【Shanghai Edition】Junior High School Science, Grade 8, Semester 1: Exploring the Microscopic World

Opening the Microscopic Eye: The Essence of Matter

Imagine splitting a drop of water in half, then into quarters… Does this process ever end? Science tells us,matter is composed of tiny particles. This is not mere imagination— a single drop of water contains approximately $1.7 \times 10^{21}$ water molecules, an scale far beyond everyday human perception.

Core Drivers: Motion and Spacing

Particles are always in constant motion: The scent of flowers reaching you from afar, or water evaporating at room temperature, are both results of continuous particle movement.
There is space between particles: The same mass of a substance occupies different volumes in solid, liquid, and gaseous states due to differences in spacing between particles—not because the particles themselves grow larger.
Hierarchical Logic: Molecules are made of atoms. For example, an $H_2$ molecule consists of two hydrogen atoms. However, there are exceptions—such as helium gas ($He$), graphite, and copper—which are directly composed of atoms.

By breaking down water molecules (through chemical reactions), we discover:the essence of chemical change is the rearrangement and recombination of atoms. Atoms cannot be further divided during chemical reactions and are the smallest indivisible units in such processes.

QUESTION 1

Which facts or phenomena in daily life can demonstrate the existence of molecules?

Flowers bloom inside the wall, yet their fragrance reaches outside

The total volume is less than 100 mL after mixing 50 mL of alcohol with 50 mL of water

Wet clothes dry under sunlight

All of the above phenomena can prove it

QUESTION 2

In the microscopic process of water electrolysis, which of the following statements is incorrect?

Water molecules split apart

Hydrogen and oxygen atoms recombine into new molecules

Atoms changed during the chemical reaction

The generated hydrogen molecules aggregated into hydrogen gas

QUESTION 3

What is the relationship between a carbon dioxide molecule and oxygen and carbon atoms? Why?

Constituent relationship. One carbon dioxide molecule consists of one carbon atom and two oxygen atoms

Mixed relationship. Carbon dioxide is a mixture of oxygen and carbon atoms

Transformation relationship. A carbon dioxide molecule can transform into any atom

QUESTION 4

Which of the following descriptions about the particles that make up matter is correct?

Only molecules can constitute matter

Metallic copper is directly composed of copper atoms

Particle motion stops at absolute zero

QUESTION 5

Explain the meaning of the following symbols and numbers: $H, H_2, 2H, 2H_2, Mg^{2+}$

H represents hydrogen element or one hydrogen atom; H₂represents hydrogen gas or one hydrogen molecule; 2H represents two hydrogen atoms; 2H₂represents two hydrogen molecules; Mg²⁺ represents one magnesium ion carrying two units of positive charge.

H represents one hydrogen molecule; H₂represents two hydrogen atoms; 2H represents two hydrogen molecules.