PSSA Grade 8 Science cheat sheet

Scientific inquiry follows a repeatable cycle: ask a question, form a testable hypothesis, design a controlled investigation, collect and analyze data, draw conclusions, and communicate results.

Independent variable = the factor the experimenter deliberately changes; dependent variable = the factor measured in response; constants = factors held the same in every trial; a control group receives no treatment and provides a baseline.

A valid hypothesis must be testable and falsifiable, meaning there must be a possible observation that would count against it.

A scientific theory is a well-supported explanation for a broad range of observations; a scientific law describes a consistent pattern in nature, often as a mathematical relationship.

Repeating trials and using larger sample sizes reduce the influence of random error and increase the reliability of results.

Percent error = |measured value − accepted value| ÷ accepted value × 100%; a smaller percent error means greater accuracy.

Precision refers to how closely repeated measurements agree with each other; accuracy refers to how close a measurement is to the true or accepted value.

Line graphs show change in a continuous variable, bar graphs compare categories, and data tables organize raw measurements before graphing.

Bias in an investigation is reduced by random sampling, blind or double-blind procedures, and review of results by other scientists.

Cell theory states that all living things are made of one or more cells, the cell is the basic unit of structure and function in life, and all cells come from existing cells.

Prokaryotic cells (bacteria) lack a nucleus and membrane-bound organelles; eukaryotic cells (plants, animals, fungi, protists) have a nucleus and organelles.

Photosynthesis equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂; it occurs in chloroplasts and stores energy in glucose.

Cellular respiration releases energy from glucose in mitochondria and is essentially the reverse of photosynthesis; both plants and animals perform it.

Diffusion moves particles from higher to lower concentration without added energy; osmosis is the diffusion of water across a selectively permeable membrane.

Mitosis produces two genetically identical body cells for growth and repair; meiosis produces four genetically varied sex cells, each with half the parent cell's chromosome number.

DNA stores genetic information; a gene is a segment of DNA that codes for a trait, and a mutation is any change in the DNA sequence.

Natural selection occurs when individuals with inherited traits that suit the environment survive and reproduce more, so those traits become more common over generations.

Energy flows from producers to consumers through food webs; only about 10% of the energy at each trophic level is passed to the next, with the rest lost mostly as heat.

A physical change alters size, shape, or state but not the identity of the substance; a chemical change produces one or more new substances with different properties.

Evidence of a chemical change includes unexpected color change, formation of a precipitate, production of a gas not caused by boiling, and release or absorption of heat or light.

Newton's second law: F = ma, so acceleration increases with net force and decreases with mass; Newton's first law says motion changes only when a net force acts.

Speed = distance ÷ time and has no direction; velocity is speed in a stated direction; acceleration = change in velocity ÷ time.

Balanced forces give a net force of zero and cause no change in motion; unbalanced forces cause acceleration.

Density = mass ÷ volume; an object floats in a fluid if its density is less than the fluid's density and sinks if it is greater.

Kinetic energy increases with both mass and speed (KE = ½mv²); gravitational potential energy increases with height and mass; energy is conserved but changes form.

Wave speed = frequency × wavelength; sound requires a medium to travel, while light can travel through a vacuum.

Amplitude relates to the energy carried by a wave; higher amplitude means more energy, and frequency determines pitch for sound and color for visible light.

Earth's layers are the crust, mantle, liquid outer core, and solid inner core; temperature and density generally increase with depth.

Plate tectonics: lithospheric plates move over the asthenosphere; divergent boundaries create new crust, convergent boundaries cause subduction or mountain building, and transform boundaries cause earthquakes.

Earthquake waves: P waves arrive first and travel through solids and liquids; S waves arrive later and cannot pass through liquids; the epicenter is the surface point above the focus.

The rock cycle converts material among igneous (cooled magma or lava), sedimentary (compacted and cemented sediments), and metamorphic (heat and pressure) rock.

Weathering breaks rock into smaller pieces, erosion transports the material, and deposition drops it in a new location.

The water cycle is driven by the Sun and includes evaporation, condensation, precipitation, runoff, and infiltration.

Seasons are caused by Earth's 23.5° axial tilt, which changes the angle and duration of sunlight, not by distance from the Sun.

Moon phases result from the changing relative positions of the Sun, Earth, and Moon; solar and lunar eclipses occur only when the three bodies align.

Earth's rotation on its axis (about 24 hours) causes day and night; revolution around the Sun (about 365.25 days) combined with axial tilt produces the seasons.