# Section 1: Properties of Matter Review

Physical properties let scientists observe and measure matter without changing it into a different substance. These properties fall into two categories. Intensive properties stay the same no matter how much of the substance you have. Density, melting point, boiling point, and solubility are all intensive. Because they are constant for a given substance, scientists use them like a fingerprint to identify what the substance is. Extensive properties change when the amount of substance changes. Mass, volume, and weight are extensive. They tell you how much matter is present, but not what the matter is.

Density is one of the most useful intensive properties. It is calculated with the formula d = m / V (density equals mass divided by volume). Density determines whether an object floats or sinks: anything with a density less than water (1.0 g/mL) will float, and anything with a density greater than water will sink.

# Section 2: States of Matter and Phase Changes Review

All matter is made of particles in constant motion, an idea known as the kinetic molecular theory. Temperature measures the average kinetic energy of those particles. In a solid, particles are tightly packed in a fixed grid and only vibrate in place, giving solids a definite shape and definite volume. In a liquid, particles are close together but slide past each other, so liquids have a definite volume but take the shape of their container. In a gas, particles are far apart and move rapidly in all directions, filling any container completely.

Matter changes state through phase changes, which are caused by adding or removing thermal energy. Adding energy causes melting (solid to liquid), evaporation or boiling (liquid to gas), and sublimation (solid to gas directly). Removing energy causes freezing (liquid to solid), condensation (gas to liquid), and deposition (gas to solid directly). During any phase change, the temperature stays constant because the energy goes into breaking or forming bonds between particles rather than changing the temperature. This creates flat plateaus on a heating curve.

# Section 3: Energy Transfer Review

Thermal energy always flows from warmer objects to cooler objects, never the other way around. This flow continues until both objects reach the same temperature, a state called thermal equilibrium. Heat moves by three methods. Conduction transfers energy through direct contact between particles and works best in solids, especially metals (good conductors). Materials that resist heat flow, such as wood, plastic, and air, are called insulators. Convection transfers energy through the circulation of fluids (liquids and gases). Warm fluid rises because it becomes less dense, and cooler fluid sinks, creating a loop called a convection current. Radiation transfers energy through electromagnetic waves and is the only method that works through empty space, which is how the Sun heats Earth.