Latent heat — phase transitions: Question bank — Hinglish deep dive

1.7.5 · D5 · Physics › Thermodynamics › Latent heat — phase transitions

Yahan use hone wale symbols exactly wahi hain jo parent note mein bane hain: $Q = m L$ (mass $m$ ka phase change karne ke liye heat), $Q = m c Δ T$ (temperature change karne ke liye heat), latent heat of fusion $L_{f}$ (solid↔liquid) aur vaporisation $L_{v}$ (liquid↔gas). Kuch naya introduce nahi kiya gaya.

True or false — justify

Kisi substance mein heat add karna usse hamesha hotter banata hai.

False. Melting ya boiling plateau par har joule potential energy ban jaata hai jo bonds todata hai, isliye temperature flat rehta hai jabki heat badhti rehti hai.

Melting ke dauran, molecules ki average kinetic energy badh jaati hai.

False. Temperature (∝ average KE) melting ke dauran constant rehta hai, isliye KE unchanged rehti hai — energy poori tarah se potential energy mein jaati hai jo lattice ko alag kheenchti hai.

Latent heat "hidden" hoti hai kyunki yeh koi aisa temperature change nahi karti jo thermometer padh sake.

True. Latent ka matlab hota hai hidden; energy substance mein broken-bond potential energy ke roop mein maujood hoti hai lekin thermometer ke liye invisible hoti hai, jo sirf temperature track karta hai.

Paani ko freeze karna exactly utni hi latent heat release karta hai jitni use absorb ki thi.

True. Fusion energy ke terms mein reversible hai:

Q = m L_{f}

melting par absorb hoti hai aur same magnitude freezing par same temperature aur pressure par baahar aati hai.

Paani ke liye

L_{v}

L_{f}

se bada hota hai kyunki gas molecules simply hotter hote hain.

False. Dono plateaus constant temperature par hote hain.

L_{v}

isliye bada hai kyunki vaporisation molecules ko attraction ke khilaaf poori tarah alag karta hai aur atmosphere ko peeche push karne ka kaam karta hai, jabki fusion sirf ek lattice ko dhila karta hai.

Apne melting point par koi substance necessarily poori tarah liquid hoti hai.

False. Melting point par solid aur liquid saath mein exist karte hain; mixture kisi bhi ratio mein ho sakta hai jabki melting puri karne ke liye heat ab bhi supply ho rahi ho.

100° C

par steam, same mass ke

100° C

par paani se zyada energy carry karta hai.

True. Woh steam banane ke liye tumhe paani ki energy ke upar

m L_{v}

add karna pada, isliye steam mein woh extra latent (potential) energy hoti hai — yehi wajah hai ki steam se burns kaafi bure hote hain.

Heating curve par plateau flat hota hai kyunki wahan heat flow karna band ho jaati hai.

False. Heat steadily flow karti rehti hai; plateau flat hota hai kyunki

Δ T = 0

— heat phase change (

Q = m L

) mein ja rahi hai, temperature raise karne mein nahi.

Spot the error

"Ice → water → steam ke liye do latent terms chahiye, isliye

Q = m L_{f} + m L_{v}

aur bas ho gaya."

Slopes missing hain. Tumhe paani ko dono plateaus ke beech

0° C

100° C

tak warm karna bhi padega (

m c_{w} Δ T

) —

m c Δ T

terms optional nahi hain.

"Melting plateau par

Δ T = 0

, isliye

Q = m c Δ T = 0

; ice melt karne ke liye koi heat nahi chahiye."

Galat formula plateau ke liye.

Q = m c Δ T

sloped regions ko describe karta hai; plateau par tumhe

Q = m L

use karna chahiye, jo nonzero hai.

Q = 0

milna ek sign hai ki tumne galat equation use ki.

"Paani ke cool hone aur freeze hone par heat released nikaalне ke liye, bas

Q = m c Δ T

0° C

tak use karo."

Tum bahut jaldi ruk gaye.

0° C

tak cool karna

m c Δ T

hai, lekin phir paani ko freeze karna constant

0° C

par extra

m L_{f}

release karta hai; dono terms count hote hain.

L_{f}

aur

L_{v}

ki units joules hain, kyunki yeh heats hain."

Units galat hain.

L

specific latent heat hai: J/kg. Actual heat

Q = m L

hai, aur sirf

Q

ki units joules hain.

"Boiling aur evaporation ek hi process hai, isliye dono ko

Q = m L_{v}

chahiye."

Same latent heat, alag process. Dono liquid→gas hain aur

L_{v}

use karte hain, lekin boiling ek fixed temperature par poore liquid mein hoti hai, jabki evaporation sirf surface par kisi bhi temperature par hoti hai — dekho Evaporation vs boiling.

"Ek calorimetry balance hai: paani dwara lost heat

= m c Δ T

of the ice."

Ice ke liye galat bookkeeping.

0° C

par ice heat gain karti hai latent fusion (

m L_{f}

) ke roop mein,

m c Δ T

ke roop mein nahi — melting ke dauran uska temperature nahi badh raha.

Why questions

Temperature kyun badhne se maana karta hai jabki ice melt ho rahi ho, chahe strong flame ho?

Kyunki temperature average kinetic energy ko reflect karta hai, aur melting ke dauran saari incoming heat crystal lattice todne ki potential energy mein convert hoti hai — koi bhi molecules ko speed up nahi karta, isliye temperature hold karta hai.

100° C

par steam se scald,

100° C

par boiling water se kyun bura hota hai?

Steam pehle tumhari skin par condense hoti hai, apni badi latent heat

m L_{v}

dump kar deti hai cool hone se pehle, isliye yeh same temperature wale paani se kaafi zyada energy deliver karti hai.

First law of thermodynamics humein kyun batata hai ki

L_{v}

mein sirf molecules ko alag kheenchne se zyada hai?

Kyunki

Q = Δ U + W

: vaporisation internal energy badhati hai aur atmosphere ke khilaaf expand karte hue kaam

W

karti hai, isliye

L_{v}

dono ke liye pay karta hai — jo ise pure bond-breaking cost se upar le jaata hai.

Hum heating-curve total ko ek formula ke bajaye paanch alag terms mein kyun split karte hain?

Kyunki sloped regions

Q = m c Δ T

(teen alag

c

values ke saath) follow karte hain aur flat plateaus

Q = m L

follow karte hain; physics har kink par change hoti hai, isliye har region independently calculate kiya jaata hai aur phir sum kiya jaata hai.

Kinetic theory of gases constant-temperature plateau ke idea ko kyun support karti hai?

Kinetic theory temperature ko directly mean molecular kinetic energy se jodhti hai; agar woh KE ek phase change ke dauran unchanged hai, toh temperature zaroor constant rahega chahe kitni bhi heat flow kare.

Sweating tumhare body ko kyun cool karta hai?

Evaporating sweat apni latent heat of vaporisation tumhari skin se kheenchti hai; woh energy escape hone wale water molecules ke saath chali jaati hai, skin ka temperature kam karti hai (dekho Evaporation vs boiling).

Edge cases

Agar tum ice mein heat add karo jo $0° C$ se neeche hai, toh kya koi bhi immediately melt ho jaati hai?

Nahi. Ice ko pehle

m c_{ice} Δ T

ke zariye

0° C

tak warm up karna hoga; sirf jab woh melting point tak pahunchti hai tab aur heat fusion mein jaati hai.

Kya hoga agar tum

0° C

par ice ko paani mein daalo aur paani itni heat supply nahi kar sakta ki sab melt ho jaye?

Mixture

0° C

par dono ice aur paani present ke saath end hoga — ek coexistence equilibrium. Sirf kuch ice melt hogi, saari available heat use karke.

Exactly melting point par heat flow karte hue, substance ka kitna fraction solid hai?

Sirf temperature se indeterminate — yeh depend karta hai ki kitni heat add ki gayi hai; thermometer same value padh raha hota hai jabki substance smoothly all-solid se all-liquid ki taraf jaati hai.

Kya pressure temperature axis par plateaus ki jagah change karta hai?

Haan. Boiling aur melting points pressure ke saath shift karte hain — exactly yahi ek phase diagram

P

–

T

space mein map karta hai;

100° C

boiling plateau

1

atm assume karta hai.

Agar koi substance sublime kare (solid seedha gas mein), uski heating curve mein kitne plateaus hote hain?

Ek — sublimation point par ek single plateau latent heat of sublimation use karke, kyunki koi alag liquid stage nahi hai jisme se warm karna ho.

Kya latent heat release ho sakti hai jabki substance ka temperature saath mein gir raha ho?

Transition ke dauran nahi. Latent release fixed transition temperature par hoti hai; koi bhi temperature fall ek alag sloped

m c Δ T

stage hai plateau se pehle ya baad mein.

Agar do objects mix karke same final temperature tak pahunchein, toh humein phir bhi kyun check karna chahiye ki saari ice melt hui ya nahi?

Kyunki energy balance system ko

0° C

par bacha hua ice ke saath stuck chhod sakta hai — poori melting assume karna exchanged heat ko overstate kar sakta hai aur galat final state de sakta hai.

Recall Traps ka ek-line summary

Slopes $Q = m c Δ T$ use karte hain, plateaus $Q = m L$ use karte hain; phase change ke dauran temperature frozen rehta hai kyunki heat potential energy ban jaati hai; aur $L_{v} > L_{f}$ kyunki full separation plus atmospheric work, sirf ek lattice ko dhila karne se zyada costly hai.

Connections

Latent heat — phase transitions — parent note jinhe yeh traps interrogate karte hain.
Specific heat capacity — sloped-region partner formula jo kaafi traps mein misuse hoti hai.
Calorimetry — method of mixtures — "kya saari ice melt hui?" edge cases.
Kinetic theory of gases — kyun constant KE ka matlab constant temperature hai.
Evaporation vs boiling — surface vs bulk vaporisation, dono $L_{v}$ use karte hain.
First law of thermodynamics — kyun $L_{v}$ atmospheric work ke liye bhi pay karta hai.
Phase diagrams — pressure plateaus ko kaise move karta hai.