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Physical properties are attributes of a substance that we can observe or measure without altering the substance itself. They help us describe the material in terms of its physical aspects and these characteristics remain constant, regardless of the substance's current state. For instance:

• Color: A blue shirt retains its blue color despite the temperature or how lighted the room is.
• Density: Gold has a high density which gives it a heavy feel compared to other metals.
• Melting and Boiling Points: The melting point of ice is 0°C, and this does not change whether the ice is melted or frozen again. Similarly, water boils at 100°C at standard pressure.

These properties are crucial for identifying and categorizing substances based on their physical characteristics. They do not involve a change in the substance's molecule arrangement or composition, so the identity remains unchanged. This makes physical properties reliable indicators for distinguishing substances in our everyday observations.

The identity of a substance refers to its fundamental characteristics that define what it is on a molecular level. These are determined by the substance's chemical composition and structure. Each substance is composed of specific atoms and molecules, arranged in a unique way to form that substance:

• The arrangement of atoms affects properties like reactivity and stability.
• Chemical composition influences how the substance interacts with others.
• Structural components determine the physical form or phase of the substance.

Knowing the identity of a substance allows chemists to predict its properties and behavior under various conditions. Maintaining the identity of a substance, despite changes like melting or boiling, highlights the distinction between physical and chemical properties. Recognizing a substance's identity is foundational in studying chemistry as it dictates potential applications and transformations.

Chemical reactions are processes where substances transform into different substances, with distinct identities from the original reactants. During a chemical reaction, the atoms in the reactants rearrange to form new products. This can be a single-step or multi-step process, often influenced by conditions such as temperature, pressure, or the presence of catalysts. Here are a few examples of chemical reactions:

• Combustion: When wood burns, it combines with oxygen in the air to form ash, water vapor, and carbon dioxide, while releasing energy as heat.
• Oxidation: Rust forms when iron reacts with oxygen, creating iron oxide, a new substance that’s different from both iron and oxygen.
• Decomposition: Water can break down into hydrogen and oxygen gas through electrolysis, showing a complete change in substance identity.

Chemical properties determine how substances react with each other. These properties are key to understanding and predicting how materials will behave during chemical reactions, whether that reaction is a slow process like rusting, or instant like an explosion. Studying chemical reactions not only helps in identifying substances but also in inventing new solutions and materials in scientific research.