Thermal Properties And Temperature

Thermal Expansion

• When a material is heated at constant pressure, its temperature increases, its volume increases (it expands), and its density decreases.
• Expansion occurs because molecules vibrate or move faster, colliding and pushing each other apart.
• Solids expand the least, gases expand the most, and liquids are in between.

Which state of matter expands the most when heated at constant pressure?

• Solid
• Liquid
• Gas
• All expand equally

Uses and Consequences of Thermal Expansion

• Liquid-in-glass thermometers use the expansion of a liquid (e.g., mercury) in a narrow capillary tube to measure temperature.
• Temperature-activated switches use a bimetallic strip that bends when heated, closing or opening a circuit.
• Unwanted expansion can cause buckling in railway tracks, roads, and bridges; gaps are built in to allow safe expansion.

A liquid-in-glass thermometer works because the liquid ____ when heated, moving up the ____ tube.

Fixed Points and Changes of State

• Pure water melts at 0 °C and boils at 100 °C at atmospheric pressure. These are called fixed points.
• During melting or boiling, temperature remains constant even though energy is added. The energy goes into overcoming intermolecular forces (potential energy), not increasing kinetic energy.
• Condensation and solidification are the reverse processes: energy is removed, and temperature stays constant until the change is complete.

When ice melts at 0 °C, what happens to the energy supplied?

• It increases the kinetic energy of the molecules.
• It increases the potential energy of the molecules.
• It increases both kinetic and potential energy equally.
• It is lost to the surroundings.

Evaporation

• Evaporation is a change of state from liquid to gas that occurs at any temperature, only from the surface of the liquid.
• Molecules with higher kinetic energy near the surface can escape, reducing the average energy of the remaining liquid, so evaporation causes cooling.
• Rate of evaporation increases with higher temperature, larger surface area, and greater air movement.

Which of these are true about evaporation?

• Evaporation occurs throughout the liquid.
• Evaporation can happen at any temperature.
• Evaporation causes cooling of the liquid.
• Evaporation only happens at the boiling point.

Boiling vs Evaporation

• Boiling occurs at a specific temperature (boiling point) and happens throughout the liquid, forming bubbles.
• Evaporation occurs at any temperature and only at the surface.
• Both are changes from liquid to gas, but boiling requires energy to overcome intermolecular forces throughout the liquid.

Which statement correctly compares boiling and evaporation?

• Both occur only at the boiling point.
• Boiling occurs throughout the liquid; evaporation occurs only at the surface.
• Evaporation occurs throughout the liquid; boiling occurs only at the surface.
• Both occur only at the surface.

Factors Affecting Evaporation Rate

• Higher temperature increases the kinetic energy of molecules, making more able to escape.
• Larger surface area allows more molecules near the surface to escape.
• Air movement (wind) removes vapour above the liquid, allowing more evaporation to occur.

Increasing the ____ of a liquid increases the rate of evaporation because more molecules have enough energy to escape.

Condensation and Solidification

• Condensation: gas to liquid. Energy is removed, potential energy decreases, temperature stays constant until all gas condenses.
• Solidification: liquid to solid. Energy is removed, potential energy decreases, temperature stays constant until all liquid solidifies.
• During both processes, the temperature remains constant because energy is being removed from the potential store, not the kinetic store.

During condensation, the temperature of the substance:

• Increases steadily.
• Decreases steadily.
• Remains constant.
• First increases then decreases.

Thermometer Sensitivity and Range

• Sensitivity: how much the liquid expands for a given temperature change. A narrower capillary tube increases sensitivity (larger movement for same expansion).
• Range: the minimum and maximum temperatures the thermometer can measure. A larger bulb or more liquid increases range.
• Fixed points of the Celsius scale: 0 °C (melting point of ice) and 100 °C (boiling point of water at standard pressure).

How does reducing the diameter of the capillary tube affect the sensitivity of a liquid-in-glass thermometer?

• Sensitivity decreases because the liquid moves less.
• Sensitivity increases because the same volume change produces a greater length change.
• Sensitivity is unaffected.
• Sensitivity increases but range decreases.

Cooling by Evaporation

• When a liquid evaporates, the most energetic molecules leave, reducing the average kinetic energy of the remaining liquid, so it cools.
• Blowing air over a liquid increases evaporation and enhances cooling.
• This is why sweating cools the body: sweat evaporates, taking heat away.

Evaporation causes cooling because the molecules with the most ____ energy escape, leaving behind molecules with ____ average energy.

Summary of Thermal Properties

• Thermal expansion: particles move apart when heated; solids expand least, gases most.
• Changes of state: temperature constant during melting, boiling, condensation, solidification.
• Evaporation: surface phenomenon at any temperature; causes cooling; rate increased by higher temperature, larger area, and air movement.
• Thermometers: use liquid expansion; sensitivity increased by narrower capillary or larger bulb.

Which of the following increases the rate of evaporation?

• Decreasing the temperature of the liquid
• Decreasing the surface area of the liquid
• Increasing air movement over the liquid
• Decreasing the temperature of the surroundings