Essential Ideas: Matter, Measurement, and Uncertainty

This is Unit One of Chemistry, Part Two of three: what a property is, what a change is, and what a measurement is made of. Part One defined matter and sorted it — elements, compounds, mixtures, atoms and molecules. This part asks a different question. Not what a sample is made of, but what can be observed about it, and what an observation is worth once written down as a number. The first move is a pair of distinctions ordinary speech runs together. A physical change and a chemical change both look like change. Only one of them leaves a different substance behind, and how dramatic the change looks does not settle which one happened. The second move separates properties that grow with the size of the sample from properties that do not. That split decides which measurements can identify a substance and which only report how much of it is on the bench. Then measurement itself. Every measurement carries three parts, and the third is the one usually dropped. The base units underneath have been moved off objects and onto physical constants, so a lab reproduces them rather than copying a standard. By the end you will be able to do five things. One: tell a physical change from a chemical one, and name the test that settles it. Two: tell an extensive property from an intensive one, and say which kind can identify a substance. Three: name the three parts of a measurement, and say why S I defines its base units by constants. Four: convert between metric prefixes, and between Celsius and kelvin, in the right direction. Five: calculate a density from a measured mass and a measured volume.

Four entries in two matched pairs. A property is what a substance can be observed to be; a change is what happens to it. Each comes in a physical version and a chemical version, and both pairs are defined against the same line — whether the chemical composition of the substance altered. Most of their warnings go to appearances that sit on the wrong side of it.

What can be read without altering it.

Chem 0061 Physical Property

A physical property is a characteristic of matter observable without changing the substance's chemical composition. The color and the density of a copper wire are both readable while the copper stays copper. Some physical properties, a melting point among them, appear only once the substance undergoes a physical change — and that change is still not a chemical one.

Form altered, identity kept.

Chem 0062 Physical Change

A physical change alters the state, form or properties of matter without changing the chemical identity of the substance present. Melting solid wax into liquid wax changes its form and leaves the same substance throughout. A dramatic-looking change is not automatically chemical — grinding a solid into powder can change its color and texture while composition stays identical.

What it can turn into.

Chem 0065 Chemical Property

A chemical property describes a substance's ability, or inability, to change into a different substance. Iron's tendency to combine with oxygen and water to form rust is one of iron's chemical properties. A chemical property is revealed only when the change is attempted or observed — flammability cannot be seen in a substance sitting still.

A different substance afterward.

Chem 0066 Chemical Change

A chemical change produces one or more substances that differ chemically from the matter present before the change. A cut apple browning qualifies, because the brown compounds differ from the substances in the fresh, uncut flesh. Color change on its own does not confirm a chemical change — food coloring in water changes color through purely physical mixing.

Two entries, and the pair does work later in the unit. One kind of property takes its value from how much matter is present; the other holds the same value at any sample size. Only the second kind can say what a substance is — the reason density arrives later in this part and mass does not.

Depends on how much.

Chem 0070 Extensive Property

An extensive property has a value that depends on how much matter is present, in direct proportion to the amount. A long coil of copper wire has more mass than a short piece cut from the same coil, because mass is extensive. Doubling a sample always changes an extensive value, so an extensive property on its own cannot identify what a substance is.

The same at any size.

Chem 0071 Intensive Property

An intensive property does not depend on how much matter is present and stays the same for any sample size. A cup of milk and a gallon of milk standing at the same room temperature read that same temperature on a thermometer. Heat and temperature are the pair most often conflated here — hot oil in a pot holds far more heat, an extensive quantity, than one drop does.

Two entries that each compress a property into a label. The first sorts every element into three classes by how well it conducts. The second puts a chemical's hazards on the outside of its container as four rated numbers. Each label is deliberately incomplete, and each entry names what it leaves out.

Three classes, by conductivity.

Chem 0075 Metals, Nonmetals, and Metalloids

Elements sort into three classes by conductivity: metals conduct heat and electricity well, nonmetals conduct poorly, and metalloids sit in between. Copper wiring conducts electricity efficiently because copper is a metal; the sulfur in a match head does not, being a nonmetal. This three-way split is a starting classification — sharper trends tied to atomic structure come later in the course.

Four colors on a container.

Chem 0079 The NFPA Hazard Diamond

The N F P A hazard diamond is a four-color symbol rating a chemical's fire, health, reactivity and special hazards from zero to four. A high number in the red diamond signals a low flash point and a fire hazard at a glance. The diamond communicates a hazard's level and not how to handle it — handling belongs to trained staff.

Two entries that set up everything after them. The first says what a measurement consists of, and the third of its three parts is the one most often dropped. The second says where the units come from — seven base units fixed by agreement, from which every other unit in chemistry is built.

Three parts, every time.

Chem 0086 The Three Parts of a Measurement

Every measurement carries three parts: a number giving magnitude, a unit giving the standard of comparison, and implied uncertainty. Reporting a dose as 100 milligrams states a magnitude and a unit; the number alone could be read as ten times what was intended. Dropping the unit is not a shortcut — a bare number can be dangerously ambiguous, as with a dose in milligrams or grams.

Seven units, fixed by agreement.

Chem 0090 SI Base Units as a System

The International System, S I, defines seven base units fixed by agreement, from which every other unit in chemistry is derived. The base units for length, mass, time and temperature combine to build a derived unit such as density, mass per volume. In casual speech S I units and metric units are treated alike — S I is the standardized subset chemistry relies on.

Four entries, and three of them tell one story. A base unit used to be an object in a vault or a fraction of a day; each is now fixed to a physical constant, so an equipped lab anywhere reproduces it instead of copying a standard. The fourth entry is a pair of temperature scales that share a degree size and disagree about where zero sits.

Length, from light.

Chem 0091 The Meter

The meter, S I's unit of length, is the distance light travels in a vacuum during a fixed fraction of a second. Because the defining constant is the speed of light, any lab with an accurate clock can reproduce the meter without a bar. Students often picture the meter as a physical bar in a vault — accurate until 1960, and not today.

Mass, since 2019.

Chem 0092 The Kilogram

The kilogram, S I's unit of mass, has been defined since 2019 by a fixed value of the Planck constant rather than by an object. Before 2019 it was tied to a platinum-iridium cylinder in France; today a lab with the right apparatus realizes the kilogram directly. Some references still describe it as a metal cylinder's mass — accurate before 2019, outdated now.

Time, by counting.

Chem 0093 The Second

The second, S I's base unit of time, is defined by counting a fixed number of radiation oscillations from a cesium atom. An atomic clock keeps time by counting those oscillations directly, with no mechanical or astronomical reference in the chain. The second is not any fraction of a day, although that is where its rough size historically came from.

Two scales, one degree size.

Chem 0094 The Kelvin and Celsius

The kelvin is S I's unit of temperature; Celsius shares its degree size but places zero at water's freezing point rather than at absolute zero. A change of one degree Celsius equals a change of one kelvin, and yet 0 degrees Celsius is roughly 273 kelvin, not 0 kelvin. Kelvin values are never written with the degree symbol Celsius and Fahrenheit carry.

Three entries carrying the base units out to the quantities a bench actually reports. The first is a procedure whose risk is direction rather than arithmetic. The other two build volume out of length, then density out of mass and volume — the first intensive property this part can calculate rather than only define.

Powers of ten, and which way.

Chem 0098 Converting Between Metric Prefixes

Converting between metric prefixes means multiplying or dividing by the power of ten each prefix represents, leaving the underlying base-unit quantity unchanged. 2,500 meters becomes 2.5 kilometers on dividing by one thousand; 3 milliliters becomes 3,000 microliters on multiplying by one thousand, since milli is one thousandth and micro is one millionth of the base unit. Direction trips up more students than the arithmetic — converting to a larger unit always shrinks the number, converting to a smaller unit always grows it, and a result that moved the wrong way signals a flipped conversion.

Volume, out of length.

Chem 0102 Volume as a Derived Unit

Volume is a derived S I quantity built from length, with the cubic meter as its standard. A liter is one cubic decimeter, the volume of a cube ten centimeters along each edge, derived entirely from the meter. Liters and milliliters feel like independent units, but both are only renamed fractions of the cubic meter.

Mass over volume.

Chem 0103 Density Defined

Density is the ratio of mass to volume, derived from the base units of mass and length. A block of lead and a block of aluminum can be the same size and differ greatly in density, because their masses differ. Density is intensive, not extensive — cutting a block of metal in half changes mass and volume equally and leaves the density unchanged.

Three entries that perform a measurement rather than define one. Two are worked procedures and they run long by design. Between them sits the technique that supplies a volume when no formula covers the object's shape — the same one the unit's capstone uses to identify a metal.

A density, worked twice.

Chem 0107 Calculating Density from Mass and Volume

Calculating density means measuring mass and volume separately — mass on a balance, volume by geometry or by displacement — then dividing mass by volume and carrying the unit through the division. The entry works a metal cube first and a liquid sample second, and the liquid's result comes out low enough to suggest an alcohol rather than water. The error to watch for is inversion: dividing volume by mass returns a reciprocal unit, cubic centimeters per gram instead of grams per cubic centimeter, and checking that the result reads mass over volume catches it immediately.

Volume when no formula fits.

Chem 0109 Volume by Water Displacement

The volume of an irregular object equals the rise in a liquid's level once the object is fully submerged, since it displaces its own volume. A stone lowered into a cylinder reading 25.0 milliliters raises the level to 32.0 milliliters, so the stone's volume is 7.0 milliliters. The object must stay fully submerged and must not dissolve or trap air, or the reading is unreliable.

One offset, both directions.

Chem 0111 Converting Between Temperature Scales

Converting between Celsius and Kelvin means adding or subtracting the fixed offset between their zero points, since the two scales share a degree size and differ only in where they start. 25 degrees Celsius plus roughly 273 gives 298 kelvin; 310 kelvin less the same offset gives 37 degrees Celsius, normal human body temperature, confirming the arithmetic runs both directions. Fahrenheit conversions are not a plain offset, because Fahrenheit degrees are a different size — the full formula scales by nine fifths, or five ninths in reverse, before shifting the zero point, and skipping that scaling factor gives a wrong result even when the offset direction is right.

Five things to carry out of Part Two. A physical change alters state or form and leaves the substance chemically the same; a chemical change leaves one or more different substances behind. How the change looks does not settle which happened — grinding shifts color and texture without touching composition, and food coloring changes a liquid's color by mixing alone. The test is whether chemical composition altered, and a chemical property shows itself only when the change is attempted or observed. An extensive property scales with the amount of matter present; an intensive property holds at any sample size. Because doubling a sample always changes an extensive value, mass on its own cannot identify a substance, while density — mass over volume, unchanged when a block is cut in half — can. Heat and temperature sit on opposite sides of that same line. Every measurement carries a number, a unit and implied uncertainty, and dropping the unit leaves a dangerously ambiguous number. S I fixes seven base units by agreement, and the meter, the kilogram and the second now rest on constants — the speed of light, a fixed Planck constant since 2019, and a count of cesium oscillations — so an equipped lab reproduces them instead of copying an object. Prefix conversion multiplies or divides by a power of ten and leaves the underlying quantity alone; direction is the part that goes wrong, since converting to a larger unit always shrinks the number. Celsius and kelvin differ by a fixed offset of roughly 273 and by nothing else. Fahrenheit is the exception: its degrees are a different size, so that conversion scales by nine fifths before shifting the zero. Density is mass divided by volume with the unit carried through the division. Mass comes off a balance; volume comes from geometry, or from displacement when no formula covers the object's shape. A result reading volume over mass means the ratio was inverted. Part Three takes up what those measured numbers are worth: significant figures, rounding, and the difference between precision and accuracy.