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The SI base unit of time is the second (s)    . Small and large time intervals can be expressed with the appropriate prefixes; for example, 3 microseconds = 0.000003 s = 3 × 10 −6 and 5 megaseconds = 5,000,000 s = 5 × 10 6 s. Alternatively, hours, days, and years can be used.

Derived si units

We can derive many units from the seven SI base units. For example, we can use the base unit of length to define a unit of volume, and the base units of mass and length to define a unit of density.


Volume is the measure of the amount of space occupied by an object. The standard SI unit of volume is defined by the base unit of length ( [link] ). The standard volume is a cubic meter (m 3 )    , a cube with an edge length of exactly one meter. To dispense a cubic meter of water, we could build a cubic box with edge lengths of exactly one meter. This box would hold a cubic meter of water or any other substance.

A more commonly used unit of volume is derived from the decimeter (0.1 m, or 10 cm). A cube with edge lengths of exactly one decimeter contains a volume of one cubic decimeter (dm 3 ). A liter (L)    is the more common name for the cubic decimeter. One liter is about 1.06 quarts.

A cubic centimeter (cm 3 ) is the volume of a cube with an edge length of exactly one centimeter. The abbreviation cc (for c ubic c entimeter) is often used by health professionals. A cubic centimeter is also called a milliliter (mL)    and is 1/1000 of a liter.

Figure A shows a large cube, which has a volume of 1 meter cubed. This larger cube is made up of many smaller cubes in a 10 by 10 pattern. Each of these smaller cubes has a volume of 1 decimeter cubed, or one liter. Each of these smaller cubes is, in turn, made up of many tiny cubes. Each of these tiny cubes has a volume of 1 centimeter cubed, or one milliliter. A one cubic centimeter cube is about the same width as a dime, which has a width of 1.8 centimeter.
(a) The relative volumes are shown for cubes of 1 m 3 , 1 dm 3 (1 L), and 1 cm 3 (1 mL) (not to scale). (b) The diameter of a dime is compared relative to the edge length of a 1-cm 3 (1-mL) cube.


We use the mass and volume of a substance to determine its density. Thus, the units of density are defined by the base units of mass and length.

The density    of a substance is the ratio of the mass of a sample of the substance to its volume. The SI unit for density is the kilogram per cubic meter (kg/m 3 ). For many situations, however, this as an inconvenient unit, and we often use grams per cubic centimeter (g/cm 3 ) for the densities of solids and liquids, and grams per liter (g/L) for gases. Although there are exceptions, most liquids and solids have densities that range from about 0.7 g/cm 3 (the density of gasoline) to 19 g/cm 3 (the density of gold). The density of air is about 1.2 g/L. [link] shows the densities of some common substances.

Densities of Common Substances
Solids Liquids Gases (at 25 °C and 1 atm)
ice (at 0 °C) 0.92 g/cm 3 water 1.0 g/cm 3 dry air 1.20 g/L
oak (wood) 0.60–0.90 g/cm 3 ethanol 0.79 g/cm 3 oxygen 1.31 g/L
iron 7.9 g/cm 3 acetone 0.79 g/cm 3 nitrogen 1.14 g/L
copper 9.0 g/cm 3 glycerin 1.26 g/cm 3 carbon dioxide 1.80 g/L
lead 11.3 g/cm 3 olive oil 0.92 g/cm 3 helium 0.16 g/L
silver 10.5 g/cm 3 gasoline 0.70–0.77 g/cm 3 neon 0.83 g/L
gold 19.3 g/cm 3 mercury 13.6 g/cm 3 radon 9.1 g/L

While there are many ways to determine the density of an object, perhaps the most straightforward method involves separately finding the mass and volume of the object, and then dividing the mass of the sample by its volume. In the following example, the mass is found directly by weighing, but the volume is found indirectly through length measurements.

Questions & Answers

what are oxidation numbers
Idowu Reply
pls what is electrolysis
Idowu Reply
Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them. ... Electricity is the flow of electrons or ions. For electrolysis to work, the compound must contain ions.
what is the basicity of an atom
Eze Reply
basicity is the number of replaceable Hydrogen atoms in a Molecule. in H2SO4, the basicity is 2. in Hcl, the basicity is 1
how to solve oxidation number
Mr Reply
mention some examples of ester
Chinenye Reply
do you mean ether?
what do converging lines on a mass Spectra represent
Rozzi Reply
would I do to help me know this topic ?
what the physic?
Bassidi Reply
who is albert heistein?
similarities between elements in the same group and period
legend Reply
what is the ratio of hydrogen to oxulygen in carbohydrates
Nadeen Reply
what is poh and ph
Amarachi Reply
please what is the chemical configuration of sodium
2, 6, 2, 1
1s2, 2s2, 2px2, 2py2, 2pz2, 3s1
what is criteria purity
Austin Reply
cathode is a negative ion why is it that u said is negative
Michael Reply
cathode is a negative electrode while cation is a positive ion. cation move towards cathode plate.
CH3COOH +NaOH ,complete the equation
david Reply
compare and contrast the electrical conductivity of HCl and CH3cooH
Sa Reply
The must be in dissolved in water (aqueous). Electrical conductivity is measured in Siemens (s). HCl (aq) has higher conductivity, as it fully ionises (small portion of CH3COOH (aq) ionises) when dissolved in water. Thus, more free ions to carry charge.
HCl being an strong acid will fully ionize in water thus producing more mobile ions for electrical conduction than the carboxylic acid
differiante between a weak and a strong acid
how can I tell when an acid is weak or Strong
an aqueous solution of copper sulphate was electrolysed between graphite electrodes. state what was observed at the cathode
Bakanya Reply
write the equation for the reaction that took place at the anode
what is enthalpy of combustion
Enthalpy change of combustion: It is the enthalpy change when 1 mole of substance is combusted with excess oxygen under standard conditions. Elements are in their standard states. Conditions: pressure = 1 atm Temperature =25°C
Observation at Cathode: Cu metal deposit (pink/red solid).
Equation at Anode: (SO4)^2- + 4H^+ + 2e^- __> SO2 + 2H2O
Equation : CuSO4 -> Cu^2+ + SO4^2- equation at katode: 2Cu^2+ + 4e -> 2Cu equation at anode: 2H2O -> 4H+ + O2 +4e at the anode which reacts is water because SO4 ^ 2- cannot be electrolyzed in the anode

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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