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Physical properties are all about what you can observe with your senses about a substance. You don't need to change the substance into something else to see these features. Some common physical properties include color, texture, boiling point, melting point, and density.

• Color: What color does the substance appear to be?
• Texture: Is it smooth, bumpy, soft, or hard?
• Boiling Point: The temperature at which a substance changes from a liquid to a gas.
• Melting Point: The temperature a substance goes from solid to liquid.
• Density: How much "stuff" is in a given space, like mass per unit volume.

These properties can be observed without altering the chemical makeup of the substance itself. For instance, chlorine gas liquefying and bromine boiling are physical changes because their core substance does not change.

Chemical properties describe a substance's potential to change into different substances. These properties are only observed during a chemical reaction that changes the substance's identity. Examples of chemical properties include flammability, reactivity with water, and acidity.

• Flammability: Can the substance catch fire?
• Reactivity with Oxygen: Does it rust or oxidize?
• Reactivity with Water: Does it react when exposed to water?

For example, when hydrogen burns in chlorine gas, it forms a new substance, showcasing its chemical properties. Similarly, iron rusting is a chemical property because it changes into a new substance, iron oxide, upon reacting with oxygen.

A substance is any kind of matter that has consistent properties and definite composition. Each substance is characterized by its own composite of physical and chemical properties, which help in identifying and distinguishing it from other materials.
Substances can be elements, like iron or lithium, or compounds, like water or chlorine gas. Each possesses unique attributes that tell us how it behaves under certain conditions.
Substances are the building blocks of everything we encounter, and understanding their properties lends insight into how they can be used or transformed. Observing properties like whether a substance is ductile, malleable, or conductive gives insight into its potential uses in manufacturing or energy applications.

State changes refer to the physical transformation of substances from one state of matter to another, such as from solid to liquid or liquid to gas. These changes occur under specific conditions, typically involving temperature and pressure shifts.

• Melting: Solid to liquid
• Freezing: Liquid to solid
• Vaporization: Liquid to gas (including boiling and evaporation)
• Condensation: Gas to liquid
• Sublimation: Solid to gas without first becoming a liquid
• Deposition: Gas to solid without first becoming a liquid

For example, chlorine gas turning into liquid at \(-35^{\circ} C\) is a case of condensation, which is a state change. These changes are crucial for understanding how substances interact in different conditions and environments, driving many natural and industrial processes.