Chemical Properties and Reactions

Atoms and the periodic table

Every atom has a tiny, heavy nucleus made of protons (+) and neutrons (no charge), surrounded by much lighter electrons (−). In a neutral atom, the number of electrons equals the number of protons.

The atomic number (the number of protons) identifies an element. Every atom with 6 protons is carbon; every atom with 8 protons is oxygen. Atoms of the same element can have different numbers of neutrons; these are isotopes (carbon-12 and carbon-14). To find neutrons, subtract: neutrons = mass number − atomic number. Sodium-23 has 23 - 11 = 12 neutrons.

Reading a periodic table square:

How the table is organized:

• Elements are in order of increasing atomic number.
• Rows are periods. Columns are groups (families), and elements in the same group have similar chemical properties because they have the same number of outer electrons. Group 1 metals (lithium, sodium, potassium) all react strongly with water. Group 18, the noble gases (helium, neon, argon), almost never react.
• Metals are on the left and center: shiny, good conductors of heat and electricity, can be bent. Nonmetals are on the upper right: dull, poor conductors, often gases. Metalloids (such as silicon) lie along the staircase line between them.

A compound has properties very different from its elements: sodium (a reactive metal) and chlorine (a poisonous gas) form table salt.

Physical vs. chemical changes

A physical change alters the form, size or state of a substance, but it is still the same substance. Melting, freezing, boiling, dissolving, cutting, bending and crushing are physical changes. Ice, liquid water and steam are all \text{H}_2\text{O}.

A chemical change (chemical reaction) produces new substances with new properties. Signs that a chemical change has happened:

• gas bubbles form (when not just boiling);
• an unexpected color change;
• light or heat is given off (or the mixture gets cold);
• a solid (precipitate) forms when two liquids are mixed;
• a new smell; the change is hard to reverse.

Physical vs. chemical properties. A physical property can be observed without changing the substance: color, density, melting point, boiling point, solubility. A chemical property describes how a substance reacts: flammability, reacting with acid, rusting.

Energy in reactions. Exothermic reactions release energy and feel warm (burning fuel, hand warmers). Endothermic reactions absorb energy and feel cold (an instant cold pack).

Chemical equations and conservation of mass

A chemical equation shows reactants on the left and products on the right:

The law of conservation of mass says matter is neither created nor destroyed in a reaction. Atoms are only rearranged. So an equation must be balanced: each kind of atom must appear in the same number on both sides.

How to balance: change only the coefficients (big numbers in front), never the subscripts. Changing a subscript changes the substance (\text{H}_2\text{O} is water; \text{H}_2\text{O}_2 is hydrogen peroxide).

Example: hydrogen burns in oxygen to make water. \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} is unbalanced: 2 oxygen atoms on the left, 1 on the right.

1. Put a 2 in front of water: \text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. Now oxygen is 2 and 2, but hydrogen is 2 on the left and 4 on the right.
2. Put a 2 in front of hydrogen: 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. Hydrogen: 4 = 4. Oxygen: 2 = 2. Balanced.

Count atoms by multiplying the coefficient by each subscript: 2\text{H}_2\text{O} has 2 \times 2 = 4 H atoms and 2 \times 1 = 2 O atoms.

When mass seems to change. If a gas escapes from an open container, the products seem lighter. If a gas from the air joins a reaction (as when iron rusts by combining with oxygen), the product seems heavier. In a closed system, the total mass never changes. GED questions often show an open container losing mass and ask you to explain it: the gas escaped; it was not destroyed.

Combustion is a common reaction type: fuel + oxygen → carbon dioxide + water + energy.

Solutions, acids and bases, and density

Solutions. A solution is an even mixture of a solute (what dissolves, like sugar) in a solvent (what does the dissolving, usually water). Concentration tells how much solute is in a certain amount of solution, for example grams per liter: 30 g of salt in 0.50 L of solution is \dfrac{30}{0.50} = 60 g/L. Adding more water dilutes a solution, so its concentration goes down; the amount of solute stays the same.

Solubility is the most solute that can dissolve at a given temperature. For most solids, solubility rises as temperature rises (sugar dissolves better in hot tea). For gases, it is the opposite: warm soda goes flat faster. A solution with the maximum dissolved is saturated; less than the maximum is unsaturated.

Acids and bases. The pH scale runs from 0 to 14:

Each pH step is a tenfold change. pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5. Mixing an acid and a base causes neutralization: the pH moves toward 7. An antacid tablet (a base) relieves heartburn by neutralizing extra stomach acid.

Density is mass divided by volume: D = \dfrac{m}{V}. It is a physical property, so it helps identify substances: every sample of pure gold has a density of about 19.3 g/cm³, whether it is a ring or a bar. Objects less dense than a liquid float in it; denser objects sink. Ice (about 0.92 g/cm³) floats on liquid water (1.0 g/cm³). To find the volume of an irregular object, use water displacement: the rise in the water level equals the object's volume.