4.3 Physical Science

Physical Science

Physical Science accounts for roughly one-third of the 55-question Praxis 5005 Science subtest. ETS draws these items from chemistry and physics: the properties and changes of matter, energy and its conservation, forces and motion, and simple machines. Questions favor conceptual understanding and the ability to spot student misconceptions over heavy calculation.

States of Matter

Matter is anything with mass and volume. The three common states differ by how tightly particles are held:

Changes of state add or remove heat: melting (solid to liquid), freezing (liquid to solid), evaporation/boiling (liquid to gas), condensation (gas to liquid), and sublimation (solid directly to gas, as with dry ice). These are physical changes, the same substance in a new form. Adding heat increases particle motion and energy; that is why temperature rises and solids loosen into liquids and gases. A subtle point: during a change of state, the temperature holds steady even as heat is added, because the energy goes into breaking bonds rather than raising temperature.

Plasma is sometimes listed as a fourth state (ionized gas, as in stars and neon signs), but elementary items focus on the familiar three.

Physical vs. Chemical Changes

The single most-tested distinction: a chemical change produces a new substance with new properties; a physical change does not.

Signs of a chemical change include color change, gas production (bubbles/odor), light or heat release, and a precipitate forming. Dissolving sugar in water is physical (you can evaporate the water and recover the sugar); burning sugar is chemical.

Properties of Matter and Density

Physical properties are observable without changing identity: color, mass, volume, texture, melting point, and density. Chemical properties describe how a substance reacts, such as flammability or reactivity with acid. Density is the workhorse formula:

Density = Mass / Volume (commonly g/cm3 or g/mL)

Worked example: a 30 g block occupying 10 cm3 has a density of 3 g/cm3. Since that exceeds water's 1 g/cm3, the block sinks. Objects less dense than a fluid float; this is why ice (about 0.92 g/cm3) floats on liquid water.

All matter is built from atoms, the smallest unit of an element, consisting of protons and neutrons in a nucleus surrounded by electrons. A molecule is two or more atoms bonded together (water is H2O). A mixture can be separated physically (sand and salt), whereas a compound requires a chemical reaction to break apart. These vocabulary distinctions show up as straightforward identification items.

Energy

Energy is the ability to do work. Core forms: kinetic (motion), potential (stored by position or chemistry), thermal (particle motion/heat), light, sound, and electrical. The Law of Conservation of Energy states energy can change form but cannot be created or destroyed. Example: a roller coaster trades gravitational potential energy at the top for kinetic energy at the bottom. Heat transfers three ways, a tested set: conduction (direct contact, a metal spoon heating in soup), convection (moving fluids, boiling water circulating), and radiation (electromagnetic waves, the Sun warming your face).

Sound needs a medium to travel and moves fastest through solids; light travels through a vacuum and far faster than sound, which is why you see lightning before you hear thunder.

Forces and Motion

A force is a push or pull. Common forces are gravity (attraction between masses), friction (opposes motion between surfaces), and magnetism. Newton's three laws are heavily tested:

1. Law of Inertia - an object at rest stays at rest and an object in motion stays in motion unless a net force acts on it.
2. F = ma - acceleration depends on force and mass; the same force accelerates a lighter object more.
3. Action-Reaction - for every action there is an equal and opposite reaction (a rocket pushes gas down; gas pushes the rocket up).

Simple Machines

Simple machines provide mechanical advantage by trading reduced force for increased distance; they do not reduce total work.

Two or more simple machines combine into a compound machine, such as a wheelbarrow (lever plus wheel and axle) or scissors (two levers plus two wedges). A common student misconception is that a machine reduces the work you do; clarify that work equals force times distance, so a ramp lowers the force but lengthens the path, leaving total work roughly unchanged (and slightly higher in reality due to friction).

Finally, expect at least one item on the scientific method and engineering design woven through these topics: forming a testable question and hypothesis, controlling variables, collecting and graphing data, and revising a design after testing. Knowing that a fair test changes only one variable at a time is frequently the key to the right answer.