Thermal properties and temperature Revision Notes

When a substance is heated at constant pressure, its particles absorb thermal energy, increasing their kinetic energy. This causes particles to move more vigorously. In solids, particles vibrate more intensely around fixed positions. In liquids, particles move faster and tend to separate slightly more. In gases, particles move rapidly and spread further apart because gases have no fixed shape or volume.

• Heating increases particle kinetic energy.
• Increased particle motion causes expansion.
• Expansion occurs at constant pressure, allowing volume to change.

In solids, particles are tightly packed in a fixed, orderly arrangement. Heating causes particles to vibrate with greater amplitude but they remain in fixed positions. This increased vibration pushes neighbouring particles slightly further apart, causing the solid to expand. The expansion is usually small because the particles cannot move freely.

For example, a metal railway track expands slightly on a hot day, which is why gaps are left between sections to prevent buckling.

• Particles vibrate more but remain fixed.
• Expansion is slight due to strong intermolecular forces.
• Example: metal rails expanding in summer.

Liquids have particles that are close together but can move past each other. When heated, particles move faster and tend to occupy more space by increasing the average distance between them. This causes the liquid to expand more noticeably than solids.

For instance, mercury in a thermometer expands when heated, causing the mercury level to rise and indicate temperature.

• Particles move faster and separate more.
• Expansion is greater than in solids.
• Example: mercury rising in a thermometer.

Gases have particles that are far apart and move freely. Heating a gas at constant pressure increases particle speed and causes particles to move further apart, greatly increasing the gas volume. This expansion is much larger than in solids or liquids because gases are compressible and have large spaces between particles.

An example is a hot air balloon: heating the air inside causes it to expand, lowering its density and allowing the balloon to rise.

• Particles move faster and spread out.
• Expansion is largest among the three states.
• Example: hot air balloon rising due to heated air expansion.

The extent of expansion when heated at constant pressure depends on the state of matter. Solids expand the least, liquids expand more, and gases expand the most. This is due to differences in particle arrangement and freedom of movement.

Understanding these differences is important in engineering, such as allowing for expansion gaps in bridges and designing thermometers.

• Solids expand slightly due to fixed particle positions.
• Liquids expand more as particles move freely but remain close.
• Gases expand greatly due to large particle separation and freedom.

In a liquid, particles are constantly moving with a range of kinetic energies. The energy distribution means some particles move faster and have more energy than others. Evaporation happens when particles at the surface of the liquid have enough kinetic energy to overcome the attractive forces holding them in the liquid and escape into the air as gas particles.

Only particles near the surface can evaporate because they are not surrounded by other liquid particles on all sides. Those with sufficient energy break free from the surface and enter the gas phase. This selective escape of higher energy particles is key to understanding evaporation.

• Particles in a liquid have varying kinetic energies.
• Only surface particles can evaporate.
• Evaporation requires particles to overcome intermolecular forces.
• More energetic particles are more likely to evaporate.

Temperature is a measure of the average kinetic energy of particles. As temperature increases, the average kinetic energy rises, so more particles have enough energy to escape the liquid surface. This increases the rate of evaporation.

For example, water left in a warm room evaporates faster than water in a cold room because more particles reach the energy threshold needed to evaporate.

This relationship explains why evaporation speeds up on hot days or when liquids are heated.

• Higher temperature means higher average particle energy.
• More particles exceed the energy needed to evaporate.
• Evaporation rate increases with temperature.
• Example: Water evaporates faster on a hot day.

The surface area of a liquid exposed to air affects evaporation. A larger surface area allows more particles to be at the surface and potentially evaporate at once, increasing the evaporation rate.

Air movement above the liquid surface removes evaporated particles, reducing the concentration of vapour near the surface. This prevents saturation and allows more particles to evaporate.

For instance, clothes dry faster on a windy day because moving air carries away water vapour, maintaining evaporation.

• Larger surface area increases evaporation rate.
• Air movement removes vapour, promoting evaporation.
• Still air slows evaporation due to vapour saturation.
• Example: Clothes dry faster with wind.

When the more energetic particles leave the liquid, the average kinetic energy of the remaining particles decreases. Since temperature relates to average kinetic energy, the liquid cools down.

This is why evaporation causes cooling, such as when sweat evaporates from skin, removing heat and helping to regulate body temperature.

This cooling effect is a direct consequence of the selective loss of higher energy particles during evaporation.

• Evaporation removes higher energy particles.
• Remaining liquid has lower average kinetic energy.
• Liquid temperature decreases (cools) after evaporation.
• Example: Sweating cools the body.

Evaporation is a surface process driven by particle energy distribution. Only particles with sufficient energy escape, and factors like temperature, surface area, and air movement influence the rate.

Understanding evaporation helps explain everyday phenomena such as drying clothes, cooling by sweat, and water cycle processes.

In practical terms, increasing temperature, spreading liquid over a larger area, or increasing airflow can speed up evaporation.

• Evaporation depends on particle energy and surface conditions.
• Temperature, surface area, and air movement affect evaporation rate.
• Evaporation causes cooling by removing energetic particles.
• Practical uses include drying and cooling mechanisms.

Evaporation is the process where molecules at the surface of a liquid gain enough energy to overcome intermolecular forces and escape into the air as gas particles. Not all molecules have the same energy; their energies vary continuously. The molecules with the highest kinetic energy are most likely to escape first.

• Molecules in a liquid move at different speeds and energies.
• Only molecules with sufficient energy can leave the liquid surface.
• Evaporation occurs at all temperatures but faster at higher temperatures.

Temperature is a measure of the average kinetic energy of molecules in a substance. When the highest energy molecules leave the liquid during evaporation, the average kinetic energy of the remaining molecules decreases. This reduction in average energy causes the temperature of the liquid to drop.

• Loss of high-energy molecules reduces average kinetic energy.
• Lower average kinetic energy means lower temperature.
• This cooling effect is noticeable in everyday situations.

A common example is sweating. When sweat evaporates from the skin, it removes heat, cooling the body. Another example is water evaporating from a wet surface, which feels cooler than dry areas. These examples illustrate how evaporation leads to temperature decrease in liquids and surfaces.

• Sweat evaporation cools the human body.
• Wet clothes dry and cool the skin by evaporation.
• Evaporation cools puddles and wet surfaces on a warm day.

The rate of evaporation, and thus cooling, depends on factors such as temperature, surface area, air movement, and humidity. Higher temperatures increase molecular energy, speeding evaporation. Larger surface areas expose more molecules to escape. Air movement removes evaporated molecules, maintaining the concentration gradient. Low humidity allows more evaporation.

• Higher temperature increases evaporation rate.
• Greater surface area allows more molecules to escape.
• Wind or air movement enhances evaporation.
• Low humidity promotes faster evaporation and cooling.

A simple experiment involves measuring the temperature change of water in an open container over time. Using a thermometer, record the initial temperature, then leave the water exposed to air and measure temperature at intervals. Repeating the experiment with different surface areas or airflow can show how these factors affect cooling.

• Apparatus: thermometer, water, container, stopwatch, fan (optional).
• Method: measure initial temperature, expose water, record temperature changes.
• Variables: surface area, air movement, temperature.
• Observations: temperature decreases as evaporation occurs.
• Safety: handle glassware carefully, avoid spills.

Thermal expansion is the increase in size, length, or volume of a material when its temperature rises. This happens because heating causes particles in the material to vibrate more vigorously and move slightly further apart, increasing the overall dimensions of the material.

All states of matter—solids, liquids, and gases—expand when heated, but the extent and nature of expansion differ. Solids mainly expand in length and volume, liquids expand mainly in volume, and gases expand significantly in volume if pressure is constant.

For example, a metal rod heated from 20°C to 50°C will become slightly longer due to the increased vibration and spacing of its atoms. This change is usually small but important in engineering contexts.

• Thermal expansion is caused by increased particle motion with temperature.
• It results in increased length, area, or volume depending on the material and state.
• Expansion is usually small but must be considered in design.

Materials used in large structures like bridges and railways expand and contract with temperature changes. If this expansion is not accommodated, it can cause warping, buckling, or cracking, leading to structural failure.

To prevent damage, engineers include expansion joints—gaps or flexible sections that allow materials to expand safely without causing stress. For example, railway tracks have small gaps between rails, and bridges have expansion joints that open or close as the temperature changes.

These joints ensure that the structure can safely handle daily and seasonal temperature variations without damage.

• Expansion joints allow controlled movement due to thermal expansion.
• Railway tracks have small gaps to prevent buckling in hot weather.
• Bridges use flexible joints to accommodate expansion and contraction.

Thermometers often use liquids such as mercury or coloured alcohol that expand when heated. As temperature rises, the liquid expands and moves up a narrow tube, indicating the temperature level.

This application relies on the predictable expansion of liquids with temperature. The length of the liquid column corresponds to the temperature, allowing accurate measurement.

For example, a mercury thermometer shows higher mercury level on a hot day because the mercury expands more than the glass tube.

• Liquids in thermometers expand with temperature, moving up a tube.
• The expansion is proportional to temperature, enabling measurement.
• Mercury and coloured alcohol are common liquids used.

Thermal expansion affects many everyday objects and processes. For example, metal lids on glass jars become easier to open if run under hot water because the metal expands more than the glass, loosening the lid.

In cooking, pots and pans expand when heated, so manufacturers design them to allow for this expansion without warping or damage.

Bimetallic strips in thermostats use two metals with different expansion rates to bend and operate switches, controlling heating or cooling devices automatically.

• Metal lids expand more than glass when heated, easing opening.
• Cookware design accounts for expansion to maintain shape and safety.
• Bimetallic strips use differential expansion to control temperature devices.

The amount a material expands can be calculated using the formula ΔL = α L₀ ΔT for linear expansion, where ΔL is the change in length, α is the coefficient of linear expansion (a material constant), L₀ is the original length, and ΔT is the temperature change.

For example, a 2.0 m steel rail (α ≈ 12 × 10⁻⁶ /°C) heated from 10°C to 40°C will expand by: ΔL = 12 × 10⁻⁶ × 2.0 × (40 - 10) = 12 × 10⁻⁶ × 2.0 × 30 = 0.00072 m or 0.72 mm.

Though small, this expansion is significant over long distances and must be accommodated in design.

• Linear expansion formula: ΔL = α L₀ ΔT
• Coefficient α varies by material (e.g., steel ~12 × 10⁻⁶ /°C)
• Small expansions accumulate over large lengths, affecting structures.

Temperature is a key factor affecting evaporation because it influences the kinetic energy of liquid molecules. When temperature increases, molecules move faster and more molecules have enough energy to overcome intermolecular forces and escape from the liquid surface into the air as vapour.

For example, water in a warm room evaporates faster than water in a cold room because the molecules have higher average kinetic energy at higher temperatures.

This relationship means that evaporation rate increases with temperature, as more molecules can leave the liquid per unit time.

• Higher temperature increases molecular kinetic energy.
• More molecules gain enough energy to evaporate.
• Evaporation rate rises as temperature increases.

The surface area of a liquid exposed to air determines how many molecules can escape at once. A larger surface area means more molecules are at the surface and able to evaporate simultaneously.

For example, water spread thinly over a plate evaporates faster than the same volume in a narrow glass because the plate exposes more surface area to the air.

Therefore, increasing surface area increases the evaporation rate by providing more opportunity for molecules to escape.

• More surface area exposes more molecules to air.
• More molecules can evaporate at the same time.
• Evaporation rate increases with surface area.

Air movement affects evaporation by removing water vapour from above the liquid surface. When air is still, water vapour accumulates near the surface, increasing local humidity and reducing the evaporation rate.

For example, a breeze over a puddle speeds up evaporation by carrying away water vapour, lowering humidity near the surface and allowing more molecules to evaporate.

Thus, increased air movement reduces the concentration of vapour above the liquid, maintaining a concentration gradient that favours evaporation.

• Air movement removes water vapour from the surface.
• Reduces local humidity and vapour saturation.
• Maintains concentration gradient, increasing evaporation rate.

These factors often work together. For instance, on a hot, windy day with a large puddle, evaporation is very rapid due to high temperature, large surface area, and strong air movement.

Understanding how these factors combine helps explain everyday phenomena such as drying clothes faster on a warm, breezy day compared to a cold, still day.

• Temperature, surface area, and air movement combine to affect evaporation.
• Higher temperature and air movement amplify evaporation from large surfaces.
• Real-life examples illustrate combined effects.