19. High-Yield Quick Facts for Revision

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Heat is energy in transit due to a temperature difference; temperature is a measure of the average kinetic energy of molecules — heat is measured in joules, temperature in kelvin/Celsius/Fahrenheit.
−40° is the unique temperature at which the Celsius and Fahrenheit scales give the same numerical reading.
Absolute zero (0 K = −273.15°C) is the theoretical temperature of minimum possible molecular motion and can never actually be reached (Third Law of Thermodynamics).
Mercury thermometers work on thermal expansion; thermocouples work on the Seebeck effect (thermo-EMF); pyrometers work on thermal radiation and are used for very high, non-contact temperature measurement.
Silver is the best conductor of heat among common metals; air (trapped, still) is a very poor conductor and is the basis of most insulating materials, including woollen clothing and thermos flasks.
Conduction needs solids mainly (particle-to-particle, no bulk movement); convection needs fluids (bulk movement of heated particles); radiation needs no medium at all and can travel through vacuum — this is how solar heat reaches Earth.
Water's unusually high specific heat capacity moderates coastal climates and drives land/sea breezes and monsoon wind patterns.
Latent heat is 'hidden' heat absorbed/released during a change of state at constant temperature; latent heat of vaporisation of water (≈540 cal/g) is much greater than its latent heat of fusion (≈80 cal/g), which is why steam burns are more severe than boiling-water burns.
Water shows anomalous expansion: it is densest at 4°C, and expands (rather than contracts) on cooling from 4°C to 0°C, and further expands markedly on freezing — this is why ice floats and why lakes freeze from the top down, preserving aquatic life beneath the ice.
Boyle's Law: P inversely proportional to V (constant T). Charles's Law: V directly proportional to T (constant P). Gay-Lussac's Law: P directly proportional to T (constant V).
Increasing pressure raises the boiling point of a liquid (principle of the pressure cooker) and lowers the boiling point at high altitude (lower atmospheric pressure); increasing pressure lowers the melting point of ice (regelation), an exception to the general rule for most solids.
A pressure cooker cooks food faster because trapped steam raises internal pressure, which raises the boiling point of water well above 100°C.
Sound is a mechanical, longitudinal wave; it cannot travel through vacuum, unlike light.
Speed of sound: solids > liquids > gases; in air it is about 331–346 m/s and increases with temperature and humidity, but is essentially unaffected by pressure alone.
Pitch depends on frequency; loudness depends on amplitude/intensity; quality (timbre) depends on the waveform/harmonic content — this lets us distinguish instruments playing the same note at the same volume.
Human audible range: 20 Hz–20,000 Hz. Below 20 Hz = infrasonic (earthquakes, elephants); above 20,000 Hz = ultrasonic (bats, dolphins, medical ultrasound, SONAR).
A distinct echo requires the reflecting surface to be at least about 17 metres away, because the human ear needs a minimum gap of 0.1 second (persistence of hearing) to distinguish two separate sounds.
Reverberation is many overlapping reflections blending into a continuous, decaying sound within an enclosed space; excessive reverberation is controlled using sound-absorbing materials in hall/auditorium design.
Doppler effect: apparent frequency (pitch) rises as a source approaches and falls as it recedes, even though the source's true frequency never changes — classic example is a passing ambulance siren.
Resonance is the dramatic build-up of vibration amplitude when a driving frequency matches an object's natural frequency (e.g., pushing a swing in rhythm, shattering a glass with sound, radio tuning); soldiers break step crossing bridges to avoid resonance-induced structural failure.
Beats arise from superposition of two close-but-different frequencies; beat frequency = difference of the two frequencies; used by musicians to tune instruments.
The ear: outer ear (pinna, ear canal, eardrum) collects and channels sound; middle ear (malleus, incus, stapes — the three smallest bones in the body) amplifies vibrations; inner ear (cochlea) converts vibrations to nerve signals sent to the brain, while the semicircular canals (also in the inner ear) govern balance, not hearing.
Noise pollution is measured in decibels; sustained exposure above roughly 85 dB risks hearing damage; India regulates permissible noise levels through the Noise Pollution (Regulation and Control) Rules under the Environment (Protection) Act.

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