Chemical reactivity isn’t a physical property: why boiling point, melting point, and solubility matter.

Understanding Physical vs. Chemical Properties: A Clear Look for SDSU Chemistry Learners

Let’s start with the basics and keep it practical. You’ve probably heard that some traits of a substance are “physical properties.” But what does that really mean, and how does it differ from chemical properties? Here’s a straightforward way to think about it, with a focus on ideas you’ll encounter in the SDSU Chemistry landscape.

What makes a property physical?

A physical property is something you can observe or measure without changing the substance’s identity. In other words, you don’t have to break or form new bonds to see it. Think of it as the substance showing off its behavior under different conditions while staying the same substance at heart.

• Boiling point: The temperature at which a liquid becomes a gas. If you heat water and it boils, you’re watching a physical change—no new substance is created; the water is still H2O, just in a different phase.

• Melting point: The temperature where a solid becomes a liquid. Ice melting to liquid water is another classic example of a physical property in action.

• Solubility: How well a substance dissolves in a solvent, like sugar dissolving in tea. Dissolution doesn’t rearrange the molecular identities of the solute and solvent into something new; it’s a physical process that describes how it disperses.

A handy, no-fuss way to remember it: physical properties describe the state and behavior of a material without changing what the material actually is.

What counts as a chemical property?

Now, a chemical property is about how a substance behaves in a way that changes its chemical identity. When a chemical property is observed, a chemical reaction is typically involved—bonds are made or broken, and new substances are formed.

Examples include:

• Chemical reactivity: How readily a substance participates in a chemical reaction with another substance. This could be rust forming on iron, or hydrogen gas reacting with oxygen to make water. The key is that something new is created—the old substance is transformed.

• Flammability: The ability to burn in the presence of oxygen. When something burns, it’s not just changing state; it’s becoming different chemicals, often releasing heat, light, and new gases.

• Acidity or basicity: How a substance donates or accepts protons in a reaction, which changes the chemical environment around it.

Think of chemical properties as the substances’ “choices” when faced with a reactant. Do they stay the same, or do they rearrange into something new? That decision—change at the molecular level—is what makes it a chemical property.

Let’s apply this to a specific question you’ll see in the SDSU setting

Question: Which is NOT a physical property of compounds and molecules?

A. Boiling point

B. Solubility

C. Chemical reactivity

D. Melting point

The correct answer is C: Chemical reactivity. Here’s why, in plain terms.

• Boiling point, melting point, and solubility are all observable without changing the substance’s identity. They describe how the substance behaves under certain conditions (temperature, solvent presence) while staying chemically the same.

• Boiling point: You heat the liquid, and it becomes gas. The molecules aren’t reinvented; they’re just in a different phase.

• Melting point: The solid becomes liquid, again without altering the chemical formula or bonding in a way that creates a brand-new substance.

• Solubility: A solid dissolving in a solvent changes how the mixture looks and behaves, but the solute and solvent aren’t chemically altered into new compounds in this simple process.

• Chemical reactivity, on the other hand, is about how substances engage in reactions that forge new substances. When bonds break and new ones form, the material you started with isn’t the same at the end. That’s the hallmark of a chemical property.

If you prefer a quick mental picture: physical properties are like the weather report for a material—temperature, state, how much of it you can dissolve—without changing what it is. Chemical properties are the forecast of what happens when you mix it with something else—will it react, rot, corrode, or combust?

A little analogy to keep things relatable

Imagine you’re testing a bunch of ingredients for a kitchen project. A spice’s melting point is like seeing at which temperature it becomes a liquid when heated—just a matter of physical state under heat. But whether that spice will react with another component to form a new sauce isn’t about its state anymore; it’s about chemistry—the kind that creates new flavors, textures, or compounds. In the same way, a material’s ability to react with a oxidizer or to burn in air is a chemical property, not a physical one.

Why this distinction matters beyond the sheet of facts

Grasping the difference between physical and chemical properties is more than memorization. It helps you think like a chemist. When you predict what will happen in a reaction or when you interpret experimental results, you’re relying on this boundary. In labs and classrooms, it guides decisions like what measurements to take, what tests to run, and how to interpret changes you observe.

A practical way to solidify the idea

• Start with everyday substances and ask two questions: If I heat it, does it stay the same substance at the end? If I mix it with something else, do I get new materials? If the answer is Yes to any reaction, you’re in the chemical-property zone.

• Use simple demonstrations (always under proper supervision and safety protocols): observe a salt dissolving in water (physical) versus iron rusting in oxygen and water (chemical). The first is about solubility; the second shows a chemical change with a new substance formed.

Common pitfalls you may run into

• Confusing a change of state with a chemical change. Ice turning into water is physical; ice left out with air and moisture can eventually become a very different compound, but that process would involve chemical steps (and often catalysts) along the way.

• Thinking solubility is a chemical reaction. Solubility is about the extent to which a substance dissolves, not about creating new chemical compounds through reaction (unless a reaction actually occurs in the solvent).

• Assuming all observable changes are chemical. Some changes (like dissolving or freezing) are observations of physical processes. They don’t guarantee chemical transformation.

A few quick questions to test your intuition

• If sugar dissolves in tea, is that a chemical change? No—it's typically a physical process, as the sugar molecules disperse but don’t chemically rearrange.

• When iron rusts, does that involve chemical reactivity? Yes—rust is the result of a chemical reaction between iron, oxygen, and water that creates new substances.

• If water boils away to steam, has the substance changed chemically? No—boiling is a physical change in phase, not a chemical transformation.

Context for SDSU students: connecting the dots

In the SDSU chemistry curriculum, you’ll frequently encounter these core ideas when you’re sorting data from experiments, interpreting graphs, or discussing reaction mechanisms. Understanding which properties signal a physical change versus a chemical change helps you form correct hypotheses and explain observations clearly. It also links to broader topics—thermodynamics, phase diagrams, solution chemistry, and reaction kinetics—where recognizing the underlying property type guides your reasoning.

A smoother path to mastery (without the stress)

• Build a mini glossary in your notes: physical properties (state, phase, solubility) versus chemical properties (reactivity, flammability, acidity).

• Practice by classifying substances you encounter daily. Is boiling point the critical property for a substance’s behavior in your kitchen? If not, what is?

• Tie it to lab etiquette and safety. If you’re unsure whether a change signals a chemical reaction, treat it with caution and seek supervision. When in doubt, you’re safer learning to verify rather than assume.

A friendly recap

• Physical properties describe how a substance behaves without changing its identity: boiling point, melting point, and solubility are classic examples.

• Chemical properties describe how a substance behaves when it reacts in ways that form new substances: chemical reactivity, flammability, and acidity are the go-to ideas.

• The difference isn’t just a vocabulary game. It’s a way to reason through experiments, readings, and problems with clarity and confidence.

If you’re ever unsure which category a property falls into, bring it back to the core question: does observing or measuring this property alter what the substance is? If the answer is yes, you’re dealing with a chemical property. If the identity stays the same, you’re looking at a physical property.

In the end, it’s a practical lens for thinking about matter. Whether you’re peering at a simple salt solution or unwinding a more complex reaction mechanism, keeping physical vs. chemical properties in mind helps you see what’s happening and why it matters. And that, more than anything, is what makes chemistry approachable—one observation, one concept, one idea at a time.