Historical Context & Motivation
For thousands of years, people noticed that materials could change. Wood turns to ash in a fire. Iron slowly becomes rust. Milk left out turns sour and chunky.
But here is the big question: how do you know a new substance has formed? Early scientists struggled with this. They needed a way to tell the difference between a simple mixture and a true chemical reaction (a process where starting substances change into entirely new substances).
These scientists gave us a powerful idea: if you want to know whether a chemical reaction happened, compare the properties of what you started with to the properties of what you ended up with. If the properties are truly different, new substances formed. That is the heart of today's lesson.
Core Principles & Definitions
Before you can compare reactants and products, you need some key vocabulary. Reactants are the starting substances that go into a reaction. Products are the new substances that come out. A property is any characteristic you can observe or measure about a substance, like its color, density, or melting point.
Chemical Reaction
Physical Change
Physical Properties
Chemical Properties
Conservation of Mass
The key idea is this: during a chemical reaction, the atoms in the reactants break apart and reconnect in new ways. Because the atoms are arranged differently, the products have different properties than the reactants. During a physical change, the atoms stay connected the same way, so the properties of the substance itself do not change.
Visual Explanation — Reactants vs. Products
The diagram below shows a classic chemical reaction: iron reacting with oxygen to form iron oxide (rust). Notice how the properties listed for the reactants are completely different from the properties of the product.
Look at the diagram above. Iron is a shiny, silvery-gray metal. Oxygen is a colorless gas. But rust is a reddish-brown, crumbly solid. The color, density, and physical state all changed. These measurable property differences are strong evidence that a chemical reaction happened. The product is a completely different substance from either reactant.
How It Works — Signs of a Chemical Reaction
Scientists look for specific signs of a chemical reaction when comparing reactants and products. These signs are clues — they suggest that a new substance may have formed. However, you always need to check the actual properties to be sure.
Color Change
Gas Production
Temperature Change
Precipitate Forms
New Smell
To confirm a chemical reaction, scientists measure properties like melting point, boiling point, density, and solubility (how well something dissolves). If these measurable properties are different in the products compared to the reactants, you have strong evidence that a reaction occurred.
Detailed Breakdown — Physical vs. Chemical Changes
One of the trickiest skills in science is telling the difference between a physical change and a chemical change. The table below will help you see the pattern.
| Change | What Happens to Properties? | New Substance? | Example |
|---|---|---|---|
| Ice melting | Shape changes; density changes slightly. Melting point, boiling point, and chemical formula stay the same. | No — still H₂O | Physical change |
| Dissolving salt in water | Salt disappears. Boiling point of water rises slightly. But you can get the salt back by evaporating. | No — still NaCl and H₂O | Physical change |
| Burning wood | Color, density, texture, and chemical composition all change. You get ash, CO₂ gas, and water vapor. | Yes — ash and CO₂ are new | Chemical change |
| Baking a cake | Batter is wet and gooey. Cake is fluffy and solid. You cannot un-bake a cake. Properties are totally different. | Yes — new substances | Chemical change |
| Cutting paper | Size and shape change. The paper is still paper. Same density, same chemical formula, same flammability. | No — still cellulose | Physical change |
The flowchart shows the thinking process scientists use. First, look for observable clues. Then, dig deeper by measuring properties. This connects to the Crosscutting Concept of Patterns — scientists look for consistent patterns in property changes across many reactions to build reliable explanations.
Worked Example — Investigating a Glow Stick
Let's return to the anchoring phenomenon from the beginning of the lesson. You crack a glow stick and it starts glowing green. Is this a chemical reaction? Let's investigate by comparing properties.
Strengths and Limitations of Evidence
Not all evidence is equally strong. Some clues can be misleading. The table below shows which types of evidence are most reliable when deciding if a chemical reaction occurred.
| Type of Evidence | Strength | Limitation |
|---|---|---|
| Color change | Easy to observe. Strong clue when dramatic. | Mixing paint changes color, but no new substance forms. Color alone is not proof. |
| Gas production | Bubbles often mean a new gas substance is forming. | Boiling water makes bubbles too, but that is just a phase change (physical). |
| Temperature change | Shows energy is released or absorbed — often tied to bond breaking and forming. | Dissolving some salts in water changes temperature without a chemical reaction. |
| Measured property differences (density, melting point, etc.) | Strongest evidence. If the melting point or density of the product is different, it is a different substance. | Requires instruments and careful measurement. Takes more time than just observing. |
| Reversibility test | If you cannot easily undo the change, it is likely chemical. | Some chemical reactions can be reversed under special conditions, so this is not perfect. |
Connection to Advanced Ideas
Right now, you are learning to identify chemical reactions by comparing properties. As you continue in science, you will learn why properties change at the atomic level. The table below shows how today's ideas connect to what comes next.
| What You Learn Now | What Comes Next |
|---|---|
| Products have different properties than reactants. | Atoms rearrange during reactions. The arrangement of atoms determines properties. |
| Mass is conserved in a reaction. | Balanced chemical equations show that atoms are neither created nor destroyed. |
| Temperature changes happen during reactions. | Energy is absorbed to break bonds and released when new bonds form (endothermic vs. exothermic reactions). |
| Some changes are physical, not chemical. | Intermolecular forces (forces between molecules) explain why physical changes occur without breaking chemical bonds. |
This connects to the Crosscutting Concept of Cause and Effect. The cause is the rearrangement of atoms. The effect is the new set of properties you observe in the products. Understanding this relationship is one of the most powerful ideas in all of chemistry.
Practice Problems
Lesson Summary
A chemical reaction occurs when reactants rearrange at the atomic level to form products with different measurable properties. Observable clues like color change, gas production, temperature change, precipitate formation, and new smell are clues, but the strongest evidence comes from comparing physical properties like density, melting point, and boiling point of the products to those of the reactants.
In a physical change, the substance keeps its identity — no new substance forms. The Law of Conservation of Mass tells us that total mass stays the same during any reaction because atoms are rearranged, not created or destroyed. Use multiple lines of evidence and measured data to build the strongest scientific argument about whether a chemical reaction occurred.