# Measurement

In chemistry, time is measured to understand how fast a reaction progresses (the rate of the reaction).

Some reactions are fast (for example explosions) while others (rusting of iron in water) are very slow.

The International System of Units (SI) uses the **second** $$(s)$$ as the official unit of time. There are 60 seconds in a minute and 60 minutes in an hour.

Temperature is measured using a **thermometer**. Digital thermometers or glass thermometers are both commonly used.

A glass thermometer contains a thin glass column filled with a liquid, usually mercury or ethanol. The liquid expands when the temperature increases. The scale on the glass allows to read off the temperature.

In most thermometers, temperature is measured using the **Celsius** scale ($$^{\circ}\text{C}$$). In the US, the **Fahrenheit** scale ($$^{\circ}\text{F}$$) is more common.

However, the international standard (SI) unit for temperature is the **kelvin** ($$\text{K}$$). Note that no degree symbol ($$^{\circ}$$) is used with temperatures measured in kelvin. There is no such thing as $$^\circ\text{K}$$).

Adding **273.15** to a temperature in degree Celsius $$(^{\circ}\text{C})$$ converts it to Kelvin.

The mass of an object is the amount of matter in that object.

The SI unit for mass is the **kilogram** (kg).

The mass of one litre of water is one kilogram. That is how the kilogram was originally defined.

When small quantities are measured, it is often more convenient to measure the mass in **grams**. There are 1000 grams in 1 kilogram.

Mass is often measured using an **electronic scale**.

Volume measures the space occupied by a substance.

Volume is mainly used for measuring the amount of liquids and gases.

The official SI unit of volume is the cubic metre $$(\text{m}^3)$$.

Be careful when you use cubic metres with prefixes (like centi-, deci- etc.). Remember that the 'cubic' $$(^3)$$ not only applies to the unit ($$\text{m}$$), but also to the unit prefix (e.g. $$\text{ c }=$$ centi). So a cubic centimetre is not one hundredth of a cubic metre but one millionth $$(100^3=1,000,000)$$ of a cubic metre.

Unit | Length conversion |
---|---|

Metre | $$1 \text{ m}$$ |

Decimetre | $$1 \text{ m}=10 \text{ dm}$$ |

Centimetre | $$1 \text{ m}=100 \text{ cm}$$ |

Millimetre | $$1 \text{ m}=1,000 \text{ mm}$$ |

Unit | Volume conversion |

Cubic metre | $$1\text{ m}^3$$ |

Cubic decimetre | $$1\text{ m}^3=1,000 \text{ dm}^3$$ |

Cubic centimetre | $$1\text{ m}^3=1,000,000 \text{ cm}^3$$ |

Cubic millimetre | $$1\text{ m}^3=1,000,000,000 \text{ mm}^3$$ |

The litre is often a more convenient unit in laboratories than the cubic metre.

The symbol for the litre is $$\text{l}$$ (a lower-case L). It is often written as $$\text{ L}$$ or $$\ell$$ to distinguish it from the number $$1$$ or the upper-case letter $$\text{ i}$$.

An adult human being has a volume of 66 litres - $$66\ell= 0.066\text{ m}^3.$$

$$1 \text{ kg}$$ of water at room temperature has a volume of $$1\ell=0.001\text{ m}^3.$$

Prefixes for litres are simpler than for cubic metres. A millilitre ($$\text{ ml}$$) is one thousandth of a litre.

$$$ \begin{align*} 1 \text{m}^3&=1,000 \ell\\ 1 \text{dm}^3&=1 \ell\\ 1 \text{cm}^3&=1 \text{ m}\ell \end{align*}$$$ Volume can be converted to mass if the **density** is known.

$$$\text{Mass} = \text{Density} \times \text{Volume}$$$

The most appropriate instrument for measuring the volume of a liquid depends on the precision you need. Generally, thinner containers allow for more precise measurement. A slight increase in volume leads to a bigger rise of the liquid if it is held in a thin container.

A pipette is used to remove and transport a fixed volume of liquid from one container to another. Pipettes vary significantly in terms of accuracy and capacity.

Commonly pipettes come in standard sizes of $$5 \text{ cm}^3$$, $$10 \text{ cm}^3$$, $$25 \text{ cm}^3$$ amd $$50 \text{ cm}^3$$.

A burette is used to measure and dispense small precise volumes of liquid. The volume of liquids dispensed with a burette does not have to be a specific standard value like $$10 \text{ cm}^3$$, while basic pipettes often have just one specific volume.

A burette is a slim cylindrical glass container with markings. It is usually held on top of another container by a stand.

Liquid is filled in the burette from the opening at the top. The desired volume of the liquid is then dispensed from the bottom into the container below the burette by turning the **stopcock**.

Volume markings on the burette indicate the volume of liquid remaining in the burette. This volume decreases as more liquid is dispensed from the bottom.

A graduated cylinder measures a large volume of liquid.

Graduated cylinders are **less accurate** than burettes but are more convenient to use when a precise volume is not required. 'Graduated' means that it has 'a scale'.

A **gas syringe** is used to measure the volume of a gas produced during a reaction. Gas fills the syringe by pushing back the **plunger**. The location of the plunger indicates how much gas is in the syringe.

A gas released as a product in a chemical reaction can be collected and measured in a gas syringe.

In many reactions, a gas is released as a product. The amount of gas released is determined by measuring the volume of the released gas.

It is quite difficult to determine the mass of a gas, so volume is a more convenient measure.

Titration is a method for finding the concentration of a solution using another solution of known concentration.

The titrant is the solution of known concentration and is placed in a burette.

The analyte is the solution of unknown concentration and is placed in a conical flask. A pipette is used to measure and dispense the solution.

The titrant is added to the analyte until all of the analyte has reacted.

A change in **pH** (for acid-base reactions) or a change in **electric potential** (for redox reactions) usually indicates that all of the analyte has reacted.

At this point (known as the **equivalence point**), the total volume of titrant dispensed is recorded. This value is used to calculate the number of molecules of titrant and analyte that reacted.

The concentration of the original analyte solution can then be determined.