A good sample of benzoic acid melts at 121-122 C however, a student had a sample that melted over a range, 105-115 C what did the student conclude about this sample?
Boiling points of liquids are all lower in Denver, Colorado than in Detroit, Michigan since Denver is in a mountainous region, what can you conclude about atmospheric (external) pressure as you climb higher above sea level?
Cocaine is a white solid that melts at 98 C when pure Sucrose ( table sugar) also is a white solid, but it melts at 185-186 C A forensic chemist working for the New York Police Department has a white solid believed to be cocaine what can be the chemistry do to determine quickly whether the sample is pure cocaine, simply table sugar, or a mixture of the two? ( A good chemist never tastes any unknown chemical)
The Correct Answer and Explanation is :
Question 1: Benzoic Acid Sample Melting Range
If a student observes that the sample of benzoic acid melts over a range of 105-115°C, which is lower than the expected pure benzoic acid melting point (121-122°C), the student can conclude that the sample is impure. The presence of impurities in a substance generally lowers and broadens its melting point range, as impurities disrupt the orderly arrangement of molecules in the solid state, making it easier for the substance to transition into the liquid state. Therefore, this melting point range suggests that the benzoic acid sample is not pure and contains other substances or impurities.
Question 2: Boiling Points in Denver vs. Detroit
In Denver, Colorado, the boiling point of liquids is lower than in Detroit, Michigan, due to the difference in atmospheric (external) pressure. Denver is located at a much higher altitude than Detroit, which means there is less air pressure at the higher elevation. Atmospheric pressure decreases as you ascend, and this lower pressure causes liquids to boil at lower temperatures. This phenomenon occurs because a liquid boils when its vapor pressure equals the external atmospheric pressure. At lower atmospheric pressure, liquids reach this point at a lower temperature, explaining why boiling points are lower in Denver compared to Detroit.
Question 3: Forensic Identification of Cocaine vs. Sucrose
A forensic chemist trying to determine whether a white solid is pure cocaine, table sugar (sucrose), or a mixture of the two could quickly perform a melting point determination. Since pure cocaine melts at 98°C and pure sucrose melts at 185-186°C, the melting point of the sample can provide crucial information:
- If the sample melts around 98°C, it is likely pure cocaine.
- If the sample melts around 185-186°C, it is likely pure sucrose.
- If the sample melts at a temperature between 98°C and 185°C, it could be a mixture of cocaine and sucrose.
The chemist can also perform additional tests, such as thin-layer chromatography (TLC), to further analyze the components of the sample. In TLC, a small portion of the sample is placed on a stationary phase, and a solvent is used to move the components of the sample up the plate. Different substances will travel at different rates, creating distinct spots that can be compared to known reference standards for cocaine and sucrose. This method allows for quick and reliable identification of the components in the mixture.
By using melting point analysis and TLC, the chemist can determine with a high degree of certainty whether the white solid is pure cocaine, sucrose, or a mixture of the two. These methods are non-destructive and relatively fast, making them suitable for forensic investigations.