Chemical Reactions and Equations

When milk turns sour, iron rusts, or food is digested, something deep is happening: one set of substances is turning into a completely new set. This is a chemical reaction. Chemistry gives us a precise language, the chemical equation, to describe these changes. It also gives us a way to sort reactions into a few clear types. This is the foundation of all chemistry.

In a chemical reaction, the starting substances, called the reactants, change into new substances, called the products.

We write this as a chemical equation, with reactants on the left, an arrow, and products on the right:

reactants → products

For example, when hydrogen burns in oxygen to make water: H₂ + O₂ → H₂O. Signs of a chemical reaction include a change of colour, a change of state, a gas being given off, a change of temperature, or the forming of a precipitate. A precipitate is an insoluble solid that appears in the mixture.

A correct chemical equation must be balanced. This means the number of atoms of each element must be the same on both sides of the arrow.

The reason is the law of conservation of mass: atoms are neither created nor destroyed in a reaction, only rearranged. So an unbalanced equation like H₂ + O₂ → H₂O is wrong. There are two oxygen atoms on the left but only one on the right. Balanced, it becomes 2H₂ + O₂ → 2H₂O. We balance by changing the numbers in front of formulas, never the formulas themselves.

Chemical reactions are grouped into a few main types:

• Combination: two or more substances join to form one product (A + B → AB), as when calcium oxide and water form slaked lime.
• Decomposition: one substance breaks into two or more (AB → A + B), often using heat, light or electricity.
• Displacement: a more reactive element pushes out a less reactive one from its compound (A + BC → AC + B).
• Double displacement: two compounds exchange their ions (AB + CD → AD + CB), often forming a precipitate.

Some reactions go only one way. Many others are reversible: they can proceed in both directions. As reactants form products (the forward reaction), the products begin to re-form reactants (the backward reaction).

At first the forward reaction is fast. As products build up, the backward reaction speeds up. Eventually the two reach the same rate. This state is chemical equilibrium. At equilibrium the amounts of reactants and products stay constant. Both reactions are still happening, which is why it is called a dynamic equilibrium.

What happens if we disturb a system at equilibrium? The answer is given by Le Chatelier's Principle: if a system at equilibrium is disturbed, it shifts to oppose (counteract) the disturbance and restore balance.

• More reactant: increasing the concentration of reactants shifts the equilibrium towards more products.
• Higher pressure: for gases, raising the pressure shifts the equilibrium towards the side with fewer gas molecules.
• Temperature change: the equilibrium shifts to absorb or release heat as needed.

This principle is hugely important in industry, where it decides how much product a reaction will yield. In the Haber process, the industrial method for making ammonia, pressure and temperature are carefully chosen to push the equilibrium towards more product.

A very important pair of changes is oxidation and reduction:

• Oxidation is the gain of oxygen or the loss of hydrogen.
• Reduction is the loss of oxygen or the gain of hydrogen.

These two always happen together. When one substance is oxidised, another is reduced. Such reactions are called redox reactions. Two familiar everyday examples are corrosion and rancidity. Corrosion is the rusting of iron as it gains oxygen. Rancidity happens when fats and oils are oxidised, causing food to spoil.

Living things run on a redox reaction too. Respiration is the slow oxidation of food, mainly glucose, inside cells to release energy. The rate of these oxidation and enzyme-driven reactions depends strongly on temperature. This is why fruits and vegetables stored in a cold chamber last longer. The low temperature slows the rate of respiration, so the reactions of ripening and decay proceed more slowly and the food stays fresh for longer.

Redox can also be harnessed to make electricity. A fuel cell does this directly. In a hydrogen-oxygen fuel cell, hydrogen is oxidised and oxygen is reduced, and the energy of the reaction comes out as an electric current. Its key features are worth remembering:

• Clean by-products: the cell emits only heat and water, with no smoke or polluting gases.
• Direct current: it produces direct current (DC), not alternating current (AC).
• Portable scale: it can power small portable devices such as laptops, not only large buildings.

Oxidation also powers life itself. Respiration is the slow oxidation of food inside living cells, releasing energy. It continues in fruits and vegetables even after harvest. The same enzyme-driven oxidation reactions drive ripening and, eventually, decay. Temperature controls how fast these reactions run. This is why fruits are kept in a cold chamber: the low temperature slows the rate of respiration, so

Three great families of compounds, acids, bases and salts, are everywhere: in our stomachs, in cleaning cupboards and in industry. Acids and bases have opposite properties:

• Acids taste sour, turn blue litmus red, and react with metals to give hydrogen gas. Examples are hydrochloric acid, sulphuric acid and the citric acid in lemons.
• Bases taste bitter, feel soapy, and turn red litmus blue. A base that dissolves in water is called an alkali, such as sodium hydroxide.

We test for them with indicators, substances that change colour. Litmus and turmeric are natural indicators. Methyl orange and phenolphthalein are common lab indicators.

To measure exactly how acidic or basic a solution is, we use the pH scale, which runs from 0 to 14:

• pH 7 is neutral (pure water).
• Below 7 is acidic: the lower the number, the stronger the acid.
• Above 7 is basic (alkaline): the higher the number, the stronger the base.

The pH scale measures the concentration of hydrogen ions in a solution. Equilibrium explains the difference between strong and weak acids. In water, acids and bases ionise to different extents. A strong acid ionises almost completely. A weak acid ionises only partly, reaching an equilibrium between the molecule and its ions. Buffer solutions, which resist changes in pH, work through such acid-base equilibria. This is how living systems keep their pH stable.

pH matters in daily life. Our body works within a narrow pH range. Plants grow best in soil of the right pH. Tooth decay starts when the mouth becomes too acidic. An antacid (a base) relieves an acidic stomach.

When an acid reacts with a base, they cancel each other out in a neutralisation reaction, producing a salt and water:

acid + base → salt + water

For example, hydrochloric acid and sodium hydroxide give common salt (sodium chloride) and water. A salt is the compound formed from the positive part of a base and the negative part of an acid. Taking an antacid works by the same reaction: it neutralises excess stomach acid.

Many everyday substances are salts with important uses:

• Common salt (sodium chloride): used in food and as the raw material for many chemicals.
• Baking soda (sodium hydrogen carbonate): used in cooking and as a mild antacid.
• Washing soda (sodium carbonate): used for cleaning and softening hard water.
• Bleaching powder: used to bleach cloth and to disinfect drinking water.
• Plaster of Paris: used for plastering and for setting broken bones.

The elements that take part in all these reactions fall into two great families: metals and non-metals. Iron, copper and gold sit on one side. Oxygen, carbon and sulphur sit on the other. Their physical properties contrast sharply:

• Metals are usually shiny (lustrous), hard, malleable (can be beaten into sheets), ductile (can be drawn into wires), and good conductors of heat and electricity. Most are solid at room temperature, except mercury, a liquid.
• Non-metals are usually dull, brittle, and poor conductors (insulators). They may be solid, liquid or gas.

There are exceptions. Graphite, a non-metal, conducts electricity. But these general rules hold for most elements.

Metals differ in how readily they react. Arranging them in order of decreasing reactivity gives the reactivity series. Highly reactive metals like potassium and sodium sit at the top. Unreactive metals like gold and silver sit at the bottom.

The series explains displacement reactions: a more reactive metal can displace a less reactive one from its compound. For example, iron can displace copper from copper sulphate solution. The series also tells us how easily a metal corrodes and how it must be extracted.

Most metals are not found pure in nature. They are locked in ores, rocks containing the metal. Winning the metal from its ore is called metallurgy. The method used depends on the metal's place in the reactivity series:

• Highly reactive metals are extracted by electrolysis.
• Moderately reactive metals are obtained by roasting the ore and then reducing it, often with carbon.
• Unreactive metals like gold are often found native and need only refining.

The extracted metal is then purified before use.

Corrosion, the everyday oxidation met earlier, slowly eats metals away through reaction with the environment. The rusting of iron needs both air (oxygen) and moisture (water) together. Blocking either one stops it. Corrosion can be prevented in several ways:

• Painting, greasing or oiling: keeps out air and water.
• Galvanising: coating iron with a layer of zinc.
• Electroplating: coating with a less reactive metal.
• Alloying: stainless steel, iron mixed with chromium and nickel, does not rust.

Preventing corrosion saves enormous amounts of money and resources every year.

Chemical reactions and chemical substances are at work in the products we use daily. Many everyday items contain additives, chemicals added on purpose, or contaminants, unwanted chemicals that enter during manufacture. Exams often test which chemical belongs to which product:

• Lead in lipstick: lead is a toxic heavy metal that can occur as a contaminant in some lipsticks, usually traced to the colour pigments used.
• Brominated vegetable oils in soft drinks: these are vegetable oils bonded with bromine, used as emulsifiers in some citrus-flavoured soft drinks to keep the flavouring evenly mixed. Their safety is controversial.
• Monosodium glutamate (MSG) in fast food: MSG is a flavour enhancer common in Chinese fast food. Excessive intake is linked to health concerns.

Chemistry also keeps drinking water safe. Modern purification systems use ultraviolet (UV) radiation as a disinfection step. UV light damages the DNA of bacteria, viruses and other microorganisms, so it inactivates or kills harmful microbes in water. Its role is limited to disinfection. UV does not remove odours from water, and it does not speed up sedimentation, the settling of solid particles, or reduce turbidity, the cloudiness caused by suspended matter. Those tasks need other steps, such as filtration, settling tanks and chemical coagulants.