Physical Sciences: Matter, Forces, and Energy
Core ideas
Matter is anything that has mass and takes up space. It is made of particles too small to see, and those particles are always moving. The arrangement and motion of particles explain solids, liquids, and gases. A solid keeps its shape; a liquid takes the shape of its container; a gas spreads out to fill its container. Properties such as mass, volume, density, magnetism, conductivity, and solubility help us identify and separate materials. A mixture contains two or more materials that are not chemically combined and can often be separated by filtering, sorting, evaporating, or using magnets. In a closed system, mass is conserved during mixing, dissolving, melting, freezing, and other changes of state.
Forces are pushes or pulls. They can start, stop, speed up, slow down, or change the direction of motion. Gravity pulls objects toward Earth, friction opposes motion between surfaces, and balanced forces do not change an object's motion. Unbalanced forces do. Energy can be transferred by motion, light, sound, electricity, and heat, and it can be stored in objects and materials.
Traps to avoid
Do not assume a gas has no mass or that dissolved material disappears. Melting and dissolving do not destroy matter. Balanced forces do not always mean an object is at rest; it may move at constant speed in a straight line. Energy is transferred, not created from nothing. Speed is distance per unit time, not the same as position.
Worked example
A student mixes 20 g of salt with 80 g of water in a closed jar. The mixture has 100 g. If the water is allowed to evaporate in an open container, the salt is left behind, but the water vapor escapes, so the open system's total mass is not conserved. Rule of thumb: in a closed system, total mass before equals total mass after.