Science relies on measurements that are both precise and accurate. This is especially important in chemistry where chemical reactions need to be as controlled as possible to get accurate results.
To achieve this, scientists have devised a set of standard measurements that are internationally valid called the SI, or Système International, which is based on the metric system.
Chemistry uses SI units in many of its measurements and standard constants. This makes it easier for research chemists to communicate and share knowledge, and it overcomes language barriers.
Having a set of common measurements is also crucial for industrial chemists to establish efficient and cost-effective manufacturing methods. Without standardised measurements, chemists would have difficulty reporting and understanding research results published by their peers.
Common quantities that are measured in chemistry include mass, volume, temperature, acidity-basicity, and matter content. Continue reading to find out more about each type of common measurement used in chemistry.
In this post:
The Importance of Accurate Measurements
Accuracy refers to the correctness of a measurement. It means that it is actually measuring the intended parameter or quantity.
For example, if you’re measuring the mass of a rock, you should consider whether the rock is magnetic or not. If it is, any strong magnet near the rock can affect its measured mass – and that means it wouldn’t be accurate.
Instrumentation is also crucial in accurate measurements. You should always properly calibrate your instruments when taking measurements.
Accurate measurements provide useful and reliable data that you can use. Measurements that are both accurate and precise will provide better analytical insights and exact synthetic results. Accuracy means that your conclusions from the data you have are more likely to be correct.
While precision is often related to accuracy, it has more to do with how sensitive your measurements are; it’s the degree of refinement of measurements. However, this doesn’t necessarily mean that a precise measurement is also accurate. That all depends on the calibration of instruments based on a standard range.
For example, when you’re using a digital thermometer, you might need to calibrate it using a mercury thermometer or an analog thermometer to check it’s accurate.
Basic Units of Measurement
Just like in other sciences, units of measurement in chemistry can either be basic or derived.
Basic units, also known as base units, are simple quantifiers and can be directly assessed as they only have one parameter each. These include mass, volume, and temperature.
For reference purposes, the seven SI base units are the following:
- Metre (m) – for measuring length
- Second (s) – for measuring time
- Mole (mole) – for measuring the amount of substance
- Ampere (A) – for measuring the amount of electrical current
- Kelvin (K) – for measuring temperature
- Candela (cd) – for measuring luminous intensity
- Kilogram (kg) – for measuring mass
Meanwhile, derived units are measures that are algebraically expressed using combinations of base units. These include density, energy, force, electric charge, and electric resistance.
Measuring Mass: Grams and Kilograms
The base mass unit used in chemistry is the kilogram, which is equivalent to 1,000 grams.
Typically, for measuring small amounts of chemicals in a laboratory setting, the gram is used. However, when it comes to converting to derived units, the kilogram is used.
Balance beams are used for measuring mass instead of spring balance or pressure balances. This is to avoid inaccuracy in terms of distinguishing between mass and weight. Weight is dependent on the force of gravity, so if you use a spring balance, for example, the weight of an object at sea level is different from its weight on top of a mountain.
Volume: Litres and Millilitres
Volume is how much space that matter takes up. In chemistry, volume is measured in terms of litres and millilitres. The litre is the SI base unit, and one litre is equivalent to 1,000 millilitres.
It’s interesting to note that one litre of pure water has a mass of almost exactly one kilogram at its maximal density at 4°C. Graduated cylinders are typically used to measure volume in chemistry.
Molar Concentration: Moles per Litre
A mole is the base unit for an amount of a substance. Molar concentration or molarity, on the other hand, is a derived unit measured as moles per litre.
Mole is the quantity of elementary entities of a substance, which can be atoms, molecules, ions, electrons, or other types of particles.
One mole is exactly 6.02214076×10²³ elementary entities. This number is also known as Avogadro’s number, which was experimentally determined. It is the amount of elementary entities of a substance based on the unit mass of one of those elementary entities when converted to one gram.
A good example of this is oxygen, which has an atomic mass of 15.9994 atomic mass units, or approximately 16 atomic mass units. However, since elemental oxygen gas exists as a diatomic substance, one mole of oxygen gas has a weight of 32 grams.
If you want to measure the concentration of oxygen in 1 kg of air, you need to express it in terms of moles per litre. You first need to find out what percentage of air is composed of oxygen. This is experimentally measured as approximately 21%. Therefore, 1 kg of air has 0.21 kg (210 grams) of diatomic oxygen or 210 grams/32 grams (1 mole of diatomic oxygen) = 6.56 moles in 1 kg of air.
Under standard temperature and pressure (STP: 273.15 degrees Kelvin and 101.325 kilopascal) only about 0.0146 mg/L of oxygen can be dissolved in water. This means that one litre of water typically has about 4.56 x 10-4 M oxygen concentration.
Temperature Measurements in Chemistry
Technically speaking, the temperature of a substance is actually the indirect measurement of its particles’ kinetic energy.
Historically, temperature scales were invented based on the freezing point of water and salt-water. Water was used as a standard because it is commonly used as a solvent in chemical reactions.
That means temperature measurements in chemistry can be conveniently done in many cases by comparing the temperature changes of chemical reactions in aqueous solutions.
Celsius, Fahrenheit, and Kelvin
The Celsius scale was invented by Anders Celsius in 1742, and is based on the freezing and boiling points of pure water. The freezing point is set as 0°C, while the boiling point is set as 100°C.
The Fahrenheitbscale was invented by Daniel Gabriel Fahrenheit in 1724. It was based on the temperature of the ice-salt mixture and normal body temperature.
The ice-salt mixture was set at 0°F, while the normal body temperature was set at 90°F, which was finally adjusted to 98.6°F. The freezing and boiling points of water are at the awkward temperatures of 32°F and 212°F, respectively.
That means the conversion of Fahrenheit to Celsius was equally awkward using the following formula:
°F = (9/5 × °C) + 32
The Kelvin scale is roughly patterned on the Celsius metric scale, but with absolute zero as the basis. It was invented by William Thompson (a.k.a. Lord Kelvin) in 1848.
The scale is based on the concept of absolute zero where no energy remains in an atom and the triple point of water. The latter is the state of water where the solid, liquid, and gaseous phases are equal. The theoretical equivalent value of absolute zero is -273°C.
The Role of Temperature in Chemical Reactions
The temperature of a system may either accelerate or slow down the rate of chemical reactions.
Generally, the rate of chemical reactions is directly proportional to temperature. Higher temperatures facilitate faster rate of reaction such as in the case of combustion.
Find out more about Why Temperature Control in Chemistry is Important here.
Advanced Chemical Measurements
In many cases of chemical analysis and synthesis, more advanced measurements are needed. These include both derived units and indirect measurements and require more sophisticated instruments and an understanding of deeper concepts in chemistry.
pH Scale
Unlike many other measurement scales, the pH scale has no unit. It’s ‘simply’ a comparative scale based on the negative logarithm of hydrogen ion concentration.
The pH scale is usually illustrated as ranging from zero to 14, but it could actually go beyond 14 and way below zero. Seven is neutral, below it is acidic and above it is basic or alkaline.
Conductivity
In chemistry, conductivity is typically measured in ionic solutions. It’s a measure of how an electrolyte can facilitate the movement of electrical current.
The SI unit of conductivity is Siemens per metre (S/m), although milli-Siemens per metre (mS/m) is more commonly used. Conductance is reciprocal to resistance measured in Ohm.
Spectrophotometry
Various substances have a specific range of light absorption capacity, which serves as a chemical signature.
Spectrophotometry is a method of measuring a substance’s light absorption capacity.
A light beam with predetermined intensity is shone on a sample solution, and a certain range of wavelengths will be absorbed and transmitted in the process. This is an indirect analytical tool for determining the type of substance in a solution.
Spectrophotometry is a technique used in spectroscopy, which can also be used to analyse chemicals in distant objects, such as stars and interstellar gases.
Measurement Tools & Equipment
Many school chemistry labs have the basic equipment necessary for measuring various quantities, which can be used either directly or indirectly to measure certain parameters.
Some don’t require specialised knowledge or setup, but others are more sophisticated. They can either be digital or analog tools, as we’ll see below.
Common Laboratory Instruments
Many standard chemistry labs have all the basic measuring tools for most chemical experiments. These tools correspond to the most common parameters that are measured, including mass, volume, temperature, pH, and electrical current.
Some of the most common instruments include:
- Weighing scales, such as triple beam balance for measuring mass
- Graduated cylinders for measuring volumes
- Thermometers for measuring temperature
- pH metres for measuring alkalinity and acidity
- Electrical metres, such as a voltmeter, galvanometer, and ohmmeter
More advanced tools and equipment, such as spectrometers, ADM flow metre for gas chromatography, and molecular spectroscopy equipment are necessary for more sophisticated research.
Digital vs. Analog Measurement Tools
Laboratory measurement tools can be classified either as digital or analog devices.
Analog devices predate digital devices and rely on direct physical actions of their mechanisms in relation to the parameters being measured. An example of an analog device is a mercury thermometer. The expansion and contraction of the metallic mercury as related to temperature change has corresponding units of measurements.
Digital devices rely on the electronic signals from sensors, which are then translated into digital readouts or printouts. These devices typically have LCD screens that display the measurements such as pH, temperature, and pressure.
Conclusion
Measurements in chemistry include both base units and derived units. Various instruments can be used to measure certain parameters. Some require less direct approach and calculations. Both accuracy and precision are important in deriving reliable data and analysis. Your insights and conclusions are only as valid as the reliability of the measurements, which in turn are dependent on the instruments used.
