NCERT Class 11 Chemistry Lab Manual Basic Laboratory Techniques

                                                                   BASIC LABORATORY TECHNIQUES

The laboratory apparatus for carrying out reactions, in general, is made up of glass. It is because glass is resistant to the action of most of the chemicals. Generally, two types of glass are used for making apparatus for laboratory work. These are soda-lime glass and borosilicate glass. Soda-lime glass, which is made by heating soda, limestone and silica, softens readily at about 300-400^°C in the burner flame. Therefore, on heating glass tubings made of soda-lime glass easily softens and can be bent. Coefficient of expansion of soda glass is very high, therefore on sudden heating and cooling, it may break. To avoid breaking, it should be heated and cooled gradually. Annealing by mild reheating and uniform cooling prevents breakage. Such glass should not be kept on cold surface while it is hot, since sudden cooling may break it. Borosilicate glass does not soften below 700-800^°C and requires oxygennatural gas flame for working. Natural gas mixed with oxygen is burnt to get the oxygen-natural gas flame. Coefficient of expansion of this glass is low and apparatus made of this glass can withstand sudden changes in temperature. Therefore, apparatus used for heating purposes is made from borosilicate glass. On heating, glass apparatus made up of borosilicate glass does not distort. In the following pages you will learn about some of the techniques of handling glass tubes and glass rods without injuring yourself. Also, you will learn the techniques of using laboratory apparatus and equipments.

(ii) Keep the lower end of a triangular file with its sharp edge perpendicular to the tube to be marked and pull it towards you to make a single deep scratch on the glass tube or the glass rod at a desired length (Fig. 2.1 a).

(iii) Keep thumbs of your hands on both sides, very close and opposite to the scratch as shown in Fig. 2.1 b and break the glass tube or rod by applying pressure from your thumbs in a direction away from you (Fig. 2.1 c). Break the tube/ rod by holding it with a cloth so that hands are not harmed.

(iv) If the glass tube does not break, make a deeper scratch at the point marked earlier and make a fresh attempt.

(v) Trim any jagged edge by striking with a wire gauge (Fig. 2.2 a).

(vi) Heat the freshly cut edge of the tube gently in the flame to make the edges round and smooth (Fig. 2.2 b). This is called fire polishing. For fire polishing, first continuously warm the cut end in the Bunsen flame and then rotate it back and forth until the edge is rounded. Too much heating may distort the rounded edge (Fig. 2.2 c).

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