Investigation 9: Physical and Chemical Changes
A physical change rearranges how matter looks or is mixed without making a new substance, while a chemical change produces a substance with different properties; telling them apart takes more evidence than just a temperature change.
Mix two clear liquids and feel the container warm up, and it is tempting to call that proof of a reaction. It isn't. Dissolving releases or absorbs real heat because breaking apart a solid's lattice costs energy while forming new solute-solvent attractions releases energy, and the net of those two numbers shows up as a temperature change. Pour sulfuric acid into water and it gets hot fast, but no new substance has formed, since evaporating the water back off would leave the same acid behind. So a good test for chemical change looks past temperature and asks whether the original substances are still recoverable, and whether a new substance with different properties has actually appeared: a new color that will not wash out, a gas that was not there before, a precipitate forming, or a melting point that shifts for the product and stays shifted.
A fizzing pain-relief tablet is a clean case for this kind of reasoning. Drop it in water and you see bubbles, a color change in an indicator, and a temperature drop. The bubbling signals carbon dioxide gas, evidence of a new substance, which points to the solid acid and the bicarbonate reacting chemically. But the active pain reliever itself, often just a coating ingredient, may simply dissolve unchanged, a physical change happening alongside the chemical one in the same cup. Separating the two means testing each ingredient on its own and checking which property changes are reversible by evaporation or recrystallization and which are not.
This investigation sits right next to the unit's work on experimental design: a solid test needs a control, something that isolates one variable, like comparing the tablet in water against the solid acid alone in water, so a competing explanation, like simple dissolving, gets ruled out rather than assumed away.
Key terms
- Physical change
- A change in appearance or state that does not produce a new substance and is often reversible.
- Chemical change
- A change that produces one or more new substances with different properties than the starting materials.
- Control
- A comparison setup that isolates one variable so a competing explanation can be ruled out.
- Hydration energy
- The energy released when solvent molecules surround and attract a dissolved ion or molecule.
- Lattice energy
- The energy required to separate the ions or particles of a solid from each other.
Watch it explained
- How it worksHow do you design a test that separates a physical change from a chemical one?
- How it worksWalk me through identifying the active ingredients in a fizzing pain relief tablet.
- Common mistakeWhy isn't dissolving with a temperature change enough evidence of a chemical reaction?
- Going furtherWhat controls does an experiment need to rule out a competing explanation?
Check yourself
End of topic.