L1. What Is Chemistry?

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Callout: How can you tell whether a change made something new, or just changed how the same substance looks or feels?

Central Idea:

Chemistry studies matter and the changes it undergoes. A physical change alters the form or state of a substance without making a new substance (for example, H2O(s) -> H2O(l) as ice melts), while a chemical change produces one or more new substances (for example, iron + oxygen -> rust). Observable signs can help you decide which kind of change occurred.

How to Observe:

Watch carefully and write what you see. Perform steps in order: prepare the sample, start the change, and observe slowly (for example, heat an ice cube or leave an iron nail in damp air). Note anything new that appears (bubbles, solid forming, color shifts, warmth). Check the sample before and after with simple tests (does mass or smell change? does the formula or taste change? — do not taste unknown chemicals). Repeat the test at least once to confirm repeatability and record times and conditions (room temperature, wet/dry).

Core Diagram:

Application and Tests: When you compare two changes, look for what stayed the same (invariants) and what changed. Ask: did atoms rearrange to give new formulas, or did the same molecules only move or change state? Use this quick decision question: if a new solid appears from two clear liquids, is a new substance likely present? If yes, test further. Practical tests to try if safe in your lab: • Observe gas (bubbles) and collect/look for new odor; • Filter the mixture to see if a solid (precipitate) separates; • Measure temperature change with a thermometer to detect heat release or absorption. These tests can falsify a wrong idea — for instance, if you think only a physical change happened but a new solid forms, your idea is false.

Reinforcement:

Remember the distinction: in a physical change the chemical formula stays the same (example: H2O(s) -> H2O(l)), while in a chemical change new formulas appear (example rust formation). Useful vocabulary: physical change, chemical change, precipitate, gas evolution, exothermic, endothermic, reactant, product. Check your reasoning with a counting check: write the chemical equation for rusting, 4Fe + 3O2 -> 2Fe2O3, and count atoms on both sides (Fe:4 left =4 right; O:6 left =6 right) to confirm atoms are conserved even though a new substance formed.

Verification Checklist: Before you conclude, check these items: units and measurements recorded (time in s or min, mass in g, temperature in °C), micro-level explanation (are atoms rearranged or just moved apart?), and consistency with observations (did you see gas, precipitate, color, or temperature change?). Repeat the experiment or try a simple variation to see if results are reproducible; if observations differ, re-evaluate your classification.

Takeaways

A physical change alters form or state but not the substance (e.g., ice melting).
A chemical change creates one or more new substances (e.g., iron rusting).
Signs of chemical change include gas, precipitate, color change, or temperature change.
Write and check chemical equations to confirm atoms and charge are conserved.
Use simple tests and repeat observations to decide whether a change is physical or chemical.