Buffer Systems

Topics: Melting point, Liquid, Melting Pages: 9 (1646 words) Published: October 23, 2013

2012

Table of Contents
Page #
1.0 Design
1.1 Focus Question2
1.2 Hypothesis2
1.3 Background Theory2
1.4 Variables in the Investigation4
1.5 Controlled Variables5
1.6 Apparatus & Chemicals5
1.7 Risk Assessment6
1.8 Experimental Method8
1.9 Lab Setup9

1.0 Design
1.1 Focus Question
What is the relationship between increasing atomic number of elements along period three and melting point (oC) measured using a heating block?

1.2 Hypothesis
When atomic number along period three is plotted against the melting point, the melting point will increase until it reaches silicon then melting point will decrease.

1.3 Background Theory
Melting point is the point at which the solid phase will convert to a liquid phase. Melting points vary, depending on the strength of forces which exist between the particles that make up a substance and can be used to characterize and determine a particular substance. Melting point can be measured using a Fisher-Johns melting point apparatus which uses a sample on a lens which is placed on the apparatus and heated. The operator is able to observe the phase change through a magnifying glass.

Figure One: Fisher-Johns Melting Point Apparatus

By determining the melting point of a compound it is possible to tell if the substance is pure. Generally, pure substances have a melting range (the difference between the temperature where the sample starts to melt and the temperature where melting is complete) of one or two degrees. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample.

Physical properties such as melting point depend on the bonding between the particles of the element. From previous experiments it has been determined that at the left of the period the elements are metallic and going across the period the strength of the metallic bonding increases as there is an increase in nuclear charge, giving rise to an increase in the melting points as the metal cations are held closer and more tightly. At the center of the period silicon forms a giant covalently bonded structure where every atom is joined to all the others by very strong covalent bonds thus giving it a very high melting point. Following this, the melting points suddenly drop due to the weak van der Waals’ forces that exist between molecules . This is shown in figure two.

Figure Two: Melting Points of Elements in Period Three

It can be deduced from the graph and information above that the trend across period 3 for melting point will be that as atomic number increases from left to right melting point will increase from Na to Si with a large increase in melting point from Al to Si. Melting point will then decrease from Si to Cl with a large decrease from Si to P.

However, it is known that only the first six elements are solid at room temperature. Chlorine, unlike the others is not and therefore its melting point cannot be measured using the Fisher- Johns melting point apparatus as samples must be solids. Therefore for the purposes of this investigation it will be excluded and its melting point will not be calculated as it is not easily measureable. Sodium is a highly reactive alkali metal. To prevent contact with air and water it must be kept under kerosene.

1.4 Variables in the Investigation
Variable Identified
Type of Variable
Method of Control/Measurement
Substance Analyzed
(Na, Mg, Al, Si, P and S)
Independent
The substances will be measured in the solid phase. Cl will be excluded as it is not in the solid phase at room temperature.
Melting Point
Dependent
This will be measured in oC and identified through observation of the substance through a magnifying glass on the Fisher-Johns melting point apparatus. The temperature...


Bibliography: Bentor, Y. (2009). Periodic Table: Alkali Metals . Retrieved May 7, 2012, from Chemicalelements.com: http://www.chemicalelements.com/groups/alkali.html
Brown, C. (2009). Higher Level Chemistry . Wales : Pearson Education Limited .
Green, J. (2007). Chemistry . Victoria : IBID Press.
Saunders, N. (2008, December 29). Trends in melting and boiling points in Period 3. Retrieved May 9 , 2012, from Creative Chemistry : http://www.creative-chemistry.org.uk/alevel/module1/trends8.htm
Thelguana. (2008, October 14). Fisher-Johns Melting Point Apparatus. Retrieved May 7, 2012 , from flickr.com: http://www.flickr.com/photos/51035797337@N01/2943119729
Yoder, C. (2012 ). Determination of Melting Point . Retrieved May 7, 2012 , from Wired Chemist : http://www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/determination-of-melting-point
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