Density tells us how much matter is packed into a given space, so it is defined as mass per unit volume. The expression that matches this idea is mass divided by volume. When you have more mass in the same volume, the density increases; when you increase the volume without increasing mass, the density decreases. If you take volume divided by mass, you’re looking at the inverse relationship (specific volume), which is not density. Multiplying mass by volume doesn’t align with the density concept at all. And saying density equals weight mixes in gravity—weight depends on gravity, whereas density is the mass per volume and uses mass, not weight, in its definition.

Rocks formed directly from cooling molten material are igneous rocks. When magma cools underground, crystals form and grow slowly, producing coarse-grained rocks like granite. If lava erupts to the surface and cools rapidly, it solidifies into glassy textures such as obsidian, with little to no crystal formation. This cooling-from-molten-rock idea distinguishes igneous rocks from sedimentary rocks, which come from fragments that are deposited and lithified, and from metamorphic rocks, which start as other rocks changed by heat and pressure. A mineral, meanwhile, is a single chemical substance and not a rock.

Form fits function means an organism’s body parts are shaped to do their jobs well. A bird’s wing is a perfect example: the skeleton is light because the bones are hollow, which lowers overall weight and makes it easier to lift off. Feathers provide the necessary surface area for lift and help with maneuvering and insulation. The wing’s shape and feather arrangement work together so flight becomes efficient, clear evidence that structure is specialized to enable a specific function. Other options still show structure aligned with purpose, but not as directly or uniquely as flight. An elephant’s trunk is a versatile tool used for grasping, breathing, and sensing, which is impressive but not a single, tightly focused function. A cactus spine serves for protection and water conservation, and leaves support photosynthesis, but these illustrate broad adaptations rather than the highly specialized form-to-function pairing seen in flight.

The main idea is that a change which creates one or more new substances is a chemical change. In a chemical change, atoms break apart and recombine to form substances with new properties. That’s why you often see new odors, colors, bubbles, or a precipitate appear, and why the material may release or absorb energy as bonds are formed or broken. Physical changes, on the other hand, don’t produce new substances. They involve changes in size, shape, or state (like ice melting to water), but the chemical identity remains the same. A phase change is a specific type of physical change where a substance changes from solid to liquid or liquid to gas, again without changing its chemical composition. Reversible change describes whether the process can be undone, but it doesn’t by itself indicate whether a new substance is formed; some changes can be reversed and still involve no new substances, while some chemical changes can be reversible too. So, only a chemical change results in substances with new chemical identities.