General Science
Chemistry & Matter
Chemistry studies matter and how substances interact and change. For General Science, focus on basic relationships among atoms, elements, bonds, states of matter, changes, mixtures, reactions, acids and bases, density, gases, and pressure rather than complicated equations.
Section 1
Matter, Atoms & Subatomic Particles
Matter has mass and takes up space. Atoms are the basic units of elements and contain protons, neutrons, and electrons.
Protons have positive charge and are in the nucleus. Their number determines the element: carbon has 6 protons and oxygen has 8. Neutrons have no charge and are also in the nucleus. Electrons have negative charge and are outside the nucleus. In a neutral atom, protons equal electrons. Most atomic mass is in the nucleus.
Example 1
Which particle determines the identity of an element? A. Electron | B. Proton | C. Neutron | D. Molecule
Solution
- 1Each element has a unique number of protons.
- 2Changing the proton number changes the element.
Why: Atomic number is determined by the number of protons.
Example 2
A neutral atom has 12 protons. How many electrons does it have? A. 6 | B. 10 | C. 12 | D. 24
Solution
- 1A neutral atom has equal positive and negative charges.
- 2Protons = electrons, so 12 protons means 12 electrons.
Why: Equal protons and electrons give no overall electrical charge.
Section 2
Elements & the Periodic Table
An element is a pure substance made of one type of atom. Each has a chemical symbol, such as H for hydrogen, O for oxygen, C for carbon, and Fe for iron.
Atomic number equals the number of protons. The periodic table organizes elements by atomic structure and properties; elements in the same group often share chemical properties. Metals usually conduct heat and electricity. Nonmetals are often poorer conductors. Noble gases, such as helium, neon, and argon, are generally unreactive.
Example 1
An element has atomic number 11. What does this tell you? A. It has 11 neutrons. | B. It has 11 protons. | C. It has 11 molecules. | D. It must be a noble gas.
Solution
- 1Atomic number is defined by the number of protons.
Why: Every element is identified by its proton count.
Example 2
Which property is most commonly associated with metals? A. Poor electrical conductivity | B. Good electrical conductivity | C. Always existing as gases | D. Being chemically identical
Solution
- 1Metals commonly conduct electricity well.
Why: Conductivity is a recognizable general property of metals.
Section 3
Ions, Molecules & Chemical Bonds
Atoms combine to form substances. A molecule has two or more bonded atoms. A compound has atoms of two or more different elements chemically bonded, such as water, H₂O.
An ion is an atom or group with electrical charge. Gaining electrons makes an atom more negative; losing electrons makes it more positive. Ionic bonds are attractions between positive and negative ions after electron transfer. Covalent bonds form when atoms share electrons.
Example 1
What happens when an atom gains an electron? A. It becomes more positive. | B. It becomes more negative. | C. It becomes a different element. | D. It loses all protons.
Solution
- 1Electrons carry negative charge.
- 2Adding an electron adds negative charge.
Why: Ions form when the balance between protons and electrons changes.
Example 2
Which statement describes a covalent bond? A. Two atoms share electrons. | B. An atom loses its nucleus. | C. Two atoms share protons. | D. All electrons disappear.
Solution
- 1Covalent bonding involves atoms sharing electrons.
Why: Sharing electrons allows atoms to form stable molecules.
Section 4
States of Matter & Phase Changes
Matter commonly exists as solids, liquids, and gases. A solid has definite shape and volume; a liquid has definite volume but changes shape; a gas has neither definite shape nor volume and fills its container.
Melting changes solid → liquid. Freezing changes liquid → solid. Vaporization changes liquid → gas. Condensation changes gas → liquid. Sublimation changes solid → gas without becoming liquid first.
Example 1
Water droplets form on the outside of a cold glass. Which phase change occurs? A. Melting | B. Condensation | C. Freezing | D. Sublimation
Solution
- 1Water vapor in air cools and changes from gas to liquid.
Why: Condensation is the change from gas to liquid.
Example 2
Which state has a definite volume but takes the shape of its container? A. Solid | B. Liquid | C. Gas | D. Plasma only
Solution
- 1A liquid keeps its volume but changes shape to match its container.
Why: Liquids flow while remaining approximately the same volume.
Section 5
Physical & Chemical Changes
A physical change alters shape, size, or state without making a new substance. A chemical change creates one or more new substances.
Melting ice, cutting paper, crushing a can, and boiling water are physical changes. Rusting iron, burning wood, and cooking an egg are chemical changes. Possible chemical-change signs include unexpected color change, gas, a new solid, heat or light, or a new odor.
Example 1
Which is a physical change? A. Iron rusting | B. Wood burning | C. Ice melting | D. Food digesting
Solution
- 1Melting changes water from solid to liquid.
- 2No new substance is made.
Why: A phase change alters form but not chemical identity.
Example 2
Why is rusting a chemical change? A. Iron only changes shape. | B. A new substance forms. | C. Iron becomes colder. | D. Iron changes from solid to liquid.
Solution
- 1Rust is chemically different from the original iron.
Why: New-substance formation is the key feature of a chemical change.
Section 6
Mixtures & Solutions
A mixture contains two or more substances physically combined, with each keeping its chemical identity. A solution is a mixture with one substance evenly distributed through another.
In salt water, salt is the solute, the substance dissolved, and water is the solvent, the substance doing the dissolving. A homogeneous mixture is uniform throughout; a heterogeneous mixture is not uniform, such as oil and water or trail mix.
Example 1
In salt water, which substance is the solvent? A. Salt | B. Water | C. Sodium only | D. Air
Solution
- 1Water dissolves the salt.
- 2The substance doing the dissolving is the solvent.
Why: The solute is dissolved in the solvent.
Example 2
Which is the best example of a heterogeneous mixture? A. Salt evenly dissolved in water | B. Air | C. Trail mix | D. Pure oxygen
Solution
- 1Trail mix has visibly different components and is not uniform throughout.
Why: A heterogeneous mixture does not have the same composition everywhere.
Section 7
Chemical Reactions & Conservation of Mass
A chemical reaction rearranges atoms to form new substances. Reactants are the starting substances; products are substances formed by the reaction.
In an ordinary chemical reaction, matter is not created or destroyed. In a closed system, total mass of reactants equals total mass of products. In an open container, gases can escape or enter, making measured mass seem to change.
Example 1
In a chemical reaction, what are reactants? A. Starting substances | B. Final substances | C. Only gases | D. Only metals
Solution
- 1Reactants are present before the reaction occurs.
Why: Products are formed from reactants during a reaction.
Example 2
A reaction in a sealed container starts with 36 grams of reactants. What is the mass of the products? A. 18 g | B. 30 g | C. 36 g | D. 72 g
Solution
- 1Use conservation of mass: mass before = mass after.
Why: Atoms rearrange during a reaction but are not created or destroyed.
Section 8
Acids, Bases, Density & Gas Behavior
Acids have pH below 7, bases have pH above 7, and a neutral substance is near pH 7. Density describes mass packed into volume: density = mass / volume.
An object tends to float when its average density is lower than the fluid's density. Gas particles collide with container walls and create pressure. When the same gas is squeezed into a smaller space at similar temperature, collisions happen more often and pressure generally increases. Heating a gas makes particles move faster.
Example 1
A substance has pH 3. It is most likely: A. Acidic | B. Basic | C. Neutral | D. Metallic
Solution
- 1A pH below 7 indicates an acid.
Why: The pH scale places acids below 7 and bases above 7.
Example 2
A gas is placed in a smaller sealed container without changing the amount of gas. What happens to pressure if temperature remains about the same? A. Decreases | B. Increases | C. Becomes zero | D. Immediately becomes solid
Solution
- 1In a smaller space, particles strike container walls more frequently.
Why: More frequent particle collisions create greater pressure.
Example 3
Two objects have the same volume. Object A has more mass than Object B. Which has greater density? A. Object A | B. Object B | C. Equal density | D. Density cannot involve mass
Solution
- 1Use density = mass / volume.
- 2With equal volume, the object with greater mass has more mass in the same space.
Why: Greater mass at equal volume means greater density.
Quick Review
Quick Review
- Proton: positive; neutron: neutral; electron: negative; atomic number: number of protons.
- Ionic bonds are attractions between charged ions; covalent bonds share electrons. Physical changes make no new substance; chemical changes do.
- Solute gets dissolved; solvent does the dissolving; reactants become products; mass is conserved in a closed system.
- Acid pH is below 7, base pH above 7, neutral near 7; density = mass / volume.
The Big Picture
- Protons → element identity. Electrons → electrical charge. Atoms bond → molecules and compounds. Phase change → same substance, different state. Chemical change → new substance. Solute + solvent → solution. Reactants → products. Smaller gas volume → generally greater pressure.