PEP Edition | Junior Chemistry | Grade 9, Vol. 1: The "Balance" of Chemical Change — Experimental Investigation of the Law of Conservation of Mass

From Qualitative to Quantitative: The Foundation of Chemistry

In the world of chemistry, we care not only about "what" substances become, but also "how much" they change.Law of Conservation of Massmarks the beginning of quantitative chemistry. Using precise balances, we discover a remarkable connection: the total mass of reactants before a chemical reaction equals the total mass of products after it.They are always equal.

₁ (before reaction)m₂ (after reaction)=Chemical Reaction Process

Core Experimental Plans

Plan 1: Combustion of Red Phosphorus — Place a small pile of dry red phosphorus in a conical flask with a layer of fine sand at the bottom. Seal it with a rubber stopper fitted with a balloon, then ignite the phosphorus.Observation:A large amount of white smoke is produced; the balloon inflates then deflates. After cooling, weighing shows $m_1 = m_2$. The closed system prevents interference from oxygen in the air or loss of products.
Plan 2: Reaction of Iron with Copper(II) Sulfate — Immerse a cleaned iron nail into a conical flask containing copper(II) sulfate solution.Chemical Equation:$Fe + CuSO_4 = Cu + FeSO_4$.Observation:The iron nail becomes coated with a reddish solid (copper), and the solution changes from blue to light green. The balance remains level throughout, once again confirming the law of conservation of mass.

QUESTION 1

In the red phosphorus combustion experiment, what is the main purpose of the small balloon tied securely to the glass tube?

To collect the white smoke of phosphorus pentoxide

To regulate the internal pressure and prevent the stopper from popping out due to expansion from heating

To act as a reactant in the chemical reaction

QUESTION 2

In the experiment where an iron nail reacts with copper(II) sulfate solution, the observable chemical phenomenon is:

A large amount of bubbles forms, and the solution turns red

A red solid appears on the iron nail, and the solution changes from blue to light green

The balance pointer swings sharply to the right

QUESTION 3

The "total mass" defined by the Law of Conservation of Mass refers to:

The total mass of all substances in the container

The total mass of all reactants equals the total mass of all products formed

The total mass of liquids in the container before and after the reaction

QUESTION 4

If the red phosphorus combustion experiment is performed in an open container, what happens to the balance reading?

The reading stays the same

The reading increases because phosphorus pentoxide is produced

The reading may increase (due to oxygen intake) or decrease (due to white smoke escaping), making it impossible to verify conservation

QUESTION 5

Which of the following is guaranteed to remain unchanged before and after a chemical reaction?

The types of substances

The types of molecules

The types, number, and mass of atoms