Notes on Physical Change, Chemical Change, and Properties

Prerequisite note: before proceeding, the reader should have reviewed separating substances by physical means.
Definition: a physical change occurs when the composition or identity of the substance does not change.
Key criterion: if a change happens and the substance’s identity/composition remains the same, it is a physical change.
Examples:
- Ice cube melting at room temperature: solid to liquid, but the substance is still
 $H2O$ (ice and water are both $H2O$ ), so composition remains the same.
- Dissolving sugar in water: the system contains sugar and water; the sugar can be recovered by evaporating the water, so dissolution is a physical change; the sweetness (a property) does not vanish.
- State changes like melting, boiling, or dissolving that do not alter chemical identity.
Important nuance: melting an ice cube or dissolving sugar in water changes the state or dispersion, not the chemical identity of the substances involved.
Distinction from chemical change: chemical change alters the identity and creates new substances with different properties.

Definition: a chemical change actually changes the identity and composition of the substance, producing one or more substances that are different from the starting material.
Key criterion: if you start with one substance and end with something completely new with different properties, that is a chemical change.
Examples:
- Combustion of hydrogen gas with oxygen gas to form water:
 $2\mathrm{H}2 + \mathrm{O}2 \rightarrow 2\mathrm{H}_2\mathrm{O}$
- The starting gases have properties very different from liquid water; water is drinkable, can be swam in, etc., unlike the reactants.
Emphasis: the property of the product (water) is not the same as the properties of hydrogen or oxygen.
Practical note: chemical changes often involve energy release or absorption (e.g., explosions when there is a large amount of hydrogen/oxygen), and the resulting material has different chemical and physical properties.

Definition: physical properties describe a substance without changing its identity; you observe or measure without inducing a chemical change.
Examples:
- Odor (smell) does not change the substance into something else.
- Taste (e.g., sweetness, sourness) does not alter chemical identity.
- Color (e.g., red shirt) does not change composition.
- Melting point: e.g., ice melts at $0^\circ\mathrm{C}$ ; observing the melting point does not change the substance.
- Boiling point: similarly observed without changing identity.
- Density: a physical property (to be defined later in the course).
Summary: physical properties are descriptions obtained while keeping the same substance intact.

Definition: properties that can only be described after undergoing a chemical reaction; observing these properties requires a chemical change.
Examples:
- Corrosion (rusting) of metals: iron becomes rust, a different substance with different properties.
- Flammability: the ability to ignite under certain conditions (e.g., gasoline is flammable; lighting gasoline with a match demonstrates this).
- Acidity and toxicity: chemical properties that describe how a substance behaves in chemical reactions.
Important nuance: describing a chemical property involves describing how the substance would react chemically, i.e., the change that would occur.

Physical change:
- Identity/composition remains the same; no new substance is formed.
- Examples include melting, freezing, condensation, dissolution, and state changes.
Chemical change:
- Identity/composition changes to form new substances with different properties.
- Examples include combustion, oxidation, rusting, digestion (in biology contexts), etc.
Physical property:
- A property observed or measured without changing the substance's identity.
Chemical property:
- A property observed only after the substance undergoes a chemical change.
Core phrase from the lecture: "The change is the actual change. The property is the description of that change." In other words, a chemical property describes what a substance would do in a chemical reaction; a chemical change describes the reaction itself and the new substance formed.

Prompt described in the lecture: pause to decide which image depicts a physical change.
Correct interpretation (as stated): Picture #1 shows a physical change, where molecules are pulled apart into the gas phase without changing identity.
Molecular-level note: In the physical-change scenario, the left image shows molecules that are simply dispersed; the right image shows incorporation of oxygen atoms into metal atoms, which represents a chemical change (formation of a new compound like iron oxide).
Takeaway: when evaluating scenarios, determine whether the identity of the substance changes (chemical change) or if only separation/dispersion occurs without changing identity (physical change).
Personal reflection noted in the transcript: the presenter mentions having previously missed this type of question, illustrating that these distinctions can be tricky.

Physical change:
- No change in chemical identity or composition; examples include melting, dissolution without final chemical alteration, and other state changes.
Chemical change:
- Creates a new substance with new properties; examples include combustion (e.g., $2\mathrm{H}2 + \mathrm{O}2 \rightarrow 2\mathrm{H}_2\mathrm{O}$ ) and rusting.
Physical properties:
- Descriptions that do not alter identity (odor, taste, color, melting point, boiling point, density).
Chemical properties:
- Descriptions requiring a chemical reaction (corrosion, flammability, acidity, toxicity).
Distinction recap:
- Change = actual process leading to a new substance; Property = description of that process or of the substance in its current state.