Chemical and Physical Changes, Gas Tests, Energy, and Investigations

A comprehensive vocabulary review of physical versus chemical changes, gas identification tests, reaction energetics, and experimental data quality concepts.

Physical change

A change in which no new substance is formed, particles remain the same substance (though their arrangement, state, or shape may change), and it is often reversible.

Chemical change

A change in which new substance(s) are formed as atoms/particles are rearranged into new combinations, and it is often difficult to reverse.

Precipitate

A new solid that forms in a solution during a chemical reaction.

Reactants

The starting substances in a chemical reaction.

Products

The new substances formed during a chemical reaction.

Law of conservation of mass

A principle stating that in a closed system, the total mass of the reactants equals the total mass of the products, because atoms are rearranged rather than created or destroyed.

Oxygen ($O_2$) test

A qualitative gas test where a glowing splint is inserted into the gas; if oxygen is present, it relights or burns brighter, indicating it supports combustion.

Hydrogen ($H_2$) test

A qualitative gas test where a lit splint is held near the gas; it produces a squeaky pop, indicating the gas is flammable.

Ammonia ($NH_3$) test

A qualitative gas test using damp red litmus paper; the paper turns blue, confirming that it is an alkaline/basic gas.

Carbon dioxide ($CO_2$) test

A qualitative gas test where the gas is bubbled through limewater; it turns milky or cloudy white as $CO_2$ reacts to form a white precipitate of calcium carbonate.

Exothermic reaction

A reaction that transfers energy to the surroundings (usually heat), making the surroundings warmer, where the products have less energy than the reactants.

Endothermic reaction

A reaction that takes in energy from the surroundings (usually heat), making the surroundings cooler, where the products have more energy than the reactants.

Bond energy balance

The concept that breaking bonds requires energy and forming bonds releases energy, where the overall energy transfer depends on the balance between energy required to break bonds and energy released when new bonds form.

Random error

Unpredictable variation between repeated measurements (such as slight differences in timing or environmental conditions) that reduces repeatability (precision).

Systematic error

A consistent bias in one direction (caused by uncalibrated instruments, faulty equipment, or method bias) that reduces accuracy.

Human / procedural mistakes

Errors in method or reading (such as misreading a scale, inconsistent timing, or spills) that affect both accuracy and reliability.

Control variables

All factors in an experiment kept the same (e.g., temperature, volume, mass, concentration, type of equipment) to ensure a fair test.

Measurement resolution

The suitability of the measuring equipment's finest scale division (e.g., $± 0.1\,g$ or $± 0.1\,^\circ\text{C}$) for the measurements being taken.

Anomalies

Unusual experimental results that should be investigated and must not be removed without a valid justification.