MICRO – DETERMINATION OF BOILING POINT

OBJECTIVES: at the end of the experiment, the student should be able to:

1. Correctly fill the boiling point capillary with the unknown liquid2. Set up the assembly for boiling point determination3. Measure the correct boiling points of benzoic and an unknown liquid4. State and explain the effects of the following errors on the boiling point data:

a. The solid is not ground finely before use in melting point determinationb. The heating liquid is not stirred continuouslyc. The rate of heating is too rapidd. Water is used as the heating liquid

5. State the reasons why coconut oil is used as the heating bath for melting point determinations

6. Explain the effect of adding another pure substance on the boiling point of a certain solid

APPARATUS:

Bunsen burner iron stand wire gauze thermometerstirring rod iron ring safety goggles boiling capillary tube

MATERIALS:Distilled water Unknown100ml beaker Coconut oil

PROCEDURE:

Make a micro boiling point tube. This consists of an outer tube about 6 cm long sealed at one end. A capillary tube about 3 cm long is sealed about 2 to 3 mm from one end and placed inside the outer tube, with that open end of the capillary tube toward the bottom. Five drops of the liquid whose boiling point is to be determined are added to the outer tube. This tube is to be fastened to a thermometer with a piece of rubber band.

The liquid is heated slowly in an oil bath. The temperature is raised until a rapid and continuous stream of bubbles comes out of the small capillary tube and passes through the liquid. The flame is then removed and the oil is allowed to cool, while being stirred continuously. The stream of bubbles becomes slower and slower as the temperature drops, until a point is reached where the bubbling stops and the liquid starts to rise in the capillary. The pressure in the tube at this point is approximately atmospheric and the temperature is recorded as the boiling point of the liquid.

If a piece of dark paper or the other suitable material is placed behind the oil bath it is easier to see the bubbles coming from the capillary tube.

1. Determine the boiling point of the distilled water as directed above.

2. Determine the boiling point of an unknown organic liquid.

REFERENCES:

Shriner and Fuson,Identification of Organiccompound pp. 27.

Helmpkamp and Johnsn, Chapter 3.