Physical vs. Chemical Properties

Hello. In the previous lesson, you classified mixtures by asking whether their composition was uniform or non-uniform. This lesson shifts the question: rather than asking what kind of sample is this?, we ask what characteristics does this material have?

By the end, you will be able to classify a stated characteristic as a physical property or a chemical property, and explain your choice using one central idea: Does observing or testing the characteristic require the substance to become a new substance?

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Properties describe matter

A property is a feature of a material that can be observed, measured, or tested. For example, copper is reddish-brown, water has a particular boiling point, and iron can rust in moist air. Each statement tells us something about the material.

The two major categories are:

• A physical property can be observed or measured without changing the material’s chemical identity.
• A chemical property describes a material’s ability to undergo a chemical reaction and form new substances.

The wording “chemical identity” means what the substance fundamentally is. Ice, liquid water, and water vapour all consist of water molecules, ${\mathrm{H}}_2\mathrm{O}$. They differ in state, but they are still water. In contrast, when wood burns, substances in the wood react with oxygen and form ash, gases, and water vapour. The original material has changed chemically.

This gives you a dependable classification question:

Could I determine this characteristic while the material remains the same substance?

• If yes, it is usually a physical property.
• If the characteristic concerns reacting, burning, corroding, or otherwise making new substances, it is a chemical property.

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Physical properties: observe or measure, but keep the same substance

A physical property tells you about a material without requiring it to change into a different substance. Some physical properties can be noticed with your senses; others require measurement or a simple physical test.

Common physical properties include:

Physical property	What it describes	Example
Colour or lustre	Appearance of a material	Gold is shiny; sulfur is yellow.
State	Whether it is solid, liquid, or gas at a stated temperature	Mercury is liquid at room temperature.
Hardness	Resistance to scratching or denting	Diamond is very hard.
Density	Mass per unit volume	Oil is less dense than water.
Electrical conductivity	Ability to carry electric current	Copper conducts electricity well.
Solubility	Ability to dissolve in a solvent	Salt dissolves in water.
Malleability	Ability to be hammered into sheets	Aluminum can be made into foil.
Ductility	Ability to be drawn into wires	Copper can be made into wire.
Melting point and boiling point	Temperatures at which state changes occur	Water boils at a particular temperature.

A state change is particularly important. When ice melts, its particles move more freely, but each particle is still a water molecule. Since no new substance forms, melting point is a physical property. The same applies to freezing point and boiling point.

Notice that a physical property does not have to be visible. Density cannot usually be seen directly, but it can be calculated from mass and volume without chemically changing the sample. Likewise, electrical conductivity may need equipment to measure, yet it is still physical because the material does not need to react to reveal it.

There is also a useful distinction between a property of a material and a feature of a particular object. A stainless-steel spoon may have a curved shape and a certain mass, but another spoon made from the same steel can have a different shape or mass. In contrast, density, conductivity, and corrosion resistance help describe the material itself.

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Chemical properties: the potential to react

A chemical property describes what a substance can do chemically. You discover it by observing a chemical reaction, in which new substances form.

For instance, iron has the chemical property of reacting with oxygen, especially when water is present, to form rust. Rust is not iron; it is a new substance called iron oxide. Therefore, the ability of iron to corrode is a chemical property.

Common chemical properties include:

Chemical property	What it means
Flammability or combustibility	Ability to burn in a reaction with oxygen
Reactivity with acids	Ability to react when placed in an acid
Corrosiveness	Ability to chemically damage materials, often metals
Tendency to oxidize	Ability to react with oxygen
Toxicity	Ability to cause harmful chemical effects in living things
Explosiveness	Ability to undergo a very rapid energy-releasing chemical reaction
Acidity or basicity	Chemical behaviour associated with acids or bases

The key is that these are statements of ability or potential:

• “Gasoline is flammable” describes a chemical property.
• “Iron reacts with acid” describes a chemical property.
• “Bleach is corrosive” describes a chemical property.

You should never test flammability, toxicity, corrosiveness, or chemical reactivity yourself. In a classroom question, you classify the stated characteristic from the information given; you do not need to perform the test.

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Property versus change: a crucial distinction

The words property and change are related, but they are not the same.

A property describes what a substance is like or what it is capable of doing:

• “Paper is flammable.”
• “Iron corrodes in moist air.”
• “Water has a boiling point.”
• “Copper is ductile.”

A change describes an event that happened to a particular sample:

• “The paper burned.”
• “The iron nail rusted.”
• “The water boiled.”
• “The copper wire was bent.”

The next lesson will focus on classifying changes as physical or chemical. For now, notice the relationship:

Stated idea	What it is	Classification
Water has a boiling point of $100^{\circ }\mathrm{C}$ at standard pressure.	A property	Physical property
Water boils and becomes water vapour.	A change	Physical change
Wood is flammable.	A property	Chemical property
Wood burns and produces ash and gases.	A change	Chemical change

A property may involve a process, but the category depends on the identity of the material. Boiling involves a change of state but no new substance, so boiling point is physical. Burning forms new substances, so flammability is chemical.

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A practical method for classification

When you encounter a characteristic in a question, use this short method.

1. Identify the stated characteristic.
   Is it describing appearance, measurement, state, shape, or ability to react?

2. Ask whether a new substance must form to reveal it.
   If not, classify it as physical. If yes, classify it as chemical.

3. Give a reason tied to chemical identity.
   Do not rely only on memorizing lists.

Here is that reasoning applied to several examples:

Characteristic	Classification	Reason
A liquid has a density of $0.92g/c{\mathrm{m}}^3$.	Physical	Density is measured from mass and volume without creating a new substance.
A metal is shiny.	Physical	Shine is observed while the material remains the same substance.
A substance can be hammered into thin sheets.	Physical	Changing shape does not change its chemical identity.
A gas has a boiling point of $-196^{\circ }\mathrm{C}$.	Physical	Boiling changes state, not chemical identity.
A metal reacts with hydrochloric acid.	Chemical	The reaction produces new substances.
A fuel is combustible.	Chemical	Combustion is a chemical reaction with oxygen.
Iron can form rust in moist air.	Chemical	Rust is a new substance.
A solution is acidic.	Chemical	Acidity describes chemical behaviour and reactivity.

A strong written explanation can use either of these sentence frames:

“This is a physical property because it can be observed or measured without changing the substance into a new substance.”

“This is a chemical property because it describes the substance’s ability to react and form new substances.”

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Avoid the most common traps

“Anything involving heat is chemical.”

Not necessarily. Heating can cause melting or boiling, which are physical changes. A material’s melting point and boiling point are physical properties. Heat can also start a reaction, such as burning, but the deciding question is always whether new substances form.

“Anything that changes is chemical.”

Also not true. Cutting paper, crushing a can, dissolving sugar, and melting ice all change the sample physically. Chemical properties concern the ability to undergo a chemical reaction, not merely any kind of change.

“A property must be directly visible.”

No. Density, melting point, conductivity, and solubility are physical properties that often require measurement or testing. The test can still be physical if it leaves the substance chemically unchanged.

“Flammability is physical because you can see a flame.”

Seeing a flame is not the reason for the classification. Burning converts a fuel and oxygen into new substances, so flammability is chemical.

“Colour change is always a chemical property.”

Colour is a physical property. A colour change during an event can be evidence that a chemical change is happening, but that is a different claim. Context matters:

• “Copper is reddish-brown” describes a physical property.
• “The metal changed from grey to reddish-brown as it reacted” describes an observation of a change, possibly chemical.

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Key takeaways

• A physical property can be observed or measured without changing a substance’s chemical identity.
• Common physical properties include colour, density, hardness, conductivity, solubility, malleability, and melting or boiling point.
• A chemical property describes a substance’s capacity to undergo a chemical reaction and form new substances.
• Common chemical properties include flammability, reactivity with acid, corrosion, toxicity, explosiveness, and acidity.
• The best classification question is: Does identifying this characteristic require a chemical reaction that produces new substances?
• Distinguish a property (“iron is corrosive”) from a change (“iron rusted”).

Next, you will use this foundation to classify observed events as physical changes or chemical changes.