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84 lessons · 4th Grade
Matter exists in three common states: solid, liquid, and gas. Solids have a fixed shape and volume, liquids have a fixed volume but take the shape of their container, and gases fill any container.
Matter is made of tiny particles that are always in motion. In solids, particles vibrate in place; in liquids, they slide past each other; in gases, they move freely.
When matter changes state, its particles move differently. Heating a solid makes particles vibrate faster until they break free and become a liquid, then a gas.
Physical changes alter the appearance of matter without changing its chemical identity. Cutting paper, melting ice, and dissolving sugar are all physical changes.
Evaporation happens when liquid particles at the surface gain enough energy to escape into the air as gas. Condensation is the reverse—gas particles slow down and form liquid.
Solutions are special mixtures where one substance dissolves completely in another. Salt water is a solution where salt (solute) dissolves in water (solvent).
In a chemical reaction, substances called reactants are transformed into new substances called products. The total mass stays the same because atoms are rearranged, not created or destroyed.
Density is how much mass is packed into a given volume. Objects denser than water sink, while less dense objects float. Oil floats on water because it is less dense.
Atoms are the basic building blocks of matter. Each element is made of one type of atom. Hydrogen has the simplest atom with just one proton and one electron.
The periodic table organizes elements by their properties. Elements in the same column (group) have similar characteristics, such as the noble gases which rarely react.
Acids have a pH below 7 and taste sour, while bases have a pH above 7 and feel slippery. A pH of 7 is neutral, like pure water.
Chemical changes produce new substances with different properties. Signs of a chemical change include color change, gas production, temperature change, and formation of a precipitate.
Conservation of mass means that matter cannot be created or destroyed in a chemical reaction. If you start with 10 grams of reactants, you end with 10 grams of products.
Molecules form when two or more atoms bond together. A water molecule has two hydrogen atoms bonded to one oxygen atom, written as H₂O.
Endothermic reactions absorb heat from surroundings, making things feel cold. Exothermic reactions release heat, like combustion. Both types of reactions are essential in nature.
A compound's properties differ from its elements. Sodium is a reactive metal and chlorine is a toxic gas, but together they form sodium chloride—common table salt.
Temperature measures how fast particles are moving. At absolute zero, particles have the least possible energy. Heating matter increases particle motion and can cause state changes.
Thermal energy transfers from warmer objects to cooler ones through conduction (direct contact), convection (fluid movement), and radiation (electromagnetic waves).
Metals are generally shiny, conduct heat and electricity, and can be bent without breaking. Nonmetals are often dull, poor conductors, and brittle in solid form.
Crystallization occurs when a solution becomes supersaturated and dissolved particles form an orderly solid structure. Snowflakes, salt crystals, and rock candy form this way.
Mixtures can be homogeneous (uniform, like salt water) or heterogeneous (non-uniform, like trail mix). Colloids like milk appear uniform but contain tiny suspended particles.
Elements are pure substances made of only one type of atom. Compounds are substances made of two or more elements chemically bonded in fixed ratios.
Chemical formulas show the types and numbers of atoms in a molecule. CO₂ has one carbon and two oxygen atoms. The subscript tells how many of each atom.
Solubility describes how much of a substance can dissolve in a solvent at a given temperature. Most solids dissolve better in hot water, while gases dissolve better in cold water.
Phase diagrams show how temperature and pressure affect the state of matter. Water's triple point is where solid, liquid, and gas phases all exist simultaneously.
A mixture contains two or more substances that are not chemically combined. You can often separate a mixture using physical methods like filtering, evaporating, or using a magnet.
The law of definite proportions states that a compound always contains the same elements in the same ratio by mass. Water is always 11.2% hydrogen and 88.8% oxygen.
Plasmas form when gases are heated to extremely high temperatures, stripping electrons from atoms. Stars, lightning, and neon signs all contain plasma.
Distillation separates mixtures based on different boiling points. Heating salt water causes water to evaporate and condense, leaving salt behind and producing fresh water.
Catalysts speed up chemical reactions without being consumed. Enzymes are biological catalysts that help reactions happen in living organisms at body temperature.
