Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 16

Chemistry is a practical subject, and our present knowledge of chemical properties and principles is based on experiments. Unfortunately, we don't have space in this book to describe many of the amazing experiments that early investigators carried out to enhance our understanding and knowledge of chemistry. The majority of experiments require making and recording measurements. The laws of science operate across the globe, so it's important that scientists make measurements that can be compared with each other. Therefore, measurements must be recorded in a universal and standard way. For this reason, the metric system was developed to establish a standardised set of units. The metric system has its origins in the eighteenth century. More recently, a revised metric system was introduced and adopted by scientists across the world. This system is known as the Système Internationale d'Unités, and the units in the system are known as SI units.

There are seven fundamental SI units. Six of these are commonly used in chemistry, and you will meet all of them in this course. The six base units used in chemistry are listed in Table 0.1 and are the units of mass, length, time, electrical current, temperature, and amount of substance. All other units for physical quantities can be reduced to these base units. For example, speed is defined as the distance or length travelled divided by the time taken, so:

Table 0.1 Base SI quantities used in chemistry with symbols and units.

Physical quantity	Symbol	Base unit	Unit symbol
Mass	m	kilogram	kg
Length	l	metre	m
Time	t	second	s
Electrical current	I	ampere	A
Temperature	T	kelvin	K
Amount of substance	n	mole	mol

This defines the unit of speed as metre per second or m/s. In chemistry, as in most other scientific subjects, this would be written as: m s⁻¹, where the superscript ‘−1’ means ‘per second’.

There are many other units you will come across that can be reduced down to these base units. Units of this type are called derived units and usually have their own symbol. A list of derived units and their symbols is given in Table 0.2.

Dealing with exponents

Exponents tell us how many times a number should be multiplied by itself. For example:

A special case is a⁰ = 1 To multiply quantities with exponents just add the exponents together: To divide quantities with exponents subtract the exponents:

Table 0.2 Commonly used derived units.

Quantity	Unit name	Symbol and base units
Area	square metre	m2
Volume	cubic metre	m3
Velocity (speed)	metre per second	m s−1
Acceleration	metre per second squared	m s−2
Density	kilogram per cubic metre	kg m−3
Concentration	mole per cubic metre	mol m−3
Energy	joule	J = kg m2 s−2
Force	newton	N = J m−1 = kg m s−2
Pressure	pascal	Pa = N m−2 = kg m−1 s−2
Frequency	hertz	Hz = s−1

It is very important when carrying out calculations to keep track of the units, as you will be expected to quote the units of your answers. Units of each value in a calculation multiply or cancel with each other, as shown in the example here:

 Acceleration is defined as the change in velocity (speed) divided by the time taken and can be represented by this equation: acceleration = .

 Replacing the quantities with their units, we obtain: acceleration = .

 The unit can also be written as s−1, so the units for acceleration are m s−1 × s−1.

 To multiply s−1 by s−1, we simply add the −1 superscripts: acceleration = m s−1 × s−1 = m s−2.

 If we know the speed of an object and wish to determine how far it travels in a certain time, we multiply the speed by the time: distance = speed × time.

 Inserting the units for speed and time gives us the unit for distance = m s−1 × s.

 To multiply s−1 by s, again we add the superscripts: distance = m s−1 × s = m. This gives us the answer for distance in the correct units of metres. You should always check your answer when doing calculations to make sure that the value you obtain and the units are sensible.