Svante August Arrhenius (/əˈriːniəs, əˈreɪniəs/ ə-REE-nee-əs, -⁠RAY-, Swedish: [ˈsvânːtɛ aˈrěːnɪɵs]; 19 February 1859 – 2 October 1927) was a Swedish scientist. Originally a physicist, but often referred to as a chemist, Arrhenius was one of the founders of the science of physical chemistry. In 1903, he received the Nobel Prize in Chemistry, becoming the first Swedish Nobel laureate. In 1905, he became the director of the Nobel Institute, where he remained until his death.

Arrhenius was the first to use the principles of physical chemistry to estimate the extent to which increases in the atmospheric carbon dioxide are responsible for the Earth's increasing surface temperature. His work played an important role in the emergence of modern climate science. In the 1960s, Charles David Keeling reliably measured the level of carbon dioxide present in the air showing it was increasing and that, according to the greenhouse hypothesis, it was sufficient to cause significant global warming.

The Arrhenius equation, Arrhenius acid, Arrhenius base, lunar crater Arrhenius, Martian crater Arrhenius, the mountain of Arrheniusfjellet, and the Arrhenius Labs at Stockholm University were so named to commemorate his contributions to science.

Arrhenius was born on 19 February 1859 at Vik (which can also be spelled as Wik or Wijk), near Uppsala, Kingdom of Sweden, the son of Svante Gustav and Carolina Thunberg Arrhenius, who were Lutheran. His father had been a land surveyor for Uppsala University, moving up to a supervisory position. At the age of three, Arrhenius taught himself to read without the encouragement of his parents and, by watching his father's addition of numbers in his account books, became an arithmetical prodigy. In later life, Arrhenius was profoundly passionate about mathematical concepts, data analysis and discovering their relationships and laws.^{[citation needed]}

At age eight, he entered the local cathedral school, starting in the fifth grade, distinguishing himself in physics and mathematics, and graduating as the youngest and most able student in 1876.^{[citation needed]}

At the University of Uppsala, he was dissatisfied with the chief instructor of physics and the only faculty member who could have supervised him in chemistry, Per Teodor Cleve, so he left to study at the Physical Institute of the Swedish Academy of Sciences in Stockholm under the physicist Erik Edlund in 1881.

His work focused on the conductivities of electrolytes. In 1884, based on this work, he submitted a 150-page dissertation on electrolytic conductivity to Uppsala for the doctorate. It did not impress the professors, who included Cleve, and he received a fourth-class degree, but upon his defense it was reclassified as third-class. Later, extensions of this very work would earn him the 1903 Nobel Prize in Chemistry.

Arrhenius put forth 56 theses in his 1884 dissertation, most of which would still be accepted today unchanged or with minor modifications. The most important idea in the dissertation was his explanation of the fact that solid crystalline salts disassociate into paired charged particles when dissolved, for which he would win the 1903 Nobel Prize in Chemistry. Arrhenius's explanation was that in forming a solution, the salt disassociates into charged particles that Michael Faraday had given the name ions many years earlier. Faraday's belief had been that ions were produced in the process of electrolysis, that is, an external direct current source of electricity was necessary to form ions. Arrhenius proposed that, even in the absence of an electric current, aqueous solutions of salts contained ions. He thus proposed that chemical reactions in solution were reactions between ions.