【People's Education Press】High School Physics - Elective Compulsory Course III: Entering the Microscopic World: The Foundation of Molecular Kinetic Theory

Imagine if we shrunk Earth down to the size of an apple. Then, a small sphere with just a 1 cm diameter—scaled down proportionally—would be roughly the size of a molecule. Welcome tothe microscopic world— where matter is no longer a continuous 'solid block,' but a vivid picture composed of billions of tiny particles.

1. Enormous Numbers and Tiny Scales: Avogadro's Constant

To bridge macroscopic mass with microscopic particle count, we defineAvogadro's constant $N_A = 6.02 \times 10^{23} \text{ mol}^{-1}$. This astronomical number means that one mole of any substance contains the same number of particles. It is precisely this 'enormous' quantity that allows incredibly tiny molecules (with diameters of about $10^{-10} \text{ m}$) to form the macroscopic world visible to the naked eye.

2. 'Gaps' and 'Wandering' Between Molecules

Matter is not perfectly packed. Just as mixing 50 mL of water with 50 mL of alcohol results in a total volume less than 100 mL, this strongly demonstratesthat there are gaps between liquid molecules. Anddiffusion— such as soy sauce seeping into egg white — further tells us: molecules never remain still. They perpetually move in random motion, crossing boundaries and interpenetrating each other.

QUESTION 1

If we compare the size of Earth to that of an apple, it’s equivalent to comparing a 1 cm diameter sphere to a molecule. Clearly, molecules are extremely small. We’ve studied the motion of objects before—how do the tiny molecules composing them move?

Molecules only start moving under macroscopic external forces

Molecules constantly undergo ceaseless, random thermal motion

Molecules move only in liquids and gases, remaining stationary in solids

The motion trajectory of molecules follows regular straight or circular paths

QUESTION 2

Pour 50 mL of water into cylinder A and 50 mL of alcohol into cylinder B (see Figure 1.1-6). Then transfer the water from cylinder A into cylinder B. Observe that the total volume of the mixture is less than 100 mL. What does this experimental phenomenon indicate?

Alcohol molecules are heavier than water molecules

A vigorous chemical reaction occurred during mixing

There are gaps between liquid molecules

Some liquid evaporated during pouring

QUESTION 3

Xiao Zhang observed the motion of suspended chalk dust in water and concluded that the irregular movement of solid small particles proves the randomness of water molecules. Xiao Li argues: 'The particles move along straight broken lines, indicating that water molecules move regularly over short periods.' What is your opinion?

Xiao Zhang is correct—the zigzag line is merely a connection between recorded positions at intervals, not the actual path

Xiao Li is correct—the straight segments represent directional momentum from molecular collisions

Both are wrong—the chalk dust is too large to follow Brownian motion rules

QUESTION 4

Given copper’s density is $8.9 \times 10^3 \text{ kg/m}^3$ and its molar mass is $6.4 \times 10^{-2} \text{ kg/mol}$. If copper molecules are assumed to be tightly packed spheres, estimate the order of magnitude of their diameter:

$10^{-8} \text{ m}$

$10^{-10} \text{ m}$

$10^{-12} \text{ m}$

QUESTION 5

Two 1 L containers hold equal masses of oxygen at $0^\circ\text{C}$ and $100^\circ\text{C}$, respectively. Regarding their molecular speed distribution:

Their speed distribution curves are identical

Both follow the 'more in the middle, fewer at the extremes' pattern, but at $100^\circ\text{C}$, a higher proportion of molecules have high speeds

At $100^\circ\text{C}$, all molecules move faster than at $0^\circ\text{C}$