ACT Science: Apply Conservation Laws to Predict Outcomes in Chemical and Physical Systems

Conservation of mass: Matter is neither created nor destroyed; it is only rearranged. In a chemical reaction, the total mass of reactants equals the total mass of products. Example: When wood burns, it seems to disappear, but the ash plus gases (smoke) have the same total mass as the original wood. This is why we balance chemical equations: to ensure mass is conserved. Conservation of energy: Energy cannot be created or destroyed; it only changes form. Example: In a car engine, chemical energy (gasoline) is converted into kinetic energy (motion) and heat. The total energy is constant. These two laws explain why certain outcomes are possible (mass and energy are conserved) and why others are impossible (they would violate conservation).

Why it matters: The ACT uses conservation laws to test whether you can predict what happens in a system. If a reaction produces more mass than was input, it violates conservation and is impossible. If energy appears from nowhere, the system is impossible. Understanding conservation helps you evaluate whether proposed scenarios make sense.

Application 1: Balancing equations. If 2 hydrogen atoms and 1 oxygen atom combine, the product must contain 2 hydrogen and 1 oxygen (conservation of mass). Application 2: Predicting product mass. If 10 grams of reactant A and 15 grams of reactant B combine with no leftover, the product must weigh exactly 25 grams. Application 3: Energy transfer. If a system releases 100 joules of heat, that energy must go somewhere (into the surroundings). In every problem, mass and energy account for everything: inputs equal outputs.

Common ACT pattern: A table shows masses of reactants and asks for product mass. Apply conservation: product mass=reactant mass. If the answer choices do not match this calculation, check whether some reactant was leftover (it would not be part of the product).

Scenario 1: Reactants A (10 g) and B (5 g) combine to form product C. What is the mass of C if all reactants are consumed? Answer: 15 grams (conservation of mass). Scenario 2: An endothermic reaction absorbs 200 joules of heat from the surroundings. Where does this energy come from? Answer: From the surroundings (energy is conserved; it is transferred, not created). Scenario 3: A wood log (mass 5 kg) is burned. The ash weighs 0.5 kg. Where is the rest of the mass? Answer: In gases released (CO2, H2O vapor, etc.). Total mass is still 5 kg. Scenario 4: A reaction releases 300 joules of heat. If the system does no work, what happens to the energy? Answer: It goes to the surroundings as heat. Scenario 5: 2 grams of hydrogen and 16 grams of oxygen combine to form water. How much water is formed? Answer: 18 grams (conservation of mass). For each scenario, identify what is conserved (mass or energy) and apply the law to predict the outcome.

After solving each scenario, ask: Does this outcome violate conservation? If yes, the scenario is impossible. If no, it is consistent with the laws of nature.

Conservation laws appear in chemistry (balanced equations), physics (energy problems), and biology (nutrient cycles). Questions testing conservation appear 2-3 times per test and reward students who understand that outcomes must satisfy these laws. Once you internalize conservation, you can predict outcomes in unfamiliar systems because the laws are universal and immutable.

Spend 20 minutes this week solving 10 problems that apply conservation of mass (balancing equations) and conservation of energy (tracking energy flow). Check that your answers respect both laws. By test day, conservation will be automatic, and you will answer these questions with confidence because you understand the deeper principle, not just the surface rules.