Experimental Chemistry: Measurement and Separation Techniques

Physical Quantities and Their Measurement

The International System of Units, known as the SI units, provides a common standard for recording scientific measurements in the chemistry laboratory. Time measures how long it takes for a specific event or process to occur, and its primary SI unit is the second, represented by the symbol $s$. Other common units for time include the minute, symbol $\text{min}$, and the hour, symbol $h$. Conversions between these units follow the standard ratios: $1\,\text{min} = 60\,s$ and $1\,h = 60\,\text{min}$. Calculating the total seconds in an hour requires multiplying these factors: $60 \times 60 = 3600\,s$.

Temperature is defined as the measure of how hot or cold an object or substance is. The official SI unit for temperature is the kelvin, symbol $K$. However, the degree Celsius, symbol $^{\circ}C$, is more commonly used in daily laboratory settings. To convert a value from the Celsius scale to the kelvin scale, the value $273$ is added to the Celsius temperature. Therefore, the formula for conversion is identified as: $\text{Temperature in } K = \text{Temperature in } ^{\circ}C + 273$.

Length measures the distance between two distinct points, using the metre, symbol $m$, as the SI unit. Common sub-units and multiples include the centimetre ($cm$) and the decimetre ($dm$). The conversion factors are structured such that $1\,m = 10\,dm = 100\,cm = 1000\,mm$. Mass represents the total amount of matter present within a substance, with the kilogram, symbol $kg$, serving as the SI unit. Smaller masses are often measured in grams ($g$) or milligrams ($mg$), while larger quantities are measured in tonnes ($t$) or kilotonnes ($kt$). The conversion scales are: $1\,kg = 1000\,g = 1\,000\,000\,mg$. Additionally, $1\,t = 1000\,kg = 1\,000\,000\,g$ and $1\,kt = 1000\,t = 1\,000\,000\,kg$.

Volume describes the total amount of three-dimensional space that a substance occupies. The standard SI unit for volume is the cubic metre, symbol $m^3$. In most laboratory applications, volume is measured in cubic decimetres ($dm^3$) or cubic centimetres ($cm^3$). These units are related by the following equalities: $1\,m^3 = 1000\,dm^3 = 1\,000\,000\,cm^3$.

Measuring Time, Temperature, and Mass

Apparatus for measuring time varies in precision and mechanism. A digital stopwatch can measure time with an accuracy of $\pm 0.01\,s$, which represents a hundredth of a second. An analogue stopwatch provides a measurement accuracy of $\pm 0.01\,s$, though this is explicitly referred to as a tenth of a second in specific laboratory standards. Temperature measurement utilizes various instruments including alcohol thermometers, digital laboratory thermometers, oral digital thermometers, and sophisticated data loggers equipped with temperature sensors. Examples of observed digital readings include values such as $38.8\,^{\circ}C$. Alcohol thermometers typically feature a bulb and an alcohol thread that expands through a capillary tube to indicate heat levels.

Mass is determined using either a beam balance or an electronic balance. A beam balance utilizes calibration marks and counterweights to find equilibrium, whereas an electronic balance provides a digital readout after pressing the tare button to zero the scale. Modern electronic balances feature modes for calibration and varied units of measurement.

Measurement of Volume for Liquids and Gases

Specific apparatus is selected based on the required accuracy and the nature of the liquid volume being measured. A pipette is designed to measure accurate fixed volumes, typically in specific increments such as exactly $10.0\,cm^3$ or $25.0\,cm^3$. A volumetric flask is used to measure accurate fixed volumes that are larger in scale, such as $100\,cm^3$ or $250\,cm^3$. For measuring a range of volumes rather than a fixed amount, a measuring cylinder is employed, which can measure to the nearest $0.5\,cm^3$. Examples of volumes measured with a measuring cylinder include $31.5\,cm^3$ or $23.0\,cm^3$. A burette provides higher precision for a range of volumes, measuring to the nearest $0.05\,cm^3$, with typical readings recorded as $31.55\,cm^3$ or $23.00\,cm^3$. For the measurement of gas volumes, a gas syringe is the standard instrument used to collect and quantify variable amounts of gas released during a reaction.

Methods of Gas Collection and Drying

Gases are collected using methods that depend on their physical properties, specifically their solubility in water and their density relative to air. The displacement of water is a common technique for collecting gases. When collecting gases by air displacement, two primary orientations are used: upward delivery and downward delivery. The choice between these methods is determined by whether the gas is lighter or heavier than the surrounding air. To ensure gases are free of moisture, various drying methods are utilized, involving apparatus such as wash bottles or drying tubes that contain specific chemicals to absorb water vapor from the gas stream.

Principles of Separation and Purification

A mixture is a combination of two or more substances that are not chemically combined. Because they are not chemically bonded, the physical properties of the individual substances determine which separation techniques are appropriate. These properties include particle size, solubility, magnetism, boiling points, and states of matter. Mixtures are broadly categorized as solid-solid, solid-liquid, or liquid-liquid combinations.

Separation of Solid-Solid Mixtures

Magnetic attraction is a technique used to separate magnetic substances from non-magnetic ones. Key magnetic substances include iron, cobalt, nickel, and certain alloys. This method is used practically in sorting waste in landfills. Sieving is a physical process used to separate solids with different particle sizes. A sieve allows smaller particles to pass through while retaining larger materials.

Solids can also be separated by using suitable solvents based on their differing solubilities. In this context, a solvent is a liquid that dissolves a solid, and a solute is the solid that is being dissolved. This process relies on the fact that one solute may dissolve in a specific solvent while another remains insoluble. Sublimation is another specialized method used when one substance in a solid mixture has the unique property of changing from a solid state directly into a gaseous state upon heating, bypassing the liquid phase.

Separation of Solid-Liquid Mixtures

Filtration is the standard method for separating an insoluble solid from a liquid. An insoluble solid is defined as one that is unable to dissolve in the liquid it is placed in. During this process, the liquid that passes through the filter is called the filtrate, while the solid material that remains trapped on the filter is known as the residue.

Evaporation to dryness is used to separate a dissolved solid from its solvent by heating the mixture until all the liquid solvent has vaporised, leaving only the solid behind. Crystallisation is a more refined method used to separate a pure solid from its saturated solution. A saturated solution is one in which no more solute can be dissolved at a given temperature. Finally, simple distillation is used to separate and recover a pure solvent (the liquid) from a solution containing dissolved solids.

Separation of Liquid-Liquid Mixtures

Liquid mixtures are classified as either homogeneous or heterogeneous. A homogeneous mixture consists of miscible liquids that form a uniform solution. A heterogeneous mixture contains immiscible liquids that do not form a uniform solution and instead separate into distinct layers known as phases.

A separating funnel is the primary tool used to separate immiscible liquids by allowing the denser phase to be drained out of the bottom. Fractional distillation is utilized to separate miscible liquids that have different boiling points. This method has critical industrial applications, including the separation of crude oil in oil refineries, the separation of liquefied air into different gases, and the separation of ethanol after the fermentation of glucose.

Applications and Theory of Chromatography

Chromatography is used to separate a mixture of substances based on their different solubilities in a given solvent. The retention factor, or $R_f$ value, of a substance is a numerical measure of its solubility and can be calculated using the following equation: $R_f = \frac{\text{distance travelled by the substance}}{\text{distance travelled by the solvent}}$ . Calculating $R_f$ values allows for the comparison of chromatograms that have been run for different durations, provided they use the same solvent and temperature conditions.

When chromatography is performed on colourless substances, locating agents must be used. These are chemicals that react with the colourless substances to produce visible, coloured spots. Chromatography is widely applied to identify unauthorized substances such as pesticides or poisons in food, detect banned substances in the blood or urine of athletes, and separate biological components like DNA fragments for forensic investigations.

Determination of Substance Purity

Determining the purity of substances is vital for various applications in everyday life. A pure substance is characterized by a specific, fixed melting or boiling point under constant conditions. In contrast, mixtures do not have a sharp melting or boiling point, instead melting or boiling over a range of temperatures.

The impact of impurities on these fixed points is significant. For example, pure water has a boiling point of $100\,^{\circ}C$ and a freezing point of $0\,^{\circ}C$. Adding salt to water creates a mixture that raises the boiling point and lowers the freezing point. This principle is utilized in countries with cold climates, where salt is spread on roads during winter to prevent the formation of ice after snowfall or rain, thereby maintaining road safety.