3 Part One. The Predicate

A Kinetic Theory of Liquids
In a liquid, molecules are close to each other, and consequently the forces exerted on one molecule by its neighbours are substantial. Thus, the problem of analysing the motion o f a single molccule is exceedingly difllcult, for cach is constantly in “collision”, subject to the forces for as many as twelve nearest neighbours. What then can We say about molecular motion in liquids? One o f the most revealing observations in this respect was made by the botanist Robert Brown in 1827. Brown discovered that very tiny particles (10^-4 cm diameter) suspended in a liquid undergo inccssant randomly directed motion. These motions occur without any apparent external cause such as stirring or convcction, and are evidently associated with an intrinsic property of all liquids.

A wealth o f experimental observation has confirmed the idea of this Brownian motion being a direct manifestation o f the thermal motion of molecules. When it is suspended in a liquid, a very small particle constantly experiences collisions with all the molecules surrounding it. If the particle is small enough, so few molecules will be able to collide with it, that at any particular instant the number striking it from one side may be different from the number striking it from the other sides; consequently, the particlc will be displaced. Subsequently, another unbalance o f collisional forces may occur, this time displacing the particlc in a different direction. The great majority o f these displacements are so small that they cannot be detected individually, but the motion which is observed is a result of many of the smaller random displacements. In essence, a Brownian particlc is a “molccule” thought to be large enough to be observable, but small enough to cxccutc observable random thermal motion.

Analysis of the motion of Brownian particles shows that their average kinetic energy is 3/2 kT. Since cach particlc is to be considered as one of the molecules of the liquid We can concludc that tlic average kinetic energy of a molecule in a liquid is also 3/2 kT — exactly the same as the kinetic energy of a gaseous molecule at tlic same temperature.
Mashq 3. Matnning asosiy tarkibini bir nechta jumlalar bilan bog'lang.

Mashq 4. Matndagi qaysi so'z "fosh qilinadi" degan ma'noni anglatadi?

Mashq 5. Matnda quyidagi savollarga javob toping:

1. Nima uchun yagona suyuq molekulaning harakatini kuzatish qiyin? 2. Suyuq molekulalarning harakatini qanday tavsiflash mumkin? 3. Suyuqlikda molekulalarning o'zaro ta'siri nimaga o'xshaydi? 4. Suyuq molekulalarning o'rtacha kinetik energiyasi qanday aniqlanadi?

Mashq 6. Quyidagi jumlalarni to'ldiring:

1. The molecules in a liquid are ... 2. To analyse the motion of a singlemolecule is difficult because ... 3. Brown discovered ... 4. Brownian motion is caused ... 5. The average kinetic energy o f a molecule in a liquid is ...

Section III
Ex. 1. Make up short dialogues according to the example.

Example: — What’s the matter? (What’s the trouble?)

— It is cold in this classroom.

— It can’t be helped. You’d better go to room 312.

Ex. 2. Respond to the following statements:

1. The molecular motion in a liquid is not easy to describe.

2. Molecules in a liquid are arranged in a rather orderly manner. 3. The properties o f the liquid solvent medium influence the speed o f reactions occurring in this medium.

Ex. 3. Give detailed answers to the questions.

1. In what way are liquids and gases alike? 2. In what way do liquids resemble solids? 3. Why are the physical properties of solutions important?

4. What forces act between the molecules in a liquid?

Ex. 4. Discuss the following topics:

1. The Liquid State as an Intermediate State Between Solids and Gases.

2. Molecular Motion of Liquids.

3. Properties of Liquids.