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Bubbles, clouds of solid, sudden color shifts, and temperature changes are common signals that a chemical reaction is happening. Each signal is a macroscopic observation, meaning you can detect it with your senses or simple measuring tools, without seeing individual particles. The key question for this course is how to connect those observations to a particle model and then to calculations that predict amounts.
Look for four kinds of evidence, each tied to a specific change in matter.
A gas forms when new molecules with weak attraction to the liquid escape as bubbles. A precipitate forms when dissolved ions rearrange into an insoluble ionic solid that appears as a cloudy solid. A color change often means electrons in a new substance absorb and reflect different wavelengths of light. A temperature change means energy is transferred as bonds break and form, so the surroundings warm up or cool down.
If you see bubbles leave a container, it can feel like matter vanished. Mass does not disappear in a chemical reaction, but the measured mass can change if particles leave the system you are weighing. The difference is between the reaction itself and the boundary of what you measure.
Predict what a balance would show for an open container compared with a closed container.
Conservation idea Mass is conserved for a closed system because atoms are not created or destroyed in ordinary chemical reactions.
The particle model starts with atoms, the smallest units that keep the identity of an element such as carbon or oxygen. In a reaction, atoms rearrange because old bonds break and new bonds form, creating new molecules or ionic compounds. The total count of each type of atom stays the same before and after, even though the grouping changes.
Compare the before and after particle pictures to verify that each element has the same number of atoms on both sides.