According to the law of conservation of mass in classical physics, how does the total mass of a closed system change over time during ordinary physical and chemical processes?

Introduction / Context:
The law of conservation of mass is a foundational principle in classical physics and chemistry. It states that in a closed system undergoing ordinary physical or chemical changes, the total mass remains constant. This question checks whether you can express that idea clearly and distinguish it from incorrect statements about mass decreasing, increasing or fluctuating without rule.

Given Data / Assumptions:

• We are considering a closed system, meaning no matter enters or leaves the system.
• Processes are ordinary physical and chemical changes, not nuclear reactions.
• Classical physics approximations are used, ignoring tiny relativistic mass changes.
• The options describe different possible behaviours of total mass over time.

Concept / Approach:
In classical chemistry, Antoine Lavoisier formulated the law of conservation of mass by carefully weighing reactants and products. He found that, in a closed vessel, the mass of substances before a reaction equals the mass after the reaction, even though substances change form. The same applies to physical changes such as melting or evaporation; mass is conserved as long as the system is closed. Modern physics refines this with Einstein relation between mass and energy, but for typical school level problems involving chemical reactions, we say that mass is neither created nor destroyed, only rearranged.

Step-by-Step Solution:
Step 1: Recall the classical statement: in a closed system, the total mass of reactants equals the total mass of products.Step 2: Understand that this implies that over time, as reactions proceed, the sum of the masses of all substances remains constant.Step 3: Recognise that new substances may form, but they are made from atoms that were already present; no new atoms are created or destroyed in ordinary reactions.Step 4: Note that mass does not gradually decrease or increase simply because reactions occur; it is conserved.Step 5: Dismiss the idea that mass fluctuates randomly without rule; this would contradict many experiments.Step 6: Conclude that the correct description is that total mass remains constant over time in a closed system undergoing ordinary processes.

Verification / Alternative check:
In laboratory experiments, chemists often perform reactions in sealed containers and use sensitive balances to verify that mass is conserved to a high degree of accuracy. For example, burning magnesium in a closed container shows that the combined mass of magnesium and oxygen before the reaction equals the mass of magnesium oxide produced. Similarly, dissolving salt in water does not change the total mass of the salt water mixture. These observations confirm that mass is conserved in closed systems under classical conditions.

Why Other Options Are Wrong:
The claim that mass gradually decreases as energy is used confuses mass with energy or fuel; while energy can change form, the total mass of the closed system remains constant in classical treatments. Saying that mass increases as new substances form overlooks that those substances are made from existing atoms whose total mass is unchanged. Random fluctuations in mass contradict the strong experimental evidence supporting conservation. These statements are not consistent with the law of conservation of mass.

Common Pitfalls:
Students sometimes mix the conservation of mass with the conservation of energy and think that consuming fuel literally destroys mass. Others may have heard about mass energy equivalence in relativity and incorrectly apply it to basic chemistry problems. To avoid these mistakes, remember that for ordinary physical and chemical changes in a closed system, classical conservation of mass is an excellent approximation: mass is neither created nor destroyed.

Final Answer:
In classical physics, the law of conservation of mass states that the total mass of a closed system remains constant over time during ordinary physical and chemical processes.