Thermal Properties of Matter — NEET notes

Temperature & Thermometers

Temperature measures the degree of hotness or coldness of a body. Thermal equilibrium (zeroth law) defines temperature operationally. Three scales — Celsius, Fahrenheit, Kelvin — are inter-convertible. NEET asks scale-conversion numericals and conceptual questions on thermal equilibrium.

T_F = 9 5 T_C + 32 | T_K = T_C + 273.15

Thermal Equilibrium & Zeroth Law

Two bodies are in thermal equilibrium when no heat flows between them. The zeroth law says: if A is in thermal equilibrium with C, and B is in thermal equilibrium with C, then A and B are also in thermal equilibrium with each other.

Place a hot cup of tea ( 80° C) in a room ( 25° C). Heat flows from the tea to the room. After a long time, the tea cools down and the room air warms up (imperceptibly) until both reach the same temperature. At this point — no net heat transfer — they are in thermal equilibrium.

• Thermal equilibrium = no net heat flow between two bodies in thermal contact
• Temperature is the property that determines whether two bodies are in thermal equilibrium
• The zeroth law was formulated AFTER the first and second laws — but is logically more fundamental, hence 'zeroth'
• A thermometer works because it reaches thermal equilibrium with the body being measured

Temperature Scales

Three scales define temperature differently. Celsius and Fahrenheit use two fixed points (ice point and steam point). Kelvin is the SI absolute scale starting from absolute zero.

All temperature scales need two fixed reference points and a rule for dividing the interval between them into equal parts.

T_C - 0 100 = T_F - 32 180 = T_K - 273.15 100

From this master relation, you can derive any conversion formula. The two most commonly needed:

T_F = 9 5 T_C + 32 T_K = T_C + 273.15

• Kelvin is the SI unit of temperature — no degree symbol (write K, not ° K)
• Kelvin scale has NO negative values — 0 K is absolute zero
• A change of 1° C = a change of 1 K = a change of 9 5 ° F
• For T (temperature change), Celsius and Kelvin are identical: T_C = T_K

Thermometers

Different thermometers exploit different thermometric properties (volume, pressure, resistance) to measure temperature. Each has its own range and accuracy.

A thermometer measures temperature by using a physical property that changes uniformly with temperature — called the thermometric property. Different properties give different types of thermometers.

• Mercury thermometer: Uses thermal expansion of mercury. Range: -39° C to 357° C (mercury's freezing and boiling points)
• Constant-volume gas thermometer: Uses pressure of a gas at constant volume. Most accurate; closest to ideal gas scale. Used to calibrate other thermometers
• Platinum resistance thermometer: Uses change in electrical resistance of platinum wire. Very precise for industrial use
• Thermocouple: Uses thermo-emf generated at the junction of two different metals. Fast response; good for rapidly changing temperatures
• Pyrometer: Measures thermal radiation from very hot objects ( >1000° C). Used in furnaces and steel mills — no physical contact needed

The ideal gas temperature scale (also called the Kelvin scale or thermodynamic scale) is defined using a constant-volume gas thermometer. As the amount of gas is reduced (approaching zero pressure), all gases converge to the same temperature reading — giving a universal, gas-independent scale.

T = (273.16 K ) P P_ tr

Here P_ tr is the pressure at the triple point of water ( 273.16 K = 0.01° C). The triple point — not the ice point — is the standard fixed point because it is uniquely defined (a single temperature and pressure where ice, liquid water, and water vapor coexist).

Quick recall

Why is the triple point of water used as the standard fixed point instead of the ice point?

The ice point ( 0° C) depends on pressure and can vary slightly. The triple point ( 273.16 K) is a unique, precisely defined state where all three phases coexist — it doesn't depend on external pressure.

Thermal Expansion

Most materials expand on heating. Linear expansion ( L = L T ), area expansion ( A = 2 A T ), and volume expansion ( V = 3 V T ) are related by = 2 and = 3 . Water's anomalous expansion (maximum density at 4° C) is critical for aquatic life. NEET frequently tests the : : = 1:2:3 ratio and applications like railway track gaps.

L = L T | : : = 1 : 2 : 3

Linear Expansion

When a solid rod is heated, its length increases. The fractional change in length per degree rise in temperature is the coefficient of linear expansion alpha.

Heat a metal rod of original length L by T . Its new length L' is:

L' = L(1 + T) L = L T

Here is the coefficient of linear expansion — it tells you the fractional increase in length per degree rise in temperature. Its SI unit is K^ -1 (or equivalently °C^ -1 , since T in K = T in ° C).

• depends on the material, NOT on the original length or temperature change
• Typical values: metals 10^ -5 K ^ -1 , glass 10^ -6 K ^ -1
• If a rod has a hole drilled through it and is heated, the hole EXPANDS (not shrinks) — the material around it pushes outward
• is approximately constant over moderate temperature ranges — for large T , it may vary

Area & Volume Expansion

Superficial (area) expansion uses coefficient beta = 2alpha, and cubical (volume) expansion uses coefficient gamma = 3alpha. These relations hold for isotropic solids.

For a solid expanding isotropically (equally in all directions):

A = A T = 2 A T

V = V T = 3 V T

The factor of 2 and 3 come from the dimensionality: area involves two lengths ( A = L L ), volume involves three ( V = L L L ). When each length grows by factor (1 + T) , the area grows by (1 + T)^2 1 + 2 T (ignoring ^2 terms, which are negligibly small).

• : : = 1 : 2 : 3 — this ratio is a NEET favorite
• = 2 (superficial/area expansion coefficient)
• = 3 (cubical/volume expansion coefficient)
• For liquids, only is defined (liquids don't have fixed shape, so and are meaningless)
• Units of , , are all K ^ -1

Quick recall

A metal plate has area 100 cm ^2 at 20° C. If = 2 10^ -5 K ^ -1 , what is its area at 120° C?

A = 2 A T = 2 2 10^ -5 100 100 = 0.4 cm ^2 . New area = 100.4 cm ^2 .

Thermal Stress

If a rod is clamped at both ends so it cannot expand, heating it creates enormous compressive stress. This is why bridges have expansion joints.

If a rod of length L , cross-section A , and Young's modulus Y is firmly clamped between two rigid walls and heated by T , it cannot expand. The 'denied expansion' creates a thermal stress:

Thermal stress = Y T

Compressive force = F = YA T

This comes from combining L = L T (thermal expansion) with Y = F/A L/L (Young's modulus). The stress is independent of the length of the rod — it depends only on Y , , and T .

• Thermal stress = Y T — independent of rod length
• The stress is compressive when heated (rod wants to expand but can't) and tensile when cooled
• This is why overhead power lines are hung loose — tight wires would snap in winter due to thermal contraction
• Glass cracks when hot water is poured on it because the inner surface expands while the outer surface doesn't — creating differential thermal stress

Anomalous Expansion of Water

Water has maximum density at 4 degrees C. Between 0 and 4 degrees C, water contracts on heating (anomalous behavior). This is why aquatic life survives in frozen lakes.

Most substances expand on heating — their density decreases. Water is an exception between 0° C and 4° C: it contracts on heating (density increases). Above 4° C, water behaves normally and expands on heating.

• Water has maximum density at 4° C ( _ 1000 kg/m ^3 )
• From 0° C to 4° C: water contracts on heating (anomalous) — is effectively negative
• Above 4° C: water expands on heating (normal behavior)
• This means both ice ( 0° C) and warm water ( >4° C) are LESS dense than water at 4° C

Quick recall

At what temperature is water densest, and what is the significance for aquatic life?

4° C. In winter, the densest water ( 4° C) sinks to the lake bottom, while colder water ( 0-4° C) stays on top and freezes. The ice acts as an insulator, keeping the bottom water liquid for fish to survive.

Specific Heat & Calorimetry

The heat absorbed or released by a body is Q = mc T , where c is the specific heat capacity. Calorimetry uses the principle that heat lost = heat gained (in an insulated system). Molar specific heats C_p and C_v for gases differ by R . NEET regularly tests mixing problems and the water-equivalent concept.

Q = mc T | C_p - C_v = R

Specific Heat Capacity

Different substances need different amounts of heat to raise the temperature of the same mass by the same amount. Specific heat quantifies this.

The specific heat capacity c (or s ) of a substance is the amount of heat required to raise the temperature of 1 kg of that substance by 1 K (or 1° C).

Q = mc T c = Q m T

SI unit of c : J kg ^ -1 K ^ -1 . Water has an unusually high specific heat: c_ water = 4186 J kg ^ -1 K ^ -1 1 cal g ^ -1 ° C ^ -1 . This is why water is used as a coolant and why coastal cities have moderate climates.

• c is a material property — it depends on the substance and its state (solid/liquid/gas)
• Water: c = 4186 J kg ^ -1 K ^ -1 (highest among common substances)
• Copper: c = 387 J kg ^ -1 K ^ -1 (metals generally have low c )
• 1 calorie = 4.186 J (the heat needed to raise 1 g of water by 1° C)
• Specific heat can depend on temperature — but for NEET, treat it as constant

Molar Specific Heat

Gases have two specific heats: at constant pressure (Cp) and at constant volume (Cv). They differ by R because at constant pressure, the gas does extra work expanding.

For gases, the specific heat depends on how the heating is done — at constant pressure or constant volume. This gives two molar specific heats:

• C_v = molar specific heat at constant volume — all heat goes into raising temperature (internal energy)
• C_p = molar specific heat at constant pressure — heat goes into raising temperature AND doing PdV work
• C_p > C_v always, because at constant pressure, extra energy is needed for expansion work
• For an ideal gas: C_p - C_v = R (Mayer's relation), where R = 8.314 J mol ^ -1 K ^ -1

C_p - C_v = R (Mayer's relation — ideal gas only)

= C_p C_v C_v = R - 1 , C_p = R - 1

Calorimetry

In an insulated system, the total heat exchange is zero. The principle of calorimetry lets you find unknown specific heats, final temperatures, and phase changes in mixing problems.

Principle of calorimetry: In an isolated system (no heat escapes), the heat lost by the hot body equals the heat gained by the cold body.

Q_ lost = Q_ gained m_1 c_1 (T_1 - T_f) = m_2 c_2 (T_f - T_2)

Here T_f is the final equilibrium temperature. Hot body cools from T_1 to T_f ; cold body warms from T_2 to T_f .

• Water equivalent ( W ): mass of water that has the same heat capacity as the calorimeter. W = m_c c_c c_ water
• Always check if a phase change occurs — if mixing hot water with ice, the ice must first melt ( Q = mL ) before warming as water
• The final temperature MUST lie between T_1 and T_2 — if your answer is outside this range, you've made an error
• In a real calorimeter, account for the heat absorbed by the calorimeter itself: Q_ lost = Q_ gained by liquid + Q_ gained by calorimeter

Quick recall

200 g of water at 80° C is mixed with 300 g of water at 20° C in an insulated container. What is the final temperature?

200 (80 - T_f) = 300 (T_f - 20) . Solving: 16000 - 200T_f = 300T_f - 6000 , so 500T_f = 22000 , giving T_f = 44° C. (Note: same substance, so c cancels out.)

Change of State

Matter changes state (solid-liquid-gas) at specific temperatures. Latent heat ( Q = mL ) is the energy absorbed/released during a phase change at constant temperature. The triple point is where all three phases coexist. NEET tests latent heat calculations, the heating curve, and regelation.

Q = mL | L_f(ice) = 3.36 10^5 J/kg | L_v(water) = 22.6 10^5 J/kg

Latent Heat

During a phase change, heat is absorbed or released without any change in temperature. This 'hidden' heat is called latent heat and goes entirely into breaking or forming molecular bonds.

When ice at 0° C melts, it absorbs heat — but its temperature stays at 0° C until ALL the ice has melted. The heat goes into breaking the crystal structure of ice. This heat is called latent heat (Latin: *latere* = to lie hidden).

Q = mL

Here L is the specific latent heat — the heat required to change the state of 1 kg of a substance without changing its temperature. SI unit: J kg ^ -1 .

• Latent heat of fusion ( L_f ): solid liquid. For ice: L_f = 3.36 10^5 J/kg = 80 cal/g
• Latent heat of vaporization ( L_v ): liquid gas. For water: L_v = 22.6 10^5 J/kg = 540 cal/g
• L_v L_f because vaporization requires breaking ALL intermolecular bonds, while fusion only loosens them
• During phase change: temperature remains constant, but internal energy changes
• The reverse processes (condensation, freezing) release the same amount of latent heat

Heating Curve

The heating curve shows how temperature changes as heat is continuously added to a substance. It has sloped regions (temperature rising) and flat plateaus (phase changes at constant temperature).

Plot temperature ( y -axis) vs heat supplied ( x -axis) for ice heated from -20° C to 120° C. You get a characteristic curve with slopes and plateaus.

• Slope regions: Temperature rises. Slope = T Q = 1 mc . Steeper slope means lower heat capacity
• Flat plateau at 0° C: Ice melting. Length of plateau = mL_f . Temperature doesn't change
• Flat plateau at 100° C: Water boiling. Length of plateau = mL_v (much longer than melting plateau since L_v L_f )
• Slope of ice region > slope of water region (because c_ ice < c_ water , so ice warms faster per unit heat)

Specific heats for the three regions: c_ ice 2090 J kg ^ -1 K ^ -1 , c_ water 4186 J kg ^ -1 K ^ -1 , c_ steam 2010 J kg ^ -1 K ^ -1 .

Quick recall

On a heating curve, which plateau is longer — the melting plateau or the boiling plateau? Why?

The boiling plateau is much longer because L_v = 22.6 10^5 J/kg L_f = 3.36 10^5 J/kg. It takes about 6.7 more energy to boil water than to melt the same mass of ice.

Phase Diagram & Triple Point

A phase diagram (P-T diagram) maps out which phase exists at each pressure-temperature combination. The triple point is the unique P-T where all three phases coexist.

A phase diagram (or P-T diagram) is a graph of pressure vs temperature showing the regions where a substance exists as solid, liquid, or gas. The boundaries between regions are called phase curves.

• Fusion curve (solid-liquid boundary): slopes right for most substances, but slopes LEFT for water (anomalous!)
• Vaporization curve (liquid-gas boundary): ends at the critical point — above this, liquid and gas become indistinguishable (supercritical fluid)
• Sublimation curve (solid-gas boundary): below the triple point, solids directly become gas (sublimation)
• Triple point of water: T = 273.16 K, P = 611.73 Pa — all three phases coexist

Regelation: Passing a loaded wire through a block of ice. The wire's pressure melts the ice below it; the water refreezes above the wire as pressure is released. The wire passes through, but the ice block remains whole!

Effect of pressure on boiling point: Increasing pressure raises the boiling point. A pressure cooker traps steam, increasing internal pressure to 2 atm, which raises water's boiling point to 120° C — food cooks faster.

Quick recall

Why does ice melt under pressure but most other solids don't?

Because ice is less dense than water (water expands on freezing). Applying pressure favors the denser phase — which for water is the liquid. For most substances, the solid is denser, so pressure favors the solid (raises the melting point).

Heat Transfer: Conduction

Conduction is the transfer of heat through a material without bulk movement of matter. The rate of heat flow is given by Q/t = KA(T_1 - T_2)/L . Thermal conductivity K measures how well a material conducts heat. NEET tests steady-state conduction, series/parallel slabs, and the temperature distribution in composite walls.

Q t = KA(T_1 - T_2) L

Conduction

In conduction, heat passes from the hot end to the cold end of a solid as molecules vibrate and transfer energy to their neighbors. No material actually moves — only energy.

Hold one end of an iron rod in a flame. After some time, the other end gets hot. The heat traveled through the rod by conduction — molecules at the hot end vibrate faster, bumping into neighboring molecules and passing on their kinetic energy, one layer at a time.

Q t = KA(T_1 - T_2) L (steady state)

Here K = thermal conductivity (W m ^ -1 K ^ -1 ), A = cross-sectional area, L = length of the slab, and (T_1 - T_2) = temperature difference between the two faces. This is called Fourier's law of heat conduction.

• K is a material property: metals have high K (copper 385 W m ^ -1 K ^ -1 ), insulators have low K (wood 0.12 )
• Temperature gradient = (T_1 - T_2)/L = dT/dx — the rate at which temperature changes with distance
• In steady state, the temperature at each point remains constant with time (heat in = heat out at every cross-section)
• Before steady state, the rod is in transient state — temperatures are still changing

Thermal Resistance

Just like electrical resistance opposes current flow, thermal resistance opposes heat flow. This analogy lets you solve series and parallel slab problems using circuit techniques.

Define thermal resistance R_ th = L KA . Then Fourier's law becomes:

H = T R_ th where R_ th = L KA

This is the thermal analog of Ohm's law: I = V/R . Temperature difference T plays the role of voltage, heat current H plays the role of electric current, and R_ th plays the role of electrical resistance.

• Series slabs (layers one behind another): R_ total = R_1 + R_2 + Same heat current flows through all layers
• Parallel slabs (layers side by side): 1 R_ total = 1 R_1 + 1 R_2 + Same T across all layers
• For series: effective K = L_1 + L_2 L_1/K_1 + L_2/K_2 (harmonic mean weighted by length)
• For parallel: effective K = K_1A_1 + K_2A_2 A_1 + A_2 (arithmetic mean weighted by area)

Quick recall

Two slabs of equal area and equal thickness L have thermal conductivities K_1 and K_2 . What is the effective conductivity when placed in series?

K_ eff = 2K_1K_2 K_1 + K_2 (harmonic mean). This is derived from R_ total = R_1 + R_2 = L K_1A + L K_2A = 2L K_ eff A .

Temperature Distribution in Composite Slabs

In a composite slab in steady state, the temperature drops across each layer in proportion to its thermal resistance. You can find the junction temperature using the series circuit analogy.

Two slabs (conductivities K_1 , K_2 ; thicknesses L_1 , L_2 ; same area A ) are placed in series between temperatures T_1 (hot) and T_2 (cold). In steady state, the same heat current H flows through both. The junction temperature T_j can be found by equating heat currents:

H = K_1 A(T_1 - T_j) L_1 = K_2 A(T_j - T_2) L_2

T_j = K_1 T_1 / L_1 + K_2 T_2 / L_2 K_1/L_1 + K_2/L_2

In the circuit analogy: T_j is the voltage at the junction of two series resistors. Using the voltage divider formula: T_j = T_2 + (T_1 - T_2) R_2 R_1 + R_2 , where R_i = L_i / K_i A .

• Temperature drop across a slab is proportional to its thermal resistance: T_i = H R_i
• Higher thermal resistance larger temperature drop across that slab
• In steady state, temperature varies linearly within each uniform slab (straight line on a T-vs-x graph)
• The slope of the T-vs-x graph changes at each junction (different K means different slope)

Heat Transfer: Convection & Radiation

Convection transfers heat by bulk fluid movement (natural or forced). Radiation transfers heat via electromagnetic waves — no medium needed. Stefan's law ( P = A T^4 ), Wien's displacement law ( _m T = b ), and Newton's law of cooling ( dT/dt T ) are key NEET formulas. Newton's law of cooling is tested as both numericals and graph-based questions.

P = A T^4 | _m T = b | dT dt = -k(T - T_0)

Convection

In convection, heat is transferred by the actual movement of the heated fluid (liquid or gas). Hot fluid rises, cold fluid sinks, creating convection currents.

In convection, the heated fluid itself moves — carrying thermal energy with it. When you boil water, the water at the bottom heats up, becomes less dense, rises, and is replaced by cooler water from above. This creates a convection current.

• Natural (free) convection: Driven by density differences due to temperature — hot fluid rises, cold fluid sinks. Example: sea/land breeze, boiling water
• Forced convection: Driven by an external agent (fan, pump). Example: car radiator, air conditioner, blood circulation
• Convection requires a medium (liquid or gas) — it cannot occur in solids or vacuum
• Rate of convective heat transfer depends on: fluid velocity, viscosity, temperature difference, and surface area

Radiation & Stefan's Law

All bodies emit thermal radiation as electromagnetic waves. The total power radiated is proportional to T^4 (Stefan-Boltzmann law). For a black body, emissivity = 1.

Thermal radiation is the emission of electromagnetic waves by all bodies above absolute zero. Unlike conduction and convection, radiation needs no medium — the Sun's heat reaches Earth through the vacuum of space.

P = A e T^4 (Stefan-Boltzmann law)

Here P = total power radiated (W), = 5.67 10^ -8 W m ^ -2 K ^ -4 (Stefan-Boltzmann constant), A = surface area, e = emissivity ( 0 e 1 ), and T = absolute temperature in Kelvin.

• Black body ( e = 1 ): perfect absorber and perfect emitter — absorbs all incident radiation, emits maximum radiation at every wavelength
• Net power radiated by a body at temperature T in surroundings at T_0 : P_ net = A e (T^4 - T_0^4)
• Radiation power T^4 — doubling the absolute temperature increases radiation by 2^4 = 16 times
• Emissivity e ranges from 0 (perfect reflector) to 1 (black body). For most real surfaces, 0 < e < 1
• Kirchhoff's law: Good absorbers are good emitters. At thermal equilibrium, absorptivity = emissivity at every wavelength

Quick recall

A body's absolute temperature is doubled. By what factor does its rate of radiation increase?

Factor of 2^4 = 16 . Since P T^4 , doubling T increases P by 16 times.

Wien's Displacement Law

The wavelength at which a black body emits maximum radiation is inversely proportional to its temperature. Hotter objects glow blue; cooler objects glow red.

A black body doesn't radiate equally at all wavelengths. There's a peak wavelength _m where emission is maximum. Wien's displacement law states:

_m T = b (b = 2.9 10^ -3 m K)

As temperature increases, _m shifts to shorter wavelengths (toward the blue/UV end). This is why a heated iron rod glows dull red at 800° C, bright orange at 1200° C, and white-hot at 2000° C.

• _m 1/T — hotter objects peak at shorter (bluer) wavelengths
• Wien's constant b = 2.9 10^ -3 m K (often written as 2898 m K)
• The Sun ( T 5800 K): _m 500 nm (green-yellow visible light)
• Human body ( T 310 K): _m 9.4 m (infrared — invisible, detected by thermal cameras)
• As T increases, the peak shifts left AND the total area under the curve increases (more total power — Stefan's law)

Newton's Law of Cooling

A body cools faster when it's much hotter than its surroundings. The rate of cooling is proportional to the temperature difference. Valid only for small temperature differences.

Newton's law of cooling states: the rate of heat loss of a body is directly proportional to the difference in temperature between the body and its surroundings, provided the temperature difference is small.

dT dt = -k(T - T_0)

Here T is the body's temperature, T_0 is the surrounding temperature (constant), and k is a positive constant depending on the nature of the surface and surroundings. The negative sign indicates cooling (temperature decreasing).

T(t) = T_0 + (T_i - T_0)e^ -kt

This is the solution — temperature decays exponentially toward T_0 . The body never quite reaches T_0 but gets asymptotically close.

• Valid only when T - T_0 is small (typically < 40° C above surroundings)
• It's an approximation of Stefan's law for small temperature differences: T^4 - T_0^4 4T_0^3(T - T_0)
• On a T vs t graph: exponential decay curve — steep initially, flattening as T T_0
• On a (T - T_0) vs t graph: straight line with slope = -k

Quick recall

A cup of tea cools from 80° C to 60° C in 5 minutes in a room at 20° C. How long to cool from 60° C to 40° C?

Using the approximate method: First interval: 80-60 5 = k(70 - 20) 4 = 50k k = 0.08 . Second interval: 60-40 t = 0.08(50 - 20) = 2.4 t = 20/2.4 8.33 min. (It takes longer because the temperature difference is smaller.)

Greenhouse Effect & Applications

Greenhouse gases trap infrared radiation re-emitted by Earth, warming the atmosphere. A thermos flask minimizes all three modes of heat transfer. NEET occasionally tests greenhouse effect concepts and practical applications of thermal properties.

No major formula — conceptual topic

Greenhouse Effect

The greenhouse effect is a natural phenomenon where certain atmospheric gases trap infrared radiation, keeping Earth warm enough to support life. Enhanced greenhouse effect leads to global warming.

The Sun's radiation (mostly visible light, 500 nm) passes through the atmosphere and heats Earth's surface. The warm Earth re-emits this energy as infrared radiation ( 10 m — much longer wavelength, by Wien's law). Greenhouse gases in the atmosphere absorb this outgoing infrared radiation and re-radiate it in all directions — including back toward Earth. This traps heat and keeps Earth warmer than it would be otherwise.

• Greenhouse gases: CO _2 , CH _4 (methane), N _2 O, water vapor, ozone, CFCs
• Without the greenhouse effect, Earth's average temperature would be about -18° C instead of the current +15° C
• Enhanced greenhouse effect: Increasing CO _2 from fossil fuels traps MORE infrared radiation → global warming
• Glass in a greenhouse works similarly — transparent to visible light, opaque to infrared — trapping heat inside

Thermos Flask

A thermos flask is engineered to minimize conduction, convection, and radiation simultaneously, keeping hot things hot and cold things cold for hours.

A thermos flask (Dewar flask) has a double-walled glass vessel with a vacuum between the walls. The inner surfaces are silvered (mirror-coated). The flask sits inside a protective casing with insulating material.

• Vacuum between walls → eliminates conduction and convection (no medium for heat transfer)
• Silvered surfaces → minimize radiation (silvered surfaces have very low emissivity; they reflect rather than absorb/emit radiation)
• Insulating stopper/lid → prevents heat loss through the opening (conduction through air)
• Outer casing with insulating material → additional conduction barrier
• The same flask keeps hot drinks hot AND cold drinks cold — it minimizes heat transfer in BOTH directions

Quick recall

Why is a thermos flask equally good at keeping cold drinks cold?

The flask doesn't 'keep things hot' — it minimizes ALL heat transfer. For a cold drink, the same vacuum (no conduction/convection) and silver coating (no radiation) prevent external heat from reaching the drink. Heat transfer works both ways, and the flask blocks both.

Applications of Thermal Properties

Thermal expansion, specific heat, and heat transfer principles appear everywhere in daily life. Understanding the physics behind these applications is essential for NEET conceptual questions.

The thermal properties covered in this chapter explain a wide range of everyday phenomena. NEET occasionally tests these as application-based conceptual questions.

• Bimetallic strip (thermostat): Two metals with different bonded together. On heating, they bend because one expands more. Used in thermostats and fire alarms
• Gaps in railway tracks and bridges: Allow thermal expansion in summer without buckling
• Pyrex glass has very low — resists thermal shock (doesn't crack with sudden temperature changes). Used in lab equipment and ovenware
• Woolen clothes trap air (low K ) between fibers, reducing conduction — keeping you warm
• Two thin blankets are warmer than one thick blanket of the same total thickness — the trapped air layer between them adds thermal resistance

• Pressure cooker: Higher pressure → higher boiling point → food cooks faster at 120° C instead of 100° C
• Black-body kettles: Black surfaces emit radiation better (Kirchhoff's law), so black kettles cool faster. But they also absorb better — so a black kettle in sunlight heats up faster too
• White/light clothes in summer: Reflect more solar radiation (visible + IR), absorb less → stay cooler
• Mud houses stay cool in summer: Mud has low thermal conductivity and high specific heat — it heats slowly and acts as a thermal buffer