Chemistry: Substances, Reactions, Atoms, Acids, and Equations
Chemistry on this exam tests how you classify matter, explain reactions at the particle level, read and balance equations, and apply acid-base ideas. Many marks come from precise vocabulary and from checking that atoms are conserved.
Substances and mixtures
A pure substance has a fixed composition: an element contains one kind of atom; a compound contains atoms chemically bonded in fixed ratios. Mixtures can be separated by physical methods such as filtration or distillation. Homogeneous mixtures look uniform; heterogeneous mixtures do not.
Atoms and molecules
Atomic number = protons; mass number = protons + neutrons. Isotopes have the same atomic number but different neutron numbers. Chemical bonds form by transferring or sharing electrons.
Chemical reactions and equations
In a chemical reaction, atoms are rearranged, not created or destroyed. Balance equations by changing coefficients, never subscripts. State symbols show solid, liquid, gas, and aqueous conditions.
Acids and bases
Acids produce H⁺ in water; bases neutralize acids. pH below 7 is acidic, 7 is neutral, and above 7 is basic. pH is logarithmic, so a change of 1 unit means a tenfold change in H⁺ concentration.
Worked example
Balance H₂ + O₂ → H₂O. Put 2 before H₂O: H₂ + O₂ → 2H₂O. Now oxygen is balanced (2 on each side), but hydrogen is 2 on the left and 4 on the right. Put 2 before H₂: 2H₂ + O₂ → 2H₂O. Check: H = 4 and 4; O = 2 and 2. Coefficients are 2, 1, 2.
Traps
Do not change subscripts to balance an equation. Do not call a compound a mixture. Do not treat pH as linear. Remember that neutralization produces salt and water, not always a visible reaction.