Basic Chemistry Concepts

Matter

Matter is anything that has mass and occupies space. It exists in three common states — solid (definite shape and volume), liquid (definite volume, no fixed shape) and gas (neither) — with plasma (ionised gas, as in the Sun and stars, lightning, neon signs) and Bose–Einstein condensate as further states.

• Pure substances: elements (one kind of atom: hydrogen, iron) and compounds (two or more elements chemically combined in a fixed ratio: water H₂O, carbon dioxide CO₂).
• Mixtures: homogeneous (solutions, air, brass) and heterogeneous (sand and water, milk). The components of a mixture retain their properties and can be separated by physical methods (filtration, distillation, evaporation, sublimation, chromatography, magnetic separation, sedimentation and decantation, centrifugation).
• A molecule is the smallest particle of a substance capable of independent existence; an atom is the smallest particle of an element that takes part in a chemical reaction.

Laws of chemical combination

• Law of conservation of mass (Lavoisier): mass is neither created nor destroyed in a chemical reaction.
• Law of constant (definite) proportions (Proust): a compound always contains the same elements in the same proportion by mass (water is always 1 part hydrogen to 8 parts oxygen by mass).
• Law of multiple proportions (Dalton) and Gay-Lussac's law of gaseous volumes.
• Avogadro's law: equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. One mole of any substance contains $6.022\times 10^{23}$ particles (Avogadro's number); one mole of a gas occupies 22.4 litres at STP.

Development of atomic models

Drawbacks of Rutherford's model: it could not explain the stability of the atom (an orbiting electron should radiate energy and spiral into the nucleus). Bohr's model explained the hydrogen spectrum but failed for larger atoms.

Subatomic particles

• The nucleus is about 10⁻¹⁵ m across, about 100,000 times smaller than the atom, but holds almost all the mass.
• Protons and neutrons are nucleons; they are made of quarks. Hydrogen (protium) is the only atom with no neutron.
• An atom is electrically neutral: number of protons = number of electrons.

Atomic number, mass number, isotopes

• Atomic number ($Z$) = number of protons. It identifies the element.
• Mass number ($A$) = protons + neutrons (nucleons). Number of neutrons $=A-Z$.
• Notation: ${}_Z^{A}X$; e.g. ${}_6^{12}\text{C}$: 6 protons, 6 neutrons, 6 electrons.

Uses of isotopes: ¹⁴C for radiocarbon dating; ⁶⁰Co for cancer treatment; ¹³¹I for thyroid disorders; ²³⁵U as nuclear fuel; deuterium (heavy water) as a moderator in reactors; ²⁴Na for detecting blood clots; isotopes as tracers in agriculture and industry.

Physical and chemical change

Signs of a chemical reaction: change of colour, evolution of a gas, formation of a precipitate, change of temperature (heat or light given out), change of state.

Chemical equations

A chemical equation represents a reaction with symbols and formulae. Reactants are on the left, products on the right, with an arrow between them.

• Physical states: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. An arrow ↑ shows a gas escaping; ↓ a precipitate.
• Conditions such as heat, catalyst, light or pressure are written on the arrow.

Balancing

By the law of conservation of mass, the number of atoms of each element must be the same on both sides. Balance by adding coefficients, never by changing the formulae.

Unbalanced: Fe + H₂O → Fe₃O₄ + H₂. Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Types of chemical reactions

1. Combination (synthesis)

Two or more substances combine to form a single product: $\text{A}+\text{B}\to \text{AB}$.

• $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$ (quicklime + water → slaked lime, exothermic; used in whitewashing).
• $2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$ (burning magnesium with a dazzling white light).
• $\text{C}+{\text{O}}_2\to {\text{CO}}_2$; ${\text{N}}_2+3{\text{H}}_2\to 2{\text{NH}}_3$ (Haber process).

2. Decomposition

A single compound breaks into two or more simpler substances: $\text{AB}\to \text{A}+\text{B}$.

• Thermal decomposition: ${\text{CaCO}}_3\stackrel{\text{heat}}{\to }\text{CaO}+{\text{CO}}_2$ (limestone → quicklime; used in cement making). Also $2{\text{Pb(NO}}_3{)}_2\to 2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$ (brown fumes).
• Electrolytic decomposition: $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$ on passing electricity (hydrogen at the cathode, twice the volume of oxygen at the anode).
• Photolytic decomposition: silver chloride and silver bromide turn grey in sunlight (basis of black-and-white photography): $2\text{AgCl}\stackrel{\text{light}}{\to }2\text{Ag}+{\text{Cl}}_2$.

3. Displacement

A more reactive element displaces a less reactive one from its compound: $\text{A}+\text{BC}\to \text{AC}+\text{B}$.

• $\text{Fe}+{\text{CuSO}}_4\to {\text{FeSO}}_4+\text{Cu}$ (an iron nail in blue copper sulphate solution turns brownish and the solution turns pale green).
• $\text{Zn}+{\text{CuSO}}_4\to {\text{ZnSO}}_4+\text{Cu}$.

4. Double displacement

Two compounds exchange ions: $\text{AB}+\text{CD}\to \text{AD}+\text{CB}$.

• Precipitation: ${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{BaSO}}_4\downarrow +2\text{NaCl}$ (white precipitate).
• Neutralisation: acid + base → salt + water: $\text{HCl}+\text{NaOH}\to \text{NaCl}+{\text{H}}_2\text{O}$.

5. Oxidation and reduction (redox)

• Oxidation: gain of oxygen, or loss of hydrogen, or loss of electrons (OIL).
• Reduction: loss of oxygen, or gain of hydrogen, or gain of electrons (RIG). Remember OIL RIG.
• They occur together in a redox reaction. The substance that gets oxidised is the reducing agent; the one reduced is the oxidising agent.
• $\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$: CuO is reduced (loses oxygen), H₂ is oxidised.
• Examples of oxidising agents: oxygen, potassium permanganate, hydrogen peroxide, potassium dichromate, chlorine. Reducing agents: hydrogen, carbon, carbon monoxide, metals.

Acids

An acid is a substance that gives hydrogen ions (H⁺) (hydronium ions, H₃O⁺) in water. Acids are sour in taste, turn blue litmus red, and react with metals, carbonates and bases.

Properties

• Acid + metal → salt + hydrogen: $\text{Zn}+{\text{H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2$. (The hydrogen gas burns with a "pop" sound.)
• Acid + metal carbonate/bicarbonate → salt + water + carbon dioxide (brisk effervescence; the gas turns lime water milky).
• Acid + base → salt + water (neutralisation).
• Acids conduct electricity in solution, since they produce ions.
• Concentrated acids are corrosive; dilute an acid by adding the acid slowly to water (never water to acid), since the process gives out a lot of heat.

Common acids

Basicity: the number of replaceable H⁺ ions: HCl (monobasic), H₂SO₄ (dibasic), H₃PO₄ (tribasic).

Bases and alkalis

A base gives hydroxide ions (OH⁻) in water, or accepts H⁺. A base soluble in water is an alkali (NaOH, KOH, Ca(OH)₂, NH₄OH). Bases are bitter, feel soapy, turn red litmus blue, and conduct electricity in solution.

• Base + acid → salt + water.
• Base + ammonium salts liberates ammonia.
• Strong alkalis are corrosive.

Common bases

Antacids (sodium bicarbonate, magnesium hydroxide, aluminium hydroxide) relieve the excess acid in the stomach.

Indicators

An indicator changes colour in acid or alkaline solution.

Olfactory indicators: onion, vanilla and clove oils lose their smell in a base.

The pH scale

The pH is a measure of the hydrogen-ion concentration: $\text{pH}=-{\log }_{10}[{\text{H}}^{+}]$. The scale runs from 0 to 14:

A change of one pH unit corresponds to a tenfold change in $[{\text{H}}^{+}]$. The pH is measured with pH paper or a pH meter.

Acid rain has a pH below 5.6, caused by sulphur dioxide and nitrogen oxides; it damages buildings (the Taj Mahal), plants and aquatic life.

pH in daily life

• Tooth decay starts when the pH in the mouth falls below about 5.5; toothpastes are basic and neutralise the acid.
• Soil pH decides plant growth: acidic soils are treated with lime; alkaline soils with gypsum or organic matter.
• Indigestion is due to excess HCl in the stomach and is relieved by antacids.
• Bee sting (methanoic acid) is treated with baking soda; a wasp sting (alkaline) with vinegar.
• The pH of blood is maintained by buffers.