Time: The Second

Introduction

For most of history, time was measured by the sky: a day was one turn of the Earth, and a second was a tiny fraction of that day. But the Earth's spin is not perfectly steady — it wobbles and gradually slows. Modern science needed something far more reliable, and found it inside the atom. Today the second is the most precisely realised of all SI units, and timing is central to chemistry, from how fast a reaction goes to how long a sample is heated.

Core explanation

The definition. The second (symbol s) is the SI base unit of time. It is defined by fixing the frequency of a particular transition in the caesium-133 atom at exactly 9 192 631 770 hertz. In simple terms, caesium atoms can absorb microwave radiation of one very exact frequency. One second is the time taken for 9 192 631 770 cycles of that radiation.

From astronomy to atoms. The second was originally 1/86 400 of a mean solar day (24 × 60 × 60 = 86 400). Because the Earth's rotation varies, astronomers later based it on the Earth's orbit around the Sun. In 1967 it was redefined using caesium atoms, because every caesium-133 atom is identical and behaves the same way everywhere.

Atomic clocks. An atomic clock tunes a microwave signal until caesium atoms absorb it most strongly, then counts the cycles. The best caesium clocks would drift by less than one second in tens of millions of years. Newer optical clocks, using other atoms and visible light, are even more stable.

Non-SI units of time. Minutes, hours and days are not SI units, but they are accepted for use with SI because they are so familiar. 1 min = 60 s, 1 h = 3600 s and 1 day = 86 400 s. Note that these conversions are not decimal, so care is needed. For very short times, prefixes are used: 1 ms = 10⁻³ s, 1 μs = 10⁻⁶ s, 1 ns = 10⁻⁹ s.

Time in chemistry. Chemists time many processes: - How long it takes a solution to turn cloudy, or a colour to change. - The time for a gas syringe to collect a certain volume of gas. - How long a substance is heated or stirred. - Very fast events: some molecular vibrations last only about 10⁻¹⁴ s, studied with ultrashort laser pulses.

In school experiments a stopwatch reading to 0.01 s is common, but human reaction time — typically around 0.2 s — is usually a bigger source of uncertainty than the stopwatch itself. Data loggers with light or pressure sensors can remove this human error.

Rate needs time. The rate of a reaction is how much a quantity changes per unit time, for example cm³ of gas per second (cm³/s). Without reliable time measurement, rates could not be compared.

Step-by-step reasoning

To time a colour change in a reaction fairly:

1. Decide exactly what event marks the end, such as a cross under the flask disappearing from view. 2. Start the timer at the moment the reactants are mixed. 3. Watch continuously from the same position. 4. Stop the timer when the chosen end point is reached. 5. Record the time in seconds and repeat to check consistency.

Visual explanation

Picture a wave of microwave radiation next to a clock face. Count the peaks of the wave: after 9 192 631 770 peaks, the second hand moves on by one tick. The clock is just a very fast, very reliable counter of atomic "heartbeats".

Real-world analogy

An atomic clock is like counting the beats of a perfect drummer who never speeds up or slows down. Knowing exactly how many beats make one second, you can measure any time interval by counting beats, and every drummer of the same kind keeps exactly the same rhythm.

Real-world example

Mobile phone networks, the internet and satellite navigation all depend on atomic time. Navigation satellites carry atomic clocks because a position is found by timing signals; an error of one microsecond would place you hundreds of metres from where you really are.

Why?

Why use caesium atoms rather than a pendulum or a quartz crystal? A pendulum changes with temperature and location, and quartz crystals differ slightly from one another and age. All caesium-133 atoms are identical, so their natural frequency is the same everywhere and never changes.

Common misconception

"The minute is the SI unit of time because it is used most often." The SI base unit is the second. Minutes and hours are accepted alongside SI, but in calculations — especially of rates — times should normally be converted into seconds.

Worked example

Question: A reaction takes 2 minutes 35 seconds to produce 50 cm³ of gas. Find the time in seconds and the average rate of gas production.

Reasoning: 2 min = 2 × 60 = 120 s, so total time = 120 + 35 = 155 s. Average rate = volume ÷ time = 50 ÷ 155 = 0.32 cm³/s.

Answer: 155 s; about 0.32 cm³/s.

Quick check

1. Which atom is used to define the second? Answer: Caesium-133.

Exam focus

Convert minutes to seconds before calculating rates, and remember time conversions use 60, not 100. When evaluating an experiment, you may be asked to name human reaction time as a source of error and suggest a data logger or light sensor as an improvement.

Advanced insight

Because time can be measured so precisely, other units are linked to it. The metre is defined through the second and the speed of light, and the definitions of the kilogram, ampere, kelvin and candela also involve the second. Improving clocks therefore improves the whole SI system, and scientists expect a future redefinition of the second based on optical clocks.

Summary

The second is the SI base unit of time, defined by fixing the caesium-133 transition frequency at exactly 9 192 631 770 Hz. Atomic clocks realise it with extraordinary stability. Minutes and hours are non-decimal units accepted with SI. In chemistry, accurate timing underpins reaction rates, and human reaction time is often the largest timing uncertainty.

Practice questions

1. Convert 1.5 hours into seconds. Answer: 1.5 × 3600 = 5400 s. 2. Explain why caesium atoms make a better time standard than the Earth's rotation. Answer: All caesium-133 atoms are identical and their frequency never changes, while the Earth's rotation varies and gradually slows. 3. A student uses a stopwatch reading to 0.01 s to time a reaction. Why is the true uncertainty larger than 0.01 s? Answer: Human reaction time in starting and stopping the watch, typically about 0.2 s, is much larger than the stopwatch resolution. 4. What is 5 ms in seconds? Answer: 0.005 s (5 × 10⁻³ s).