The melting point of a substance is the temperature at which a solid, ordered crystalline substance is in equilibrium with a more random liquid state, thereby undergoing a phase change.
Since the components of the solid substances are bound by strong intermolecular attractive forces, it also reflects the energy needed to weaken these attractive forces and move freely in the liquid state.
| Physical Properties | Chemical Properties |
|---|---|---|
Meaning | Physical properties are those properties that can be observed or measured without changing the substances' identity; that is, there is no change in the substance's chemical structure. It does not undergo any chemical reaction that results in a structural transformation, and no new substance is formed. |
Two ways physical properties respond when you change how much substance is present: one scales with the amount, the other holds fixed.
Double the sample and the property doubles. Halve the sample and the property halves. Examples: mass, volume, total heat released.
Independent of sample size. Density, boiling point, refractive index, melting point, and colour all keep their values whether you have a drop or a litre.
Properties that change when the size or amount of the sample changes. Dependent on the amount of matter: doubling the quantity of matter must also double the property.
Properties that do not change with the size or amount of the sample. Entirely independent of the amount of matter present.
Linear scaling. The value changes in direct proportion to the amount of substance. If you have 100 mL of benzene with mass 87.9 g, then 200 mL has mass 175.8 g.
No change. The value is characteristic of the substance itself, not the amount. Pure ethanol boils at 78.4 °C whether you have 5 mL or 5 L.
Mentally split the sample in half. If the property's value also halves, it is extensive. Volume of acetone halves when you pour half of it away. So does total mass.
Split the sample in half. If the property's value stays the same, it is intensive. The density of acetone (0.791 g/mL) is the same whether you measure it in the original beaker or in a smaller dish poured from it.
Mass of a sample of ethanol, volume of benzene, total heat released in a combustion reaction, total number of moles, total enthalpy.
The boiling point of acetone, the refractive index of toluene, the density of chloroform, the melting point of urea, and the colour of acetic acid.
Extensive properties can help calculate the stoichiometry required for reactions to proceed and control the quantity of the product formed. Moles, mass, and volume drive every yield calculation.
Intensive properties, such as boiling point or melting point, help identify a substance and evaluate the purity of organic compounds. Two samples of the same compound match on intensive properties even if their masses differ.
Mnemonic: Intensive = Inherent Nature stays The Same. Boiling point, density, refractive index, melting point: each is an inherent fingerprint of the substance.
Why this works: Intensive properties are characteristic constants of the substance itself. Like fingerprints, they identify what something is, not how much of it you have. Extensive properties, by contrast, answer "how much" rather than "what."
A chemist measures the boiling point and total mass of 100 mL of pure ethanol, then pours half of it into a new beaker. Which value is exactly half of the original?
Every physical property of a substance falls into one of two camps based on a single test: does its value depend on how much substance is present? Properties that scale with the amount of matter are called extensive. Properties that hold a fixed value regardless of how much substance is sitting in front of you are called intensive [1].
| Qualitative Analysis | Quantitative analysis |
|---|---|---|
Aim | Identifies what compounds or functional groups are present. Therefore, the type of information obtained is descriptive. For example, whether the compound changed color, evolved gas, or formed a precipitate.
| Measures how much of a compound is present and, therefore, numerically describes its quantity. |
Learning Objective: To study how to determine by qualitative analysis the physical properties of the state, nature, odor, color, solubility, and physical constants of an unknown organic compound.
Skill Level - Intermediate
Prerequisites:
Learning Objective: To study how molecular weight, molecular shape, functional group, and carbon number impact density.
Skill Level - Intermediate
Prerequisites:
Learning Objective: To study which intermolecular forces and structural features affect the solubility of organic compounds in water.
Skill Level - Intermediate
Prerequisites:
Hydrophobicity
Learning Objective: To investigate which intermolecular forces are predominant in a liquid state and to study the impact of molecular weight, molecular shape, and polarity on the boiling point of liquids.
Skill Level - Intermediate
Prerequisites:
Learning Objective: To study the impact of symmetry, carbon numbers, geometry, and polarity on the melting point of solids.
Skill Level - Intermediate
Prerequisites:
Learning Objective: To learn about the intermolecular forces and how they impact an organic compound's physical properties.
Skill Level - Intermediate
Prerequisites:
Physical properties identify the substance's unique nature by subjecting the substance to qualitative and quantitative measurements. In these experiments, the substance does not undergo destruction or reconstitution of its composition but can change states/phases.