Freezing Mixtures Acetone–Dry Ice System Liquid–Liquid Mixtures: Ideal liquid mixtures

1. Thermodynamics
By Kirti V.J
ED24B1019.

2. About
● Freezing Mixtures
● Acetone–Dry Ice
System
● Liquid–Liquid
Mixtures: Ideal liquid
mixtures

3. Freezing Mixtures

4. 1. A freezing mixture is a combination of two or more substances that,
when mixed, produce a temperature lower than that of any of the
components individually.
2. The cooling effect arises because certain processes — like melting or
dissolving — are endothermic, meaning they absorb heat from the
surroundings.
3. When heat is absorbed, the temperature of the system decreases,
leading to cooling.

5. Thermodynamic Explanation
1. When a solid (ex. ice) melts or dissolves in another substance (ex.
salt), it requires heat energy — the latent heat of fusion or solution.
2. This heat is taken from the surroundings, thus lowering the
temperature of the mixture.
3. The phenomenon can be explained using the depression in freezing
point concept:
ΔTf= Kf m
⋅
where Kf is the molal freezing point constant and m is molality.

6. Cooling Curve for a Freezing Mixture

7. Common Examples:
1. Ice + Salt (NaCl): Used for freezing ice cream and rapid cooling of
beverages; can reach about –20 °C.
2. Ice + Ammonium Chloride (NH Cl):
₄ Often used in laboratories for
controlled cooling.
3. Dry Ice + Acetone: Achieves very low temperatures (around –78 °C).

8. Acetone–Dry Ice
System

9. Acetone-Dry Ice is a special type of freezing mixture widely used in
laboratories for cooling and condensation purposes.
Components:
● Dry ice: It is a solid carbon dioxide (CO ) with a sublimation point of –
₂
78.5 °C.
● Acetone: An organic solvent with a very low freezing point (–95 °C)
and high thermal conductivity.

10. Working Principle
When dry ice is added to acetone…..
1. The solid CO₂ sublimates (i.e. changes directly from solid to gas),
absorbing latent heat of sublimation from the surrounding acetone.
2. Acetone, having a low freezing point, remains in the liquid state and
efficiently distributes the cooling effect.
3. The temperature of the mixture stabilizes around –78 °C which is the
sublimation temperature of CO at 1 atm.
₂

11. Applications
1. Cooling chemical reactions that are highly exothermic.
2. Creating low-temperature baths in laboratories.
3. Preserving biological samples or refrigerating sensitive chemicals.
4. Used in vacuum traps and cryogenic experiments.

12. Liquid–Liquid
Mixtures

13. A liquid–liquid mixture is a system formed when two or more liquids
are mixed together. Depending on the nature of the liquids and their
molecular interactions, the mixture may be miscible (completely soluble
in each other in all proportions) or immiscible (separate into layers).
● Miscible liquids such as Ethanol and water, benzene and toluene.
● And Immiscible liquids such as Water and oil.

14. Ideal Liquid Mixture

15. An ideal liquid mixture (or ideal solution) is one that obeys Raoult’s
Law throughout its composition range.
According to Raoult’s Law:
PA = XAPA
0
and PB = XBPB
0
where,
PA,PB = partial vapour pressures of components A and B,
XA,XB = their mole fractions,
PA
0
,PB
0
= vapour pressures of pure components.
The total vapour pressure of the mixture is:
Ptotal = PA+ PB

16. Raoult’s Law Graph

17. Characteristics of Ideal Mixtures
1. No heat is absorbed or evolved on mixing
i.e. ΔHmix=0
2. No volume change on mixing
i.e. ΔVmix=0
3. The intermolecular forces between unlike molecules (A–B) are equal to
those between like molecules (A–A and B–B).

18. Examples:
● Benzene + Toluene
● Hexane + Heptane
● Ethanol + Methanol
These liquids are chemically similar and have comparable molecular forces.
Applications of Ideal Liquid Mixtures:
1. Used in distillation processes for separation of volatile liquids.
2. Important in chemical engineering and thermodynamic calculations.
3. Helps in determining molecular interactions and activity coefficients.

19. QUESTIONS
Raoult’s law is obeyed by which of the following liquid pairs? (IIT JAM
2018)
A) Benzene–Acetone
B) Chloroform–Acetone
C) Benzene–Toluene
D) Ethanol–Water

20. In an ideal liquid mixture, what
is the enthalpy of mixing (ΔHₘᵢₓ)?
(IIT JAM 2016)
A) Positive
B) Negative
C) Zero
D) Cannot be predicted
A mixture of acetone and
chloroform shows a negative
deviation from Raoult’s law.
Which statement explains this
correctly? (IIT JAM 2020)
A) A–B interactions < A–A and B–B
B) A–B interactions > A–A and B–B
C) No interactions between A and B
D) Vapour pressure is higher than
ideal

21. References
Atkins, P. & de Paula, J. – Physical Chemistry, 11th Edition, Oxford University
Press (2018). (Chapter 5; Section 5.3; Pages 142–149)
P.W. Atkins & Julio de Paula – Elements of Physical Chemistry, 7th Edition,
Oxford University Press. (Chapter 3; Pages 88–96)
O.P. Tandon – Physical Chemistry, G.R. Bathla Publications (Chapter 2; Pages
73-85)
NCERT Class XII Chemistry Part I – (Chapter 2: Solutions; Pages 24-33)