General Chemistry for Safety

1. Why General Chemistry Matters for the Safety Professional

Nearly every serious workplace hazard outside of mechanical and ergonomic injury has a chemical foundation. Fires and explosions are rapid oxidation reactions. Toxic exposures are governed by how a substance vaporizes, dissolves, and reacts inside the body. Corrosive burns depend on pH and concentration. Confined space fatalities are often caused by displacement of oxygen, hydrogen sulfide generation, or carbon monoxide from incomplete combustion. Reactive incidents (runaway polymerization, water-reactive releases, incompatible storage) have killed workers at countless facilities.

The BCSP places General Chemistry for Safety within the Industrial Hygiene and Occupational Health domain of the ASP blueprint because an Associate Safety Professional must be able to read a Safety Data Sheet (SDS) critically, anticipate how a chemical will behave when released, convert exposure measurements between units, understand flammability data, recognize incompatible chemicals, and communicate hazards accurately to workers. You do not need to be a chemist, but you must speak the language fluently enough to make sound risk decisions.

2. What General Chemistry for Safety Is

For ASP purposes, general chemistry is the set of foundational concepts that describe the composition, properties, and behavior of matter as they relate to hazard recognition, evaluation, and control. The core content areas are:

• Atomic structure, elements, compounds, and mixtures
• The periodic table and chemical families
• Chemical bonding (ionic, covalent, metallic) and why it matters for reactivity
• States of matter and phase changes
• The mole concept, molecular weight, and stoichiometry
• Gas laws and vapor behavior
• Solutions, concentration units, and dilution
• Acids, bases, pH, and corrosivity
• Oxidation-reduction and combustion chemistry
• Flammability properties (flash point, LEL/UEL, autoignition, vapor density)
• Chemical reactivity and incompatibility
• Basic organic chemistry and functional groups
• Hazard communication systems (GHS, NFPA 704, HMIS)

3. How It Works: Core Concepts Explained

3.1 Atoms, Elements, Compounds, and Mixtures
An atom is the smallest unit of an element that retains its chemical identity; it consists of protons and neutrons in the nucleus with electrons in orbitals around it. The atomic number equals the number of protons and defines the element. The atomic mass (mass number) is approximately protons plus neutrons. An element cannot be broken down chemically (e.g., chlorine, Cl). A compound is two or more elements chemically bonded in fixed proportions (e.g., sodium chloride, NaCl; water, H2O). A mixture contains substances that are physically combined but not chemically bonded, and can be separated physically (e.g., air, gasoline, most solvent blends in the workplace). Most workplace products are mixtures, which is why the SDS lists multiple ingredients.

3.2 The Periodic Table
Elements are arranged by increasing atomic number. Groups (columns) share similar chemical behavior because they have the same number of valence (outer-shell) electrons. Key families for safety:
• Alkali metals (Group 1: Li, Na, K) are extremely reactive, especially with water, producing hydrogen gas and heat; sodium and potassium can ignite on contact with moisture.
• Alkaline earth metals (Group 2: Mg, Ca) are reactive; magnesium burns intensely and cannot be extinguished with water or CO2.
• Halogens (Group 17: F, Cl, Br, I) are strong oxidizers and highly toxic gases or volatile liquids; fluorine is the most reactive element.
• Noble gases (Group 18: He, Ne, Ar) are chemically inert but are simple asphyxiants that displace oxygen in enclosed spaces.
• Transition and heavy metals (Cr, Ni, Cd, Pb, Hg) are of concern for chronic toxicity, carcinogenicity, and sensitization.

3.3 Chemical Bonding
Ionic bonds form when electrons transfer from a metal to a nonmetal (NaCl). Ionic compounds are typically solids with high melting points, dissolve in water to form conductive solutions, and are generally non-flammable. Covalent bonds form when atoms share electrons (most organic compounds, water, CO2). Covalent compounds are often gases or volatile liquids with lower boiling points, and the majority of flammable and toxic vapors encountered in industry are covalent organic molecules. Polarity matters: polar molecules (water, alcohols) dissolve polar substances, while nonpolar molecules (hydrocarbons) dissolve nonpolar substances, summarized as like dissolves like. This principle explains why nonpolar solvents defat skin and why lipid-soluble chemicals cross the skin and accumulate in fatty tissue.

3.4 States of Matter and Phase Changes
Matter exists as solid, liquid, or gas (plasma is a fourth state). Phase changes include melting, freezing, vaporization (evaporation/boiling), condensation, and sublimation (solid directly to gas, as with dry ice). Important terms:
• Vapor: the gaseous form of a substance that is normally a liquid or solid at room temperature.
• Gas: a substance that exists in the gaseous state at normal temperature and pressure.
• Boiling point: the temperature at which vapor pressure equals atmospheric pressure. Lower boiling point means higher volatility and greater inhalation hazard.
• Vapor pressure: the pressure exerted by a vapor in equilibrium with its liquid; higher vapor pressure means faster evaporation. Substances with vapor pressure above about 760 mmHg at room temperature are gases.
• Melting point and freezing point are relevant to storage conditions and to the hazards of cryogenic liquids.

3.5 The Mole and Molecular Weight
A mole is 6.022 × 10^23 particles (Avogadro's number). The molecular weight (MW) of a compound is the sum of the atomic weights of its atoms, expressed in grams per mole. Example: water (H2O) = 2(1.008) + 16.00 ≈ 18 g/mol. Carbon dioxide (CO2) = 12.01 + 2(16.00) ≈ 44 g/mol. The MW is essential for converting between ppm and mg/m3 and for assessing vapor density. One mole of any ideal gas occupies 22.4 liters at STP (0°C, 1 atm) and 24.45 liters at NTP (25°C, 1 atm). Industrial hygiene calculations almost always use 24.45 because occupational exposure limits are referenced to 25°C.

3.6 Gas Laws
Gases are the most mobile and often the most immediately dangerous form of a chemical. Key relationships:
• Boyle's Law: at constant temperature, pressure and volume are inversely related (P1V1 = P2V2). A compressed gas cylinder released to atmosphere expands enormously.
• Charles's Law: at constant pressure, volume is directly proportional to absolute temperature (V1/T1 = V2/T2). Temperatures must be in Kelvin (K = °C + 273.15).
• Gay-Lussac's Law: at constant volume, pressure is directly proportional to absolute temperature (P1/T1 = P2/T2). This is why heated cylinders and sealed drums can rupture.
• Combined Gas Law: P1V1/T1 = P2V2/T2.
• Ideal Gas Law: PV = nRT, where n is moles and R is the gas constant (0.0821 L·atm/mol·K).
• Dalton's Law: total pressure of a gas mixture equals the sum of partial pressures. Oxygen partial pressure falls at altitude even though the percentage stays 20.9%.
• Graham's Law: lighter gases diffuse faster than heavier gases (rate inversely proportional to the square root of MW).

3.7 Vapor Density and Specific Gravity
Vapor density compares the weight of a vapor or gas to an equal volume of air (air = 1, effective MW about 29). Approximate vapor density = MW / 29. Gases with vapor density less than 1 (hydrogen, methane, ammonia, helium) rise; those greater than 1 (propane, gasoline vapor, hydrogen sulfide, chlorine, carbon dioxide) sink and collect in low areas, pits, trenches, and confined spaces. Carbon monoxide (MW 28) is essentially the same density as air and mixes uniformly. Specific gravity compares the density of a liquid or solid to water (water = 1). Liquids with specific gravity less than 1 and low water solubility (most hydrocarbons) float on water, which is why water can spread a fuel fire rather than extinguish it.

3.8 Solutions, Concentration, and Dilution
A solution consists of a solute dissolved in a solvent. Concentration units you must know:
• Percent by weight or volume.
• Molarity (M): moles of solute per liter of solution.
• Parts per million (ppm): for airborne gases and vapors, 1 ppm = 1 part per 1,000,000 parts of air by volume. 1% = 10,000 ppm.
• Milligrams per cubic meter (mg/m3): mass per volume of air, used for particulates and also for gases/vapors.

The single most tested chemistry calculation on the ASP exam is the ppm to mg/m3 conversion at 25°C and 1 atm:
mg/m3 = (ppm × MW) / 24.45
ppm = (mg/m3 × 24.45) / MW
Example: Toluene (MW 92.14) at 50 ppm = (50 × 92.14) / 24.45 ≈ 188 mg/m3. If a question specifies 0°C, use 22.4 instead. Note that ppm applies only to gases and vapors; particulates, fumes, and fibers are expressed in mg/m3 or fibers/cc.

Dilution problems use C1V1 = C2V2. Example: how much water must be added to 500 mL of 10% solution to make a 2% solution? 10 × 500 = 2 × V2, so V2 = 2,500 mL total, meaning 2,000 mL of water is added. Always add acid to water, never water to acid, because dilution of concentrated acids is strongly exothermic and can cause violent spattering.

3.9 Acids, Bases, and pH
An acid donates hydrogen ions (H+); a base accepts them or releases hydroxide (OH−). The pH scale runs 0 to 14 and is logarithmic: each whole number represents a tenfold change in hydrogen ion concentration. pH 7 is neutral; below 7 is acidic; above 7 is basic (alkaline). A pH 2 solution is 100 times more acidic than a pH 4 solution. pH = −log[H+]. Regulatory note: EPA defines a corrosive hazardous waste as having pH ≤ 2 or ≥ 12.5. Strong acids (sulfuric, hydrochloric, nitric, hydrofluoric) and strong bases (sodium hydroxide, potassium hydroxide) cause chemical burns. Alkali burns are often more damaging than acid burns because bases saponify fats and penetrate deeper. Hydrofluoric acid is uniquely dangerous because the fluoride ion penetrates tissue and binds calcium, requiring calcium gluconate treatment. Acids plus bases produce a neutralization reaction yielding salt and water, releasing heat. Acids reacting with certain metals release hydrogen gas; acids reacting with cyanide salts release hydrogen cyanide; acids reacting with hypochlorite (bleach) release chlorine gas, and bleach with ammonia releases chloramine gas, two of the most common household and janitorial chemical incidents.

3.10 Oxidation, Reduction, and Combustion
Oxidation is loss of electrons; reduction is gain of electrons (mnemonic: OIL RIG). An oxidizer supplies oxygen or accepts electrons, enabling or intensifying combustion (oxygen, chlorine, nitrates, perchlorates, peroxides, permanganates, chromates). Oxidizers must be stored away from flammables and organic materials. Combustion is rapid oxidation that releases heat and light. Complete combustion of hydrocarbons produces CO2 and water; incomplete combustion produces carbon monoxide, soot, and other toxic byproducts. The fire triangle (fuel, oxygen, heat) and fire tetrahedron (adding the uninhibited chemical chain reaction) describe what must be present and what can be removed to extinguish a fire. Rust is slow oxidation; spontaneous combustion of oily rags is self-heating oxidation that reaches ignition temperature.

3.11 Flammability Properties
These appear repeatedly on the exam and in every SDS:
• Flash point: the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near its surface. Lower flash point = greater fire hazard. OSHA and GHS classify flammable liquids by flash point and boiling point (GHS Category 1: flash point below 23°C and boiling point ≤ 35°C; Category 2: flash point below 23°C and boiling point above 35°C; Category 3: flash point 23°C to 60°C; Category 4: flash point above 60°C to 93°C).
• Fire point: the temperature at which the vapor will continue to burn after ignition, usually a few degrees above the flash point.
• Autoignition temperature: the temperature at which a substance ignites spontaneously without an external ignition source. It is typically much higher than the flash point and is not to be confused with it.
• Lower Explosive (Flammable) Limit (LEL/LFL): the minimum concentration of vapor in air that will burn. Below the LEL the mixture is too lean.
• Upper Explosive (Flammable) Limit (UEL/UFL): the maximum concentration that will burn. Above the UEL the mixture is too rich.
• Flammable range: the span between LEL and UEL. A wider range means greater hazard (hydrogen 4% to 75%; acetylene 2.5% to 100%; gasoline roughly 1.4% to 7.6%; methane 5% to 15%).
• Combustible gas indicators read in percent of LEL, not percent by volume. 10% LEL for methane equals 0.5% by volume or 5,000 ppm. OSHA confined space entry requires atmospheres below 10% of LEL. A reading above 100% LEL may mean the atmosphere is actually above the UEL, which is still extremely dangerous because ventilation will bring it back into the flammable range.
• Flammable liquid vs combustible liquid: historically, flammable liquids had flash points below 100°F (37.8°C) and combustible liquids had flash points at or above 100°F; current OSHA HazCom aligned to GHS uses the four categories above.

3.12 Chemical Reactivity and Incompatibility
Reactive hazards cause some of the most catastrophic industrial events. Key categories:
• Exothermic reactions release heat; endothermic reactions absorb heat. Runaway exothermic reactions increase in rate as temperature rises, roughly doubling with every 10°C increase.
• Water-reactive materials (sodium, potassium, lithium, calcium carbide, sulfuric acid, metal hydrides) release heat, flammable gas, or toxic gas when contacting water.
• Pyrophoric materials (white phosphorus, finely divided metals, silane, some organometallics) ignite spontaneously in air.
• Peroxide formers (ethyl ether, tetrahydrofuran, isopropyl ether) form shock-sensitive peroxide crystals over time and must be dated and disposed of on schedule.
• Polymerization: monomers such as styrene, acrylonitrile, and vinyl chloride can undergo uncontrolled polymerization releasing enormous heat; inhibitors are added and must not be depleted.
• Unstable or explosive materials decompose rapidly with heat, shock, or friction.
• Toxic gas generation: acids + cyanides = HCN; acids + sulfides = H2S; bleach + ammonia = chloramines; bleach + acids = chlorine; nitric acid + organics = nitrogen oxides.
Safe storage requires segregation by hazard class rather than alphabetically: separate acids from bases, oxidizers from flammables and organics, water-reactives from aqueous solutions, and store nitric acid apart from other acids. The NFPA 704 diamond flags reactivity (yellow quadrant, 0 to 4), flammability (red), health (blue), and special hazards (white: OX for oxidizer, W with a slash for water-reactive, SA for simple asphyxiant).

3.13 Basic Organic Chemistry
Organic compounds contain carbon, usually bonded to hydrogen. Recognizing functional groups helps predict hazards:
• Alkanes (single bonds: methane, propane, hexane) are flammable; n-hexane causes peripheral neuropathy.
• Alkenes (double bonds: ethylene, propylene) are flammable and prone to polymerization.
• Alkynes (triple bonds: acetylene) are highly flammable with wide explosive ranges.
• Aromatics (benzene ring: benzene, toluene, xylene) are flammable; benzene is a known leukemogen.
• Alcohols (−OH: methanol, ethanol, isopropanol) are flammable and water-miscible, so alcohol fires require alcohol-resistant foam; methanol causes blindness.
• Ketones (C=O: acetone, MEK) are highly volatile, flammable solvents.
• Aldehydes (formaldehyde, glutaraldehyde) are irritants and sensitizers; formaldehyde is a carcinogen.
• Ethers (diethyl ether) are extremely flammable with very low flash points and form peroxides.
• Esters (ethyl acetate) are flammable solvents with fruity odors.
• Halogenated hydrocarbons (trichloroethylene, methylene chloride, carbon tetrachloride) are generally less flammable or non-flammable but are toxic to the liver, kidneys, and nervous system, and decompose in heat or flame to form phosgene and hydrogen chloride. Methylene chloride is metabolized to carbon monoxide in the body.
• Amines and nitro compounds are often toxic and can be explosive.
• Isocyanates (TDI, MDI) are potent respiratory sensitizers found in polyurethane work.

3.14 Hazard Communication Chemistry
The OSHA Hazard Communication Standard (29 CFR 1910.1200), aligned with GHS, requires a 16-section SDS. Sections most relevant to chemistry are Section 2 (hazard identification and pictograms), Section 3 (composition), Section 7 (handling and storage), Section 8 (exposure limits and PPE), Section 9 (physical and chemical properties: appearance, odor, pH, melting/boiling point, flash point, evaporation rate, flammability, LEL/UEL, vapor pressure, vapor density, specific gravity, solubility, autoignition, decomposition temperature, viscosity), and Section 10 (stability and reactivity, incompatibilities, hazardous decomposition products). GHS pictograms include the flame (flammables, pyrophorics, self-reactives), flame over circle (oxidizers), exploding bomb (explosives), corrosion (skin/metal corrosives), skull and crossbones (acute toxicity), health hazard (carcinogens, sensitizers, target organ toxicity), exclamation mark (irritants, less severe hazards), gas cylinder (gases under pressure), and environment (aquatic toxicity).

4. Linking Chemistry to Toxicology and Exposure Assessment
Physical and chemical properties dictate exposure routes. High vapor pressure and low boiling point favor inhalation. Lipid solubility favors skin absorption (the SDS skin notation). Water solubility affects where a gas deposits in the respiratory tract: highly soluble gases (ammonia, HCl, SO2) irritate the upper airways and give warning; poorly soluble gases (phosgene, nitrogen dioxide, ozone) penetrate deep into the alveoli and can cause delayed pulmonary edema with little warning. Particle size, not chemistry alone, determines respirability of dusts. Chemical form matters: hexavalent chromium is far more hazardous than trivalent chromium; metallic mercury vapor is readily absorbed by inhalation while liquid mercury is poorly absorbed through the gut. Understanding these connections allows you to answer scenario-based questions that blend chemistry, industrial hygiene, and toxicology.

5. Essential Formulas and Constants to Memorize
• mg/m3 = (ppm × MW) / 24.45 (25°C, 1 atm)
• ppm = (mg/m3 × 24.45) / MW
• 22.4 L/mol at STP (0°C); 24.45 L/mol at 25°C
• 1% by volume = 10,000 ppm
• % LEL reading × LEL (vol %) = actual vol %
• Vapor density ≈ MW / 29
• K = °C + 273.15; °F = (°C × 1.8) + 32
• P1V1 = P2V2 (Boyle); V1/T1 = V2/T2 (Charles); P1V1/T1 = P2V2/T2 (Combined); PV = nRT
• C1V1 = C2V2 (dilution)
• pH = −log[H+]; each pH unit = 10× change
• 1 atm = 760 mmHg = 14.7 psi = 101.3 kPa
• Avogadro's number = 6.022 × 10^23

6. Exam Tips: Answering Questions on General Chemistry for Safety

Tip 1: Master the ppm to mg/m3 conversion cold. This is the most predictable calculation in the domain. Read whether the question gives 25°C (use 24.45) or 0°C/STP (use 22.4). Write the formula first, plug in values, and check that your answer is reasonable: for a compound with MW near 24, ppm and mg/m3 will be roughly equal; for heavier molecules, mg/m3 will be larger than ppm.

Tip 2: Always convert temperature to Kelvin in gas law problems. Charles's, Gay-Lussac's, and the Ideal Gas Law all require absolute temperature. A distractor answer will almost always correspond to using Celsius directly.

Tip 3: Do not confuse flash point with autoignition temperature. Flash point requires an ignition source and is low (often below room temperature for flammable liquids); autoignition is spontaneous and high (hundreds of degrees). When a question asks which property best indicates the fire hazard of a liquid at ambient temperature, the answer is flash point.

Tip 4: Know the direction of hazard for each property. Lower flash point = more hazardous. Lower LEL = more hazardous. Wider flammable range = more hazardous. Higher vapor pressure = more volatile = greater inhalation hazard. Lower boiling point = more volatile. Vapor density above 1 = sinks and accumulates in low areas.

Tip 5: Translate combustible gas meter readings correctly. Meters read percent of LEL. If methane LEL is 5% and the meter reads 20% LEL, the actual concentration is 1% by volume or 10,000 ppm. Remember the OSHA confined space action level is 10% of LEL.

Tip 6: Remember pH is logarithmic. A change of 3 pH units is a 1,000-fold change in hydrogen ion concentration, not threefold. Also recall EPA corrosivity thresholds of pH ≤ 2 or ≥ 12.5.

Tip 7: Memorize the classic incompatibility pairs and their products. Bleach + ammonia → chloramines; bleach + acid → chlorine; acid + cyanide → hydrogen cyanide; acid + sulfide → hydrogen sulfide; water + alkali metals → hydrogen and heat; oxidizers + organics → fire or explosion. Questions often describe a janitorial or maintenance scenario and ask what gas was produced.

Tip 8: Use the periodic table families as shortcuts. If a question involves an unfamiliar Group 1 metal, assume water reactivity. If it involves a halogen, assume strong oxidizer and respiratory toxicity. If it involves a noble gas, the hazard is simple asphyxiation, not toxicity.

Tip 9: Apply like dissolves like. Water-miscible flammables (alcohols, acetone) require alcohol-resistant foam and will not float on water; nonpolar hydrocarbons float and spread. Lipid-soluble solvents penetrate skin and defat tissue.

Tip 10: Recognize halogenated solvents as the exception to flammability. Many chlorinated hydrocarbons are non-flammable or have no flash point, but they are toxic and generate phosgene and HCl in fires or when drawn through hot work. A question pairing a chlorinated degreaser with welding is pointing at phosgene.

Tip 11: Read SDS-based questions by section number. Know that physical properties are in Section 9 and reactivity information is in Section 10. Exam items may ask where a given piece of information is found.

Tip 12: Watch units and magnitude in every answer choice. Eliminate answers with wrong units (ppm for a dust, mg/m3 when ppm was requested) or implausible magnitudes. Carry three significant figures through the math and round at the end.

Tip 13: Link chemistry to control strategy. When a question asks for the best control, chemistry informs the answer: high vapor pressure flammables need grounding and bonding, ventilation, and ignition source control; water-reactives need dry storage and Class D extinguishers; oxidizers need segregation from fuels; acids need to be added to water, with eyewash and shower nearby.

Tip 14: Expect scenario questions that mix domains. A confined space question may require you to recognize that hydrogen sulfide (vapor density 1.19) settles at the bottom, that it deadens the sense of smell at high concentration, and that the correct instrument reading order is oxygen first, then flammables, then toxics. Pull the chemistry thread through the scenario.

Tip 15: Practice worked problems until they are automatic. Sample: An SDS lists the TLV for xylene (MW 106.16) as 100 ppm. What is the TLV in mg/m3? Answer: (100 × 106.16) / 24.45 ≈ 434 mg/m3. Sample: A meter reads 15% LEL for propane (LEL 2.1%). What is the concentration in ppm? Answer: 0.15 × 2.1% = 0.315% = 3,150 ppm. Sample: A gas occupies 10 L at 25°C; what volume at 75°C at constant pressure? Answer: V2 = 10 × (348.15 / 298.15) ≈ 11.7 L.

7. Summary
General chemistry underpins hazard recognition across the ASP exam. Focus on the properties that predict behavior (vapor pressure, boiling point, vapor density, specific gravity, solubility, flash point, LEL/UEL, pH, reactivity), be fluent in the handful of formulas that convert and compare those properties, memorize the common incompatibilities and chemical families, and always connect the chemistry back to exposure routes, fire and explosion potential, and the controls that reduce risk. With those fundamentals in place, chemistry questions become some of the most reliable points you can earn on exam day.