18 Questions
What happens to the temperature of a substance during the melting process?
It remains constant
How accurate is usually the melting range that chemists can obtain?
+/- 2-3°C accuracy
What is the temperature at which a substance changes from a solid to a liquid state known as?
Melting point
What effect do impurities have on the melting range of a substance?
Decrease and broaden the range
What type of forces must be overcome for a substance to change from solid to liquid during melting?
Intermolecular forces
Why is determining the melting point of a compound considered important?
To test for impurities
What happens to the energy added to a substance during the melting process?
It is consumed as heat of fusion
Why is it difficult to find the exact melting point of a substance?
Impurities in the substance affect the accuracy
How does the presence of impurities affect the melting range of a substance?
Depresses and broadens the melting range
What is the significance of determining the melting range of a compound in terms of purity?
To check for impurities
What type of property is the melting point of a substance?
Physical property
What does a smaller and higher melting range indicate about a sample compared to an impure sample?
Higher purity
Why is determining the melting point particularly important for organic chemists?
To determine the purity of solid molecular compounds
What does a wide melting range of a sample suggest?
The sample is not pure
What is the purpose of determining a 'mixed' melting point?
To establish that two substances are identical
How can the identity of two substances with the same melting point be established?
By mixing them in equal quantities to form a 50:50 mixture
What happens to the melting range of mixtures of different substances?
It broadens and melts over a range of temperatures
When does the absence of a depression in melting point suggest nonidentity between two substances?
When an intimate mixture melts over a wide range of temperatures
Study Notes
Melting Point
- The melting point of a substance is the temperature at which it changes from a solid to a liquid state.
Characteristics of Melting Point
- Pure crystalline substances have a clear, sharply defined melting point.
- During the melting process, all of the energy added to a substance is consumed as heat of fusion, and the temperature remains constant.
Determining Melting Point
- Determining the melting point of a compound is one way to test if the substance is pure.
- A pure substance generally has a melting range of one or two degrees.
- Impurities tend to depress and broaden the melting range.
Importance of Melting Point in Organic Chemistry
- The determination of melting points is particularly important to organic chemists, since they generally work with solid molecular compounds that have low melting points (below 300°C) and which can be conveniently measured.
Melting Point as a Criterion of Purity
- A sharp melting point (actually, a melting range of less than about 1°C) is often taken as evidence that the sample is fairly pure, and a wide melting range is evidence that it is not pure.
- The presence of an impurity causes a reduction of the freezing point of the sample.
Mixture Melting Points
- An identical or near identical temperature range of melting is not, in itself, proof of the sameness of two organic chemical samples.
- The melting point of a mixture will be identical to that of the known compound if the samples are the same, or lowered and much broadened if they are not.
- The determination of a "mixed" melting point is used to prove or disprove the identity of two substances.
Test your knowledge on the concept of melting point, where a substance transitions from solid to liquid state. Understand how energy is consumed during the melting process and why chemists often report a range of melting points instead of a specific value.
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