tube

13- Fix the capillary to a thermometer (use a rubber

14- Next fix the apparatus as shown in the picture. The

15- Heat strongly until the thermometer reaches 65 – 70°C. 16- Now heat very slowly so the temperature rises at a

17- Keep an eye always on the sample (if you have a

18- Record this temperature and the temperature at which the naphthalene is totally molten.