Dissolving sugar is a physical change! In a physical change, the substance changes form but not its chemical identity. Examples of physical changes: melting ice, boiling water, crushing a can, dissolving sugar. In a chemical reaction, new substances are formed. Signs of a chemical reaction: color change (iron rusting from silver to brown), temperature change (exothermic releases heat, endothermic absorbs heat), gas production (bubbles when vinegar reacts with baking soda), precipitate formation (cloudy solid appears when two solutions are mixed), and light emission (glow sticks, fire). The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total mass of reactants equals the total mass of products.

Reactants are vinegar and baking soda! The chemical equation is: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The reactants are on the left of the arrow; products on the right. The fizzing (CO₂ gas) is a sign of a chemical reaction. In a chemical equation, the number of atoms of each element must be the same on both sides (balanced equation). Lavoisier discovered the Law of Conservation of Mass, which states that mass is conserved in chemical reactions. This means the total mass of reactants equals the total mass of products. In the vinegar and baking soda reaction, if you did it in a closed container, the mass would stay the same. In an open container, the CO₂ gas escapes, so the mass appears to decrease. That is why the reaction seems to lose mass – the gas leaves the container.

Exothermic releases heat! The word "exothermic" comes from "exo" (out) and "thermic" (heat). Combustion (burning) is highly exothermic. The burning of fuel releases heat that we use for heating, cooking, and engines. Endothermic reactions absorb heat; the reaction vessel becomes cold. For example, when baking soda reacts with citric acid in water, the mixture gets cold. Some chemical cold packs use an endothermic reaction (often ammonium nitrate dissolving in water). Photosynthesis is endothermic – plants absorb light energy (from the sun) to convert carbon dioxide and water into glucose and oxygen. The energy change in a chemical reaction is due to the difference in bond energies between reactants and products. Exothermic reactions have more energy in the bonds of the products (energy is released). Endothermic reactions have more energy in the bonds of the products (energy is absorbed).

The fire triangle is fuel, oxygen, and heat! Fire extinguishers work by removing one of these: water removes heat (cools the fire), carbon dioxide removes oxygen (smothers the fire), and some extinguishers remove fuel (foam blankets the fuel). Combustion is a rapid oxidation reaction. Slow oxidation (like rusting) produces heat slowly and does not create flame. Complete combustion (enough oxygen) produces carbon dioxide and water. Incomplete combustion (limited oxygen) produces carbon monoxide (a poisonous gas) and soot (carbon particles). That is why you should never run a generator or car in an enclosed space – carbon monoxide can kill you. The "fire tetrahedron" adds a fourth element: chemical chain reaction. Some extinguishers (like halon) interrupt the chain reaction. Always practice fire safety!

OIL RIG: Oxidation Is Loss, Reduction Is Gain! Oxidation and reduction always occur together. If one substance is oxidized (loses electrons), another substance must be reduced (gains those electrons). In a zinc-copper battery (Daniel cell), zinc atoms lose electrons (oxidized) and become Zn²⁺ ions, while Cu²⁺ ions gain electrons (reduced) and become copper metal. The flow of electrons through an external wire is electricity. Rusting is a slow oxidation of iron in the presence of oxygen and water. Iron loses electrons to become Fe²⁺ and eventually Fe³⁺ (iron(III) oxide, Fe₂O₃, rust). The Statue of Liberty originally was copper-colored (copper metal). Over time, the copper oxidized to form a green patina (basic copper carbonate). The patina protects the underlying metal from further corrosion. Not all oxidation is bad – the formation of the protective patina on copper and the oxide layer on aluminum (aluminum oxide) prevents further corrosion.

Fe + S → FeS is a synthesis reaction! Two elements combine to form a compound. The opposite of synthesis is decomposition (AB → A + B). Synthesis reactions are common in industry: ammonia synthesis (Haber process: N₂ + 3H₂ → 2NH₃) produces fertilizer. The formation of water (2H₂ + O₂ → 2H₂O) is a synthesis reaction. The reaction of iron and sulfur to form iron sulfide is exothermic – the mixture glows red hot. After the reaction, the iron sulfide is not magnetic (unlike iron). Synthesis reactions are also called "combination reactions." In a more general sense, any reaction where two or more reactants combine to form one product is a synthesis. This is one of the five main types of chemical reactions (synthesis, decomposition, single replacement, double replacement, combustion).

2H₂O → 2H₂ + O₂ is a decomposition reaction! Water decomposes into hydrogen and oxygen when electricity is passed through it (electrolysis). The bubbles of hydrogen and oxygen gas can be collected. Decomposition reactions are the opposite of synthesis reactions. Another example: hydrogen peroxide (H₂O₂) naturally decomposes into water and oxygen (2H₂O₂ → 2H₂O + O₂). That is why hydrogen peroxide bottles are brown – light accelerates the decomposition, so brown bottles block light. A fun demonstration: mix hydrogen peroxide with a catalyst (manganese dioxide or yeast) – it rapidly bubbles (oxygen gas). The decomposition of ammonium dichromate produces a "volcano" of green chromium(III) oxide. Decomposition reactions are often endothermic (require energy to break bonds). In chemistry, decomposition can be thermal (heat), electrolytic (electricity), or photolytic (light).

Zinc will displace copper! The reactivity series: potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), aluminum (Al), zinc (Zn), iron (Fe), tin (Sn), lead (Pb), hydrogen (H), copper (Cu), silver (Ag), gold (Au). A more reactive metal will displace a less reactive metal from its compound. That is why iron nails can be coated with copper by placing them in a copper(II) sulfate solution (the iron displaces copper). The "activity series" also includes halogens: fluorine is most reactive, then chlorine, then bromine, then iodine. A more reactive halogen will displace a less reactive halide. For example, chlorine gas will displace bromine from sodium bromide solution. Single replacement reactions are often redox reactions. They are used in the extraction of metals (e.g., aluminum displaces iron in the thermite reaction). The reaction of magnesium with hydrochloric acid (Mg + 2HCl → MgCl₂ + H₂) is also a single replacement (magnesium replaces hydrogen).

A precipitate is the cloudy solid! Double replacement reactions often produce a precipitate (solid), a gas, or water. The reaction of silver nitrate and sodium chloride is used in chemistry labs to test for chloride ions (white precipitate). The reaction of vinegar (acetic acid) and baking soda (sodium bicarbonate) is a double replacement reaction that produces carbon dioxide gas (the fizz). Neutralization (acid + base → salt + water) is a double replacement. Precipitation reactions are used in water treatment to remove contaminants (e.g., adding alum to water to precipitate dirt). In a double replacement reaction, all compounds are usually in aqueous solution (dissolved in water). The driving force is the formation of a precipitate, a gas, or a weak electrolyte (like water). Double replacement reactions are common in aqueous solutions. They are also called "ionic reactions" because they involve ions in solution.

A catalyst speeds up reactions without being used up! In the reaction, a catalyst lowers the activation energy, allowing more reactant molecules to have enough energy to react. Enzymes are biological catalysts that are essential for life. For example, catalase in your cells breaks down hydrogen peroxide (a toxic byproduct) into water and oxygen. Without catalase, hydrogen peroxide would damage cells. Catalysts are highly specific: each enzyme catalyzes only one reaction (like a lock and key). In industry, catalysts save energy and reduce waste. The catalytic converter in a car contains platinum, palladium, and rhodium; it converts carbon monoxide (CO) to carbon dioxide (CO₂), unburned hydrocarbons to CO₂ and water, and nitrogen oxides (NOx) to nitrogen (N₂) and oxygen (O₂). Catalysts are also used in the production of plastics, fertilizers, and pharmaceuticals. Some catalysts are poisoned by impurities (lead, sulfur), which is why leaded gasoline was phased out (lead destroyed catalytic converters).