Directly Into A Gas Without Passing Through The Liquid Phase. (1) in This Process, The Students
Directly Into A Gas Without Passing Through The Liquid Phase. (1) in This Process, The Students
Directly Into A Gas Without Passing Through The Liquid Phase. (1) in This Process, The Students
Determination
Abstract:
The experiment aims to determine the purity of impure Benzoic acid through sublimation
and melting point determination. Sublimation is a transition phase in which a solid turns
directly into a gas without passing through the liquid phase. [1] In this process, the students
placed the impure benzoic acid in an evaporating dish covered with a perforated filter paper and
topped with an inverted watch glass. The set-up was cooled by placing wet tissue above the
inverted watch glass. After heating the set-up, the students observed that there is a formulation of
crystals in the inverted watch glass. The set-up was cooled and the sublimate was weighed. The
retrieved sublimate was four grams (4g) of sublimate. The crystals were collected and grinded
with the use of mortar and pestle. The grinded sublimate was then individually packed in a
capillary tube. The tubes packed with sublimate were placed beside a thermometer, near the
mercury bulb. The capillary tube containing the crystals and the thermometer were then
immersed in an oil bath until the sublimate melted. The students noted the temperature at which
the sublimate starts to melt and when it is completely melted. The sublimate of the impure
benzoic acid starts to melt at 120C and fully melts at 122C. This process is use to determine
the melting point of the product and compare it with the standard value of pure benzoic acid
which is 122C. [2] From the gathered data, the researchers were able to identify the purity of the
compound and calculate the percentage recovery in the test. The group retrieved four grams (4g)
of sublimate and got a percentage recovery of 80 %.
Introduction:
Sublimation is the conversion of substance from the solid phase to the gas phase without
passing through the intermediate liquid phase. [1] Sublimation is a purification method for solid
materials. This occurs if the vapor substance is greater than the atmospheric pressure at the
melting point. The solid will sublime due to this occurrence. However, this purification method
is limited only to the impurities, which are non-volatile or have lower vapors pressure than the
pure compound. [3] Sublimation is also used to explain the reverse process of the gas state
changing directly to solid state again by cooling. This happens when air saturated water changing
from the gaseous to the solid phase. [4]
Melting point or also known as freezing point is the temperature at which the solid and
liquid phases of a substance coexist or in equilibrium at a specific pressure. The terms melting
and freezing point are use depending on whether the substance is being heated or cooled. A pure
substance has a single melting point if it us under the standard condition of pressure. [5] Melting
point determination can be use to determine the identity and purity of an unknown compound.
[3] Benzoic acid is the compound that the researchers used for this experiment. Its molecular
formula is C7H6O2.Benzoic acid is a white, crystalline organic compound belongs to the family
of carboxylic acids, and is widely used for food preservative and for manufacturing various
cosmetics, dyes, plastics, and insect repellants. Pure benzoic acid is slightly soluble in water and
melts at 122C. [2]
At the end of the experiment, the researchers were able to accomplish the following
objectives: (1) Purify benzoic acid by sublimation, (2) Determine and compare the melting point
of the product with a standard. (3) Calculate the percentage recovery.
Methodology:
The compound tested for this experiment was impure benzoic acid. The materials needed
for this experiment were evaporating dish, perforated filter paper, watch glass, Bunsen burner,
beaker, thermometer, iron stand and clamp, iron ring, wire gauze, capillary tubes, rubber
band/thread, unused cooking oil, and tripod.
Purification of the Impure Benzoic Acid
The students placed 5g of impure benzoic acid in an evaporating dish. The evaporating
was then covered with a perforated filter paper. After covering the evaporating dish, the students
placed the inverted pre-weighed watch class on top of the filter paper. The openings of the filter
paper was then sealed using a masking tape and the set-up was heated for 10 to 15 minutes using
a hot plate. The inverted watch glass was kept cool by placing tissue papers moisten with water.
The students waited until most of the sample has vaporized, then the set-up was stopped from
heating and then it was cooled down. The students carefully inverted the pre-weighed watch
glass. After that, the sublimate including the crystals adhering to the perforated filter paper were
collected in the pre-weighed watch glass then weighed. The percentage recovery was calculated
base on the data gathered.
Preparation of the Capillary Tube
The students heated one side of the capillary tube using a Bunsen burner. The capillary
tube is being rotated to melt the glass evenly. Continue heating until the other end of the capillary
tube is closed.
Preparation of the Hot Oil Bath
The unused cooking oil is poured about half-full in a clean beaker. The beaker was then
placed on top of the tripod with wire gauze, and a Bunsen burner was placed underneath the setup.
5.00g.
97.00g.
93.00g.
4.00g.
The original amount of the impure benzoic acid was 5g. The weight of the watch glass is
93g. The watch glass is used as a covering and later on used as a container for sublimate. After
undergoing the sublimation process and collecting the entire sublimate, the weight of the watch
glass and the sublimate was weighed and was found to be weighing 97g. Subtracting the weight
of watch glass from the weight of the watch glass with sublimate, the total sublimate from the
process is 4.00g.
From the gathered data, we can compute the Percentage Recovery using this formula:
recovery =
recovery =
4.00 g .
100
5.00 g .
The compound use in this experiment is the crude form of benzoic acid, which is the
impure benzoic acid. Impure benzoic acid contains the impurities phthalic acid and
benzylbenzoate. [6]
The purpose of computing the percentage recovery in the experiment is to find out how
impure the sample compound is. The percentage recovery was computed based on the collected
data before and after the sublimation process.
Based on the gathered data shown in Table 1, the initial weight of the compound is 5g.
and after the sublimation process the gathered sublimate is 4g. Using the percentage recovery
formula, the percentage recovery was 80%. This means to say that 20% of the initial sample was
made up of impurities.
Melting Point Determination
Melting point is the temperature where the compound starts to melt until it has
completely melted. It is the temperature at which the solid and liquid phases of a substance
coexist or in equilibrium at a specific pressure. [4]
In the melting point determination, the set-up used in the experiment was the hot oil bath
since the melting point of benzoic acid is higher than the boiling point of water. Pure benzoic
acid melts at 122 C (252 F). [2]
In doing the melting point determination, the capillary tubes filled with grinded sublimate
and the thermometer were immersed in the oil bath. The temperature at which they started to
melt and completely melted where noted. Table 2 shows the difference of the temperature of the
sublimate and the pure benzoic acid at which they started to melt and completely melted.
Substance
Sublimate
Pure Benzoic Acid
Based on Table 2, the temperature the sublimate started to melt was 120C. The
sublimate was completely melted at the temperature of 122C. On the other hand, the pure
benzoic acid started to melt at NC and completely melted at NC.
Pure benzoic acid has a standard melting point of 122C. [2] Given this fact, there is no
difference of temperature between the gathered sublimate and the pure benzoic acid. Meaning to
say the gathered data here shows that the sublimate gathered is a pure benzoic acid.
Reference:
http://chemistry.about.com/od/chemistryglossary/a/sublimationdef.htm
http://www.britannica.com/science/benzoic-acid
http://www.infoplease.com/encyclopedia/science/sublimation-chemistry.html
http://glossary.periodni.com/glossary.php?en=melting+point
http://editions.sciencetechnologyaction.com/lessons2-32889.php