Arsenic (As, atomic number 33) is a naturally occurring metalloid element. It belongs to the pnictogen group on the periodic table, sharing chemical similarities with phosphorus and antimony. Arsenic is widely recognized as a potent toxin. However, its thermal behavior is complex and defies the typical understanding of how a solid changes state upon heating. Understanding arsenic’s unique phase transitions is necessary to accurately determine its “boiling point.”
Understanding Arsenic’s Phase Transition: Sublimation
When most solids are heated, they melt into a liquid before boiling into a gas. Arsenic, however, bypasses this intermediate liquid state under standard atmospheric pressure. This unusual process is known as sublimation, where the element transitions directly from a solid into a gaseous vapor.
This behavior is related to the element’s triple point—a specific temperature and pressure where the solid, liquid, and gas phases of a substance coexist in equilibrium. For most elements, the triple point occurs at a pressure far below the standard atmospheric pressure (1 atmosphere). Arsenic’s triple point, by contrast, is located at a much higher pressure.
Since the pressure at the triple point is greater than the surrounding atmosphere, heating solid arsenic causes its vapor pressure to equal the atmospheric pressure before the melting point is reached. At this temperature, the solid-to-gas phase transition begins, and the substance sublimates instead of melting. The liquid phase can only exist when the surrounding pressure is artificially increased.
The Specific Numerical Data
The temperature at which solid arsenic transforms into a gas under standard atmospheric pressure (1 atmosphere) is its sublimation point. This occurs at approximately 614 °C (1137 °F). This value is often mistakenly cited as the element’s boiling point, but it represents a solid-to-gas change, not a liquid-to-gas one.
To observe a true melting and subsequent boiling point, arsenic must be placed in a confined system under significantly increased pressure. The triple point, the lowest temperature at which liquid arsenic can form, is approximately 817 °C (1503 °F). This temperature is achieved at a pressure of about 28 atmospheres (3628 kPa). Under these extreme conditions, arsenic melts at 817 °C, and its true boiling point occurs at an even higher temperature, following the expected phase diagram.
Physical Forms and Industrial Applications
Elemental arsenic exists in several structural forms called allotropes. The most common and stable form is metallic gray arsenic, which is a brittle, crystalline solid with a steel-gray appearance and poor electrical conductivity. Yellow arsenic is a less stable allotrope that forms when arsenic vapor is rapidly cooled. It slowly converts back to the gray form at room temperature or when exposed to light.
Despite its toxic nature, arsenic is used in specialized industrial applications. It is used to create alloys, such as those with lead, utilized in car batteries and ammunition. Small amounts added to copper can also increase the metal’s resistance to corrosion.
The most significant modern application is in the semiconductor industry, utilizing the compound gallium arsenide (GaAs). Gallium arsenide is used in the manufacture of high-speed electronic devices, light-emitting diodes, and solar cells due to its superior electron mobility compared to silicon. Historically, inorganic arsenic compounds were used as pesticides, herbicides, and in wood preservation products like chromated copper arsenate, though these uses have been restricted.
Health Hazards and Environmental Concerns
The thermal property of sublimation is directly relevant to arsenic’s health hazard profile. Since arsenic turns directly into a toxic vapor upon heating at 614 °C, any industrial process involving heating the solid metal risks immediate, high-level exposure via inhalation. The vapor consists of toxic arsenic molecules, making the heating process dangerous to anyone nearby.
The primary routes of human exposure are through the ingestion of contaminated drinking water and food grown in contaminated soil. Inorganic arsenic is a confirmed carcinogen, and long-term exposure can lead to a condition known as arsenicosis. Symptoms of chronic poisoning include distinctive skin lesions, such as hyperpigmentation and keratosis, and an increased risk of developing cancers of the skin, bladder, and lungs.
Acute exposure from high doses can cause immediate symptoms such as severe abdominal pain, vomiting, and diarrhea, potentially leading to death in extreme cases. Chronic low-level exposure has been linked to cardiovascular diseases, diabetes, and negative impacts on cognitive development in children. Arsenic is a naturally occurring component of the Earth’s crust. It becomes a major public health threat when it leaches into groundwater at high concentrations, affecting millions globally.