Lessons 1–2 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
1
Molecular-kinetic theory of the structure of matter
Textbook: pp. 4–7
GoalKnow the three basic statements of molecular-kinetic theory (MKT) and explain how diffusion and Brownian motion support them.
New words
molecular-kinetic theory (MKT) · molekulyar-kinetik nazariya (MKN)diffusion · diffuziyaBrownian motion · Broun harakatichaotic (random) motion · xaotik harakat
Explanation
Molecular-kinetic theory explains the structure and properties of matter by the motion of its particles and the interaction between them. It rests on three experimentally confirmed statements: matter consists of atoms and molecules; the particles move continuously and randomly; attractive and repulsive forces act between the particles. The ancient Greek thinker Democritus called the smallest indivisible particle of matter an “atom”, and the theory developed steadily from the 18th century. The best evidence for particle motion is diffusion, the spontaneous mixing of different substances: a smell spreads through a room, and tea slowly colours the water evenly. In 1827 the Scottish botanist R. Brown saw under a microscope the endless random jerking of pollen grains in water; this is called Brownian motion. Its cause is the random impacts of liquid molecules on the grain from all sides (explained by A. Einstein in 1905), so it also proves that matter consists of molecules.
Worked examples
The smell of freshly baked bread reaches another room, but it takes several minutes. Reason: the odour molecules move randomly and collide countless times with air molecules, changing direction again and again, so they do not fly in a straight line.
A smoke particle or pollen grain jerks about under a microscope because liquid (gas) molecules hit it randomly from all sides. If the particle is tiny, the impacts from one side can outnumber those from the other and it moves; a large body is hit equally from all sides, so it stays still.
Class activity
Model: 5–6 students act as “molecules”, walking slowly and randomly with eyes open in a marked area of the classroom; one large soft ball is the “Brownian particle”. Watch how the ball moves as the molecules bump it. Then replace it with a much smaller object and discuss the difference. Walk slowly, do not run.
Practice
1
State the three basic statements of MKT.
1) Matter consists of atoms and molecules; 2) the particles move continuously and randomly; 3) attractive and repulsive forces act between them.
2
How does MKT explain sugar dissolving and mixing faster in hot tea?
In hot water molecules move faster, so diffusion is faster and the sugar molecules spread through the water quickly.
3
Name three features of Brownian motion.
The particle moves continuously and randomly; its path is a complicated broken line; the motion depends on particle size (smaller particles move more).
4
Why do two very smooth, clean glass plates pressed tightly together become hard to separate, while ordinary rough pieces of glass do not stick?
Smooth surfaces come close enough over a large area for the distance between molecules to be tiny, so molecular attraction becomes noticeable. Rough surfaces touch only at a few points; elsewhere the molecules stay far apart, and the attraction falls off very quickly with distance.