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Melting and Boiling Point - Introduction, Definition, Differences, FAQs

Melting and Boiling Point - Introduction, Definition, Differences, FAQs

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Team Careers360Updated on 02 Jul 2025, 04:36 PM IST

Introduction

Colour, hardness, malleability, solubility, electrical conductivity, density, freezing points, melting points, and boiling points are all physical qualities of matter. The freezing point of a liquid is the temperature at which it turns solid. The temperature at which a solid becomes a liquid is known as the melting point. Because the change of state involves the same two states, melting and freezing points occur at the same temperature (liquid-solid; solid-liquid). The temperature at which a solid transforms into a liquid or a liquid transforms into a solid.

Evaporation is the loss of water from a material. When wet clothing is stretched out to dry on a clothesline on a sunny day, the water has evaporated off the surface of the clothes after a few hours. Warmer water molecules leave the liquid, removing heat. As the heat escapes, the remaining liquid water cools, a process known as evaporative cooling. The molecules gather energy as the temperature of the water rises, and they move faster and faster until they have enough energy to overcome the attractive interactions between the molecules and form a gas. The boiling point is the temperature at which a liquid's molecules turn into gas.

Background wave

As a solid absorbs kinetic energy, its particles vibrate more rapidly as it heats up. Eventually, the solid structure's particle order begins to break down, and the solid begins to melt. The melting point is the temperature at which a solid melts into a liquid. At the melting point, the disruptive vibrations of the solid's particles overwhelm the attraction forces at work within the solid. The intensity of the attraction forces determines a material's melting point, just as it does its boiling point.

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Commonly Asked Questions

Q: How does hydrogen bonding affect melting and boiling points?
A:
Hydrogen bonding significantly increases melting and boiling points. These strong intermolecular forces require more energy to overcome, leading to higher temperatures for phase changes. This is why water, which exhibits hydrogen bonding, has unusually high melting and boiling points compared to similar-sized molecules.
Q: What is the relationship between intermolecular forces and the energy required for phase changes?
A:
The strength of intermolecular forces directly correlates with the energy required for phase changes. Stronger forces require more energy to overcome, resulting in higher melting and boiling points. The energy needed for these phase changes is quantified as the heat of fusion (for melting) and heat of vaporization (for boiling).
Q: How do melting and boiling points change across a period in the periodic table?
A:
Across a period in the periodic table, melting and boiling points generally increase from left to right. This trend is due to increasing nuclear charge and decreasing atomic size, which lead to stronger intermolecular forces. However, this trend can be affected by other factors such as bonding type and molecular structure.
Q: What is the relationship between a substance's critical point and its boiling point?
A:
The critical point is the temperature and pressure above which a substance cannot exist as separate liquid and gas phases. The critical temperature is always higher than the boiling point at any pressure. Above the critical point, the distinction between liquid and gas phases disappears, forming a supercritical fluid.
Q: How does the concept of London dispersion forces relate to melting and boiling points?
A:
London dispersion forces are weak intermolecular forces present in all substances. They become more significant as molecular size increases, leading to higher melting and boiling points. In non-polar molecules, London dispersion forces are often the primary intermolecular force determining these properties.

What is Melting point of water?

Melting Point Definition or Melting Definition: The melting point of a solid is the temperature at which it transforms into a liquid.

The melting point of a liquid is the temperature at which it transforms from a solid to a liquid at atmospheric pressure. This is the point where the liquid and solid phases are in balance. The substance's melting point varies with pressure and is reported at standard pressure.

Agar, for example, melts at 85 degrees Celsius (185 degrees Fahrenheit; 358 degrees Kelvin) and solidifies at 31 degrees Celsius (88 degrees Fahrenheit; 304 degrees Kelvin); this direction dependency is known as hysteresis.

The word "freezing point" refers to the temperature at which a liquid transforms into a solid and is thus the polar opposite of the term "melting point." Substances, on the other hand, can be chilled beyond their freezing temperatures without forming a solid. These liquids are referred to as super cooled liquids.

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Commonly Asked Questions

Q: What is the heat of fusion, and how does it relate to melting point?
A:
The heat of fusion is the amount of energy required to change a substance from solid to liquid at its melting point. It represents the energy needed to overcome the intermolecular forces holding the solid structure together. Substances with higher heats of fusion generally have higher melting points.
Q: What is supercooling, and how does it relate to melting point?
A:
Supercooling occurs when a liquid is cooled below its freezing point without solidifying. This metastable state can exist because the formation of a solid requires a nucleation site. Supercooling demonstrates that the melting point is not always a strict boundary between liquid and solid states.
Q: What is the triple point of a substance?
A:
The triple point of a substance is the unique combination of temperature and pressure at which all three phases (solid, liquid, and gas) of that substance coexist in equilibrium. It's a specific point on a phase diagram where the melting, boiling, and sublimation curves intersect.
Q: How does molecular size affect melting and boiling points?
A:
Generally, as molecular size increases within a homologous series, both melting and boiling points increase. Larger molecules have stronger van der Waals forces between them, requiring more energy to separate. However, this trend can be affected by other factors like molecular shape and polarity.
Q: How do melting and boiling points affect a substance's volatility?
A:
Volatility refers to a substance's tendency to vaporize. Substances with lower boiling points are generally more volatile because they require less energy to transition from liquid to gas. The melting point indirectly affects volatility by influencing whether a substance is liquid (and thus able to evaporate) at a given temperature.

What is the boiling point of water?

Boiling Point Definition or Boiling Definition: The temperature at which a liquid's vapour pressure equals the atmospheric pressure of the liquid's environment is called the boiling point. The liquid turns into a vapour at this temperature. The boiling point of water is determined by the surrounding pressure. The boiling point of a liquid at high pressure is higher than the boiling point at normal atmospheric pressure.

For a given pressure, various liquids have different boiling points. In 1982, the International Union of Pure and Applied Chemistry (IUPAC) established the standard boiling point of a liquid as the temperature at which the liquid boils at 1 bar of pressure.

A pure material's boiling and melting points are physical constants that characterize the substance in its pure condition. Although these physical constants cannot be predicted for a given material, they can be rationalized on a relative basis for certain substances by taking into account the type of chemical bonding, intermolecular forces, and other factors.

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Commonly Asked Questions

Q: What is the relationship between vapor pressure and boiling point?
A:
Vapor pressure and boiling point are inversely related. The boiling point is reached when the vapor pressure of a liquid equals the atmospheric pressure. Substances with higher vapor pressures at a given temperature will have lower boiling points because their molecules can more easily escape into the gas phase.
Q: How does the strength of intermolecular forces affect melting and boiling points?
A:
Stronger intermolecular forces lead to higher melting and boiling points because more energy is required to overcome these forces. For example, substances with hydrogen bonding (like water) have higher melting and boiling points compared to similar molecules without hydrogen bonding.
Q: How do impurities affect the melting and boiling points of a substance?
A:
Impurities generally lower the melting point and raise the boiling point of a substance. They disrupt the crystal structure, making it easier to melt, and increase the energy required for molecules to escape into the gas phase. This principle is used in freeze distillation and fractional distillation.
Q: What is the difference between evaporation and boiling?
A:
Evaporation occurs at the surface of a liquid at any temperature, while boiling occurs throughout the liquid at a specific temperature (the boiling point). Evaporation is a gradual process that doesn't require the liquid to reach its boiling point, whereas boiling involves rapid vaporization and bubble formation.
Q: How do melting and boiling points relate to a substance's physical state at room temperature?
A:
A substance's physical state at room temperature depends on how its melting and boiling points compare to room temperature. If both points are below room temperature, it's a gas; if both are above, it's a solid. If room temperature is between the melting and boiling points, it's a liquid.

Difference between melting point and boiling point.

MELTING POINT

BOILING POINT

  • Melting point is defined as the point at which a solid state changes to a liquid condition.

  • Boiling point is defined as the temperature at which air pressure equals the vapour pressure of a liquid.

  • This is primarily used to describe solid-state chemicals or molecules.

  • It refers to substances or molecules that exist in a liquid condition.

  • It also varies in terms of pressure, which is described as standard pressure.

  • It varies depending on the pressure in the environment.

  • Because the interactions between the ions are strong, ionic compounds have high melting points.

  • The boiling point of a molecule is determined by the structure of the molecule.

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Commonly Asked Questions

Q: What is the difference between melting point and boiling point?
A:
Melting point is the temperature at which a solid changes to a liquid, while boiling point is the temperature at which a liquid changes to a gas. Melting involves breaking the rigid structure of a solid, whereas boiling involves overcoming intermolecular forces in a liquid to form a gas.
Q: Why do different substances have different melting and boiling points?
A:
Different substances have different melting and boiling points due to variations in their intermolecular forces. Stronger intermolecular forces require more energy to overcome, resulting in higher melting and boiling points. Factors like molecular size, shape, and polarity affect these forces.
Q: How does pressure affect the boiling point of a liquid?
A:
Pressure directly affects the boiling point of a liquid. As pressure increases, the boiling point increases because more energy is required for the vapor pressure to overcome the external pressure. Conversely, as pressure decreases, the boiling point decreases.
Q: What happens to the temperature of a substance during melting or boiling?
A:
During melting or boiling, the temperature of a substance remains constant. This is because the energy being added is used to break intermolecular bonds rather than increase kinetic energy. This phenomenon is known as a phase change or latent heat.
Q: Can a substance have the same melting and boiling point?
A:
In theory, a substance could have the same melting and boiling point, but this is extremely rare. Such a substance would transition directly from solid to gas (sublimation) or gas to solid (deposition) at that temperature. In practice, most substances have distinct melting and boiling points.

Frequently Asked Questions (FAQs)

Q: What is the relationship between melting point, boiling point, and a substance's molar mass?
A:
Generally, within a homologous series or similar compounds, melting and boiling points increase with increasing molar mass. This is because larger molecules typically have stronger van der Waals forces between them. However, this trend can be affected by other factors such as molecular shape, polarity, and the ability to form hydrogen bonds.
Q: How do melting and boiling points relate to a substance's surface tension?
A:
Surface tension, the cohesive forces at a liquid's surface, is indirectly related to melting and boiling points. Substances with stronger intermolecular forces tend to have higher surface tension, melting points, and boiling points. During boiling, bubbles must overcome surface tension to form and rise, so liquids with higher surface tension often have higher boiling points.
Q: What is the significance of the freezing point depression and boiling point elevation in solutions?
A:
Freezing point depression and boiling point elevation are colligative properties that occur when a solute is added to a pure solvent. The solute particles interfere with the formation of solid crystals, lowering the freezing point, and require more energy for vaporization, raising the boiling point. These effects are proportional to the concentration of the solute and are used in various applications, such as de-icing roads and determining molecular masses.
Q: How do melting and boiling points relate to a substance's heat capacity?
A:
While heat capacity (the amount of heat required to raise the temperature of a substance by one degree) is not directly related to melting and boiling points, there are correlations. Substances with higher heat capacities often have stronger intermolecular forces, which can lead to higher melting and boiling points. However, this relationship is not universal and can vary depending on the substance's structure and composition.
Q: What is the relationship between melting point, boiling point, and intermolecular potential energy?
A:
Melting and boiling points are directly related to the intermolecular potential energy of a substance. Higher potential energy barriers between molecules result in higher melting and boiling points because more kinetic energy (i.e., higher temperature) is required for molecules to overcome these barriers during phase transitions.
Q: How do melting and boiling points relate to a substance's critical pressure?
A:
The critical pressure is the pressure required to liquefy a gas at its critical temperature. It's the highest pressure at which a distinct liquid phase can exist. Above the critical pressure, a substance cannot be liquefied by increasing pressure alone. The boiling point curve on a phase diagram ends at the critical point, where critical pressure and temperature intersect.
Q: What is the effect of pressure on the melting point of a substance?
A:
Pressure typically has a much smaller effect on melting point compared to its effect on boiling point. For most substances, increasing pressure slightly increases the melting point. However, water is a notable exception; increasing pressure lowers its melting point due to its unusual property of expanding when it freezes.
Q: How do melting and boiling points relate to a substance's enthalpy of fusion?
A:
The enthalpy of fusion is the energy required to melt a substance at its melting point. Generally, substances with higher melting points have higher enthalpies of fusion because more energy is needed to overcome the stronger intermolecular forces. Boiling points often correlate with this trend but are more directly related to the enthalpy of vaporization.
Q: How do melting and boiling points relate to the concept of cohesive forces?
A:
Cohesive forces are the attractive forces between molecules of the same substance. Stronger cohesive forces result in higher melting and boiling points because more energy is required to overcome these forces during phase changes. The strength of cohesive forces is directly related to the types and strength of intermolecular interactions present.
Q: What is the relationship between melting point, boiling point, and the kinetic molecular theory?
A:
The kinetic molecular theory explains melting and boiling points in terms of molecular motion. At the melting point, molecules gain enough kinetic energy to overcome the forces holding them in a fixed position. At the boiling point, they have sufficient energy to overcome the liquid's surface tension and escape into the gas phase.
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