3.2 Physical and Chemical Properties

Physical and chemical properties are the two main ways scientists describe and identify matter. Physical properties tell you what a substance looks, feels, or measures like without changing what it's made of. Chemical properties tell you how a substance behaves when it reacts or transforms into something new. Together, they give you a complete picture of any material you encounter.

Physical Properties

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Observable Characteristics

Physical properties are characteristics you can observe or measure without changing the substance's chemical composition. When you measure the density of a block of iron, the iron is still iron afterward. That's the key idea: observation without transformation.

• Malleability is a material's ability to be hammered or pressed into thin sheets without breaking. Gold is one of the most malleable metals; a single gram can be beaten into a sheet covering about one square meter.
• Ductility describes how easily a material can be drawn into wires without snapping. Copper is highly ductile, which is why it's the standard material for electrical wiring.
• Conductivity is a substance's ability to transmit heat or electricity. Metals generally have high conductivity, while materials like rubber and glass are poor conductors (called insulators).

Thermal Properties

Thermal properties are especially useful for identifying unknown substances, since each pure substance has its own characteristic melting and boiling points.

• Melting point is the temperature at which a solid changes to a liquid. Water melts at 0°C, while iron melts at 1,538°C.
• Boiling point is the temperature at which a liquid changes to a gas at standard atmospheric pressure. Water boils at 100°C; ethanol boils at 78.37°C. If you have an unknown liquid and it boils at 78.37°C, that's strong evidence it could be ethanol.
• Specific heat capacity measures the amount of heat energy needed to raise one gram of a substance by one degree Celsius. Water has an unusually high specific heat capacity ($4.18 \, \text{J/g°C}$), which is why large bodies of water help moderate coastal climates. It takes a lot of energy to change water's temperature, so oceans heat up and cool down slowly compared to land.

Additional Physical Properties

• Density is mass per unit volume ($D = \frac{m}{V}$), often expressed in g/cm³. Lead has a density of about 11.3 g/cm³, while aluminum sits around 2.7 g/cm³. This is why a small piece of lead feels surprisingly heavy compared to the same-sized piece of aluminum.
• Solubility describes how well a substance dissolves in a given solvent. Sugar dissolves readily in water, but sand does not. Solubility often changes with temperature; most solids dissolve more easily in warmer water.
• Hardness measures resistance to scratching or indentation. The Mohs hardness scale ranks minerals from 1 (talc, very soft) to 10 (diamond, the hardest natural substance). A harder mineral can scratch a softer one, so you can use this scale to help identify unknown minerals.
• Color results from how a substance absorbs and reflects different wavelengths of light. Chlorophyll absorbs red and blue light but reflects green, which is why plants look green.

Chemical Properties

Reactivity and Interactions

Chemical properties describe how a substance behaves when it undergoes a chemical change. Unlike physical properties, you can only observe chemical properties when the substance actually transforms into one or more new substances. You can't tell that iron rusts just by looking at a shiny iron nail; you have to expose it to water and oxygen and watch the change happen.

• Reactivity measures how readily a substance undergoes chemical reactions. Sodium is so reactive that it reacts vigorously with water, producing hydrogen gas and heat. Noble gases like helium and neon, by contrast, have extremely low reactivity because their outer electron shells are already full.
• Oxidation involves a substance combining with oxygen. Iron oxidizing in the presence of water and oxygen produces rust (iron oxide), a completely different substance from the original iron.
• Acidity or basicity describes how a substance behaves in water-based solutions, measured on the pH scale (0–14). A pH below 7 is acidic, 7 is neutral, and above 7 is basic. Lemon juice is acidic (pH around 2), while baking soda dissolved in water is basic (pH around 8–9).

Combustion and Stability

• Flammability indicates how easily a substance catches fire and sustains burning in the presence of oxygen. Gasoline is highly flammable, which makes it useful as fuel but dangerous to store carelessly. Concrete, on the other hand, has very low flammability.
• Combustibility is related to flammability but refers more broadly to a material's ability to burn. Wood is combustible (it will burn if heated enough) but not as easily ignited as gasoline. The distinction: flammability is about how easily something ignites, while combustibility is about whether it can burn at all.
• Stability describes how resistant a substance is to decomposition or chemical change over time. Table salt is very stable under normal conditions. Nitroglycerin is notoriously unstable and can decompose explosively with even a small shock.

Chemical Behavior in Reactions

• Redox potential measures a substance's tendency to gain or lose electrons during chemical reactions. Substances with a high tendency to lose electrons are strong reducing agents; those that readily gain electrons are strong oxidizing agents.
• Catalytic behavior refers to how some substances speed up chemical reactions without being consumed in the process. Enzymes in your body are biological catalysts that make essential reactions happen fast enough to sustain life. Without them, digestion and other processes would be far too slow.
• Photochemical properties describe how a substance reacts when exposed to light. Photosynthesis is a photochemical process where plants convert light energy, water, and carbon dioxide into glucose and oxygen.
• Radioactivity involves the spontaneous emission of particles or energy from unstable atomic nuclei. Uranium-238, for example, slowly decays over billions of years, releasing radiation in the process. Radioactivity is a chemical property of the nucleus itself, not something caused by an outside reaction.