Composition, Solutions, and Stoichiometry

This is Unit Three of Chemistry, Part Two: equations and reaction types. Balancing an equation is allowed to change one thing and one thing only: coefficients. Changing a subscript to make an element balance silently changes which substance the formula names, so the equation stops describing the reaction you started with. The same reaction can then be written three ways. A molecular equation keeps every compound intact, which is a bookkeeping convenience rather than a claim about what is in the beaker. A complete ionic equation separates the dissolved strong electrolytes into ions while leaving solids whole. A net ionic equation removes the spectators and leaves only what actually reacts. Then three reaction families. Precipitation, where ions swap partners and one pairing turns out insoluble. Acid-base, where a proton transfers — and never as a bare hydrogen ion, since it moves straight onto a water molecule. And redox, where oxidation numbers change. The part closes on classification, and there the ordering matters: a reaction can show more than one surface feature at once, and a change in oxidation number is the test that settles it. By the end you will be able to do five things. One: balance without changing the substance. Two: say what a net ionic equation removes. Three: predict a precipitate. Four: tell strong from concentrated. Five: classify a reaction.

Two entries. The notation first, then the procedure — and the procedure's warning is about the one move that looks like it should work and destroys the equation's meaning.

The notation.

Chem 0423 Writing a Chemical Equation

A chemical equation places reactant formulas left of an arrow and product formulas to its right, with coefficients showing relative amounts — methane burning in oxygen runs one to two to one to two. A formula with no written coefficient carries an implied one, and treating it as zero or overlooking it while tallying atoms undercounts them.

Making it balance.

Chem 0426 Balancing Chemical Equations

Balancing means adjusting only coefficients, never subscripts, until every element's count matches on both sides. For propane's combustion, match carbon first with a coefficient of three on carbon dioxide, then hydrogen with four on water — which fixes ten oxygen atoms on the product side. Five on oxygen gas supplies exactly ten on the reactant side. Changing a subscript instead, writing oxygen gas as something else to make the count work, silently changes which substance the formula names.

Three entries taking one reaction from its most compact form to its most informative. Each level strips away something the previous one carried, and the last is the one that says what the reaction actually is.

Everything intact.

Chem 0429 Molecular Equations

A molecular equation writes every reactant and product as a complete neutral formula rather than as separated dissolved ions — silver nitrate plus sodium chloride giving silver chloride plus sodium nitrate, all in intact formulas. It is a bookkeeping convenience, not a claim that dissolved sodium chloride exists as intact units.

Dissolved ions separated.

Chem 0430 Complete Ionic Equations

A complete ionic equation rewrites every dissolved strong electrolyte as its separated ions while keeping solids and liquids whole — so the silver nitrate and sodium chloride reaction expands into four separate ions on the left. The solid silver chloride stays written as one formula, since it is not dissolved.

Only what reacts.

Chem 0431 Net Ionic Equations and Spectator Ions

A net ionic equation removes the spectator ions — those unchanged on both sides — leaving only the species that react, so cancelling sodium and nitrate leaves silver ion plus chloride ion giving solid silver chloride. Cancelling an ion that appears on both sides in different amounts or states, or cancelling the precipitate itself, is not valid.

Three entries. A property, an empirical scheme describing it, and a procedure using that scheme to predict a solid. The procedure's warning names the mistake that makes the prediction come out empty.

A maximum.

Chem 0434 Solubility

Solubility is the maximum concentration a substance reaches in a solvent under given conditions — add solvent to excess solid and dissolving stops at that maximum, leaving undissolved solid however long it is stirred. Insoluble describes very low solubility rather than an absolute zero; a genuinely zero-solubility substance is essentially unknown.

The scheme.

Chem 0435 Solubility Rules

Solubility rules are an empirical classification naming which ionic compounds are generally soluble or insoluble in water — all common nitrate salts are soluble, while most common carbonates are not, apart from the group one metals and ammonium. Skipping a listed exception misapplies a rule as though it had none.

Will a solid form.

Chem 0437 Predicting Precipitation Reactions

Combine the two solutions' ions in every possible new pairing, then check each pairing against the solubility rules. Mixing potassium chloride with silver nitrate offers two new pairings: potassium with nitrate, and silver with chloride. The rules mark nitrates and potassium salts soluble but silver chloride insoluble, so silver chloride precipitates while potassium and nitrate stay dissolved as spectators. Checking only the original pairings misses the new pairing the rules actually flag.

Five entries. What each yields in water, the strong-and-weak distinction that repeats Part One's point in a new setting, what neutralisation produces, and a procedure whose result is the same for every strong acid and every ionic hydroxide.

What each yields.

Chem 0440 Acids and Bases

An acid yields hydronium ions in water and a base yields hydroxide ions — hydrogen chloride gas reacts with water to form hydronium, while solid sodium hydroxide dissolves to release hydroxide directly. Dissolved acids never release a bare hydrogen ion into solution; that proton transfers immediately to a water molecule.

Essentially complete.

Chem 0443 Strong Acids and Strong Bases

A strong acid or base reacts with, or dissociates in, water essentially completely, leaving almost none of the starting compound intact — hydrochloric acid and the ionic hydroxides behave this way. Assuming every acid dissociates completely, or reading strong as meaning concentrated or dangerous rather than fully ionised, misapplies the term.

Only partial.

Chem 0444 Weak Acids and Weak Bases

A weak acid or base reacts with water only partially, leaving most of the original compound un-ionised at any moment — dissolved ammonia forms only about one percent ions, the rest staying intact. Calling a weak acid merely dilute confuses incomplete reaction with low concentration; a concentrated solution can still be a weak acid.

Acid meets base.

Chem 0447 Neutralization Reactions

A neutralisation reaction combines an acid with a base other than water itself, producing a salt and usually water — magnesium hydroxide neutralising hydrochloric acid gives magnesium chloride and water. Salt here names any ionic compound formed this way, a broader sense than the kitchen meaning.

The general result.

Chem 0450 Writing Net Ionic Equations for Acid-Base Reactions

Dissociate every strong acid and strong base into ions, then cancel the spectators. Neutralising barium hydroxide with nitric acid gives a complete ionic form with barium, hydroxide, hydronium and nitrate — and the water product stays intact. Cancelling barium and nitrate leaves hydronium plus hydroxide giving water, which is what any strong acid neutralised by any ionic hydroxide reduces to. Writing the acid's dissociation as a bare hydrogen ion, or dissociating the water product, destroys that cancellation.

Two entries introducing a tool rather than a phenomenon. An oxidation number is assigned rather than measured, and the procedure for assigning them works through a fixed order — with the unknown solved last rather than guessed first.

An assigned charge.

Chem 0453 Oxidation Numbers

An oxidation number is an assigned charge tracking how electrons are distributed among an atom's bonds, whether or not those bonds are ionic — in water, oxygen is assigned minus two and each hydrogen plus one, summing to zero for the neutral molecule. It is an accounting convention, not necessarily an atom's real measured charge in a covalent molecule.

Working through the order.

Chem 0456 Assigning Oxidation Numbers

Apply a fixed priority: free elements are zero, then fixed values for a handful of common elements, then whatever value balances the formula's known overall charge. In sulfate, oxygen takes its usual minus two, so four oxygens give minus eight; the ion carries two minus overall, so sulfur must be plus six. Sulfur's value is found last rather than assumed first. Applying oxygen's or hydrogen's usual value without checking the guideline's own exceptions, such as oxygen in a peroxide, gives a number that fails to balance.

Three entries. The class itself, defined entirely by whether oxidation numbers change, and then two named patterns within it. The first entry's warning is what makes the classification procedure at the end possible.

Numbers change.

Chem 0459 Oxidation-Reduction (Redox) Reactions

A redox reaction is one where at least one element's oxidation number rises while another's falls, tracking an electron transfer — sodium going from zero to plus one while chlorine goes from zero to minus one. A reaction with no oxidation-number change, such as most precipitation or acid-base reactions, is not a redox reaction at all.

Fuel and oxidant.

Chem 0462 Combustion Reactions

Combustion is a vigorous redox reaction between a fuel and an oxidant, usually oxygen, releasing heat and light — ethylene's combustion oxidises carbon while oxygen is reduced. Combustion names a broad redox pattern rather than one fuel or one oxidant, covering a candle in air and a rocket burning aluminium in perchlorate alike.

One element out.

Chem 0465 Single-Displacement Reactions

A single-displacement reaction is a redox reaction where one element displaces another from a compound and emerges as a free element — a more reactive metal in a dissolved metal salt displacing the less reactive one, which plates out as solid. It always changes oxidation numbers, unlike a precipitation reaction where ions swap partners unchanged.

One procedure closing the part. Three tests applied in order — and the warning is the reason the order exists, because a reaction can look like it belongs to two categories at once.

Three tests.

Chem 0468 Classifying a Reaction as Precipitation, Acid-Base, or Redox

Check in order whether ions combine into an insoluble solid, whether a hydrogen ion transfers between species, or whether any oxidation number changes. Silver nitrate with sodium chloride forms a solid with no charge changes — precipitation. Hydrochloric acid with sodium hydroxide transfers a proton with no oxidation-number change — acid-base. Zinc metal with copper sulfate changes both metals' oxidation numbers — redox, and single displacement specifically. A reaction can show more than one surface feature, and oxidation-number change is the deciding test.

Five things to carry out of Part Two. Balancing adjusts coefficients and never subscripts. A coefficient states how many units of a substance take part; a subscript states what the substance is. Change a subscript to make an element balance and the equation now describes a different reaction, correctly balanced and about something else. A net ionic equation removes the spectator ions — the ones appearing unchanged on both sides — leaving only what reacts. What may not be cancelled: an ion appearing on both sides in different amounts or different states, and the precipitate itself, which is a product rather than a spectator. To predict a precipitate, pair each solution's ions in every new combination — not the pairings you started with — and check each against the solubility rules, exceptions included. Any pairing the rules call insoluble is the precipitate, and the remaining ions are spectators. Strong is about completeness, concentrated is about amount. A strong acid ionises essentially completely; a weak one leaves most of itself intact, as dissolved ammonia does at about one percent. A concentrated solution of a weak acid is an ordinary thing, and so is a dilute strong one. And oxidation number is the deciding test between reaction types. Precipitation swaps partners with no charge change. Acid-base transfers a proton with no charge change. Redox is defined by the change itself — so when a reaction shows features of two categories, that is what settles it. Part Three turns to stoichiometry — what a balanced equation lets you calculate.