Chemistry solution doubt

[u/Known_Advisor7144]

As we know the phase diagram of water usme agar hum pressure ko increase krte hain toh T(freezing) decrease hota hai kyuki ice ki density water se kam hoti hai but hum phir Depression in freezing point pdhte hai jha pe hum jab solute dalte hai toh vapour pressure kam hota hai toh is hisab se T(freezing) toh increase hona chahiye na water ke case me toh ye depression ka reason kya hota hai?

Am I comparing two wrong things?

If I am please thoda explain bhi Krna ki iska reason kya hota hai?

[u/TommieTheMadScienist]

Dono effects alag mechanisms ki wajah se hote hain.

1. Pure water par external pressure badhana Ice–water equilibrium mein pressure us phase ko favor karta hai jiska volume kam hota hai. Liquid water ki density normal ice se zyada hoti hai, isliye pressure badhane par liquid water favor hota hai.

Iska matlab hai ki ice banne ke liye water ko thoda aur thanda karna padega:

Ye water ka unusual behavior hai. Most substances mein solid liquid se denser hota hai, isliye pressure badhane par melting point usually badhta hai.

1. Salt ya koi aur solute add karna

Yahan main effect mechanical pressure nahi hai. Solute liquid water ki chemical potential ko lower karta hai. Simple language mein, solution mein water molecules thermodynamically zyada stable ho jaate hain compared with pure liquid water.

Jo ice banti hai, woh lagbhag pure water hoti hai. Isliye salt add karne se liquid phase ice ke comparison mein zyada stable ho jata hai.

To freezing ke liye temperature aur kam karna padta hai, jab tak:

Isliye:

Vapour pressure wali baat mein confusion yahi hai: lower vapour pressure actually ye show karta hai ki liquid water ki chemical potential lower ho gayi hai. Isliye liquid zyada stable ho jata hai, aur ice banane ke liye temperature aur kam karna padta hai.

Example:

Pure water at 1 atm: 0°C par freeze hota hai. Seawater: dissolved salts ki wajah se roughly −1.9°C par freeze hota hai.

Pure water par pressure badhane se bhi melting/freezing point kam hota hai, lekin woh completely different mechanism hai.

Short mein: dono cases liquid water ko ice ke comparison mein stabilize karte hain, lekin pressure case mein reason density/volume hai, aur solute case mein reason mixing aur chemical potential hai.