Hydrazine

Hydrazine (ヒドラジン) is an inorganic chemical compound with the chemical formula N2H4. It is a liquid fuel apparently still in use in Academy City for rockets, despite being highly explosive and toxic.

Principles
In Academy City, due to the dangerous nature of the chemical it can only be handled in a limited number of districts such as District 10, and most likely  District 23 as well. Regardless, it is still used as fuel for rockets. Rockets are constructed in District 23, and its fuel is loaded there. However, the container portion where the fuel for auxiliary boosters used for altitude control is loaded in District 10. It is used for Academy City's secret operation of sending out materials into space to build the space port of the Endymion, under the guise of participating in the Venus Probe Contest. It is also used for the rocket that will carry an additional module for the Hikoboshi II.

Toaru Kagaku no Railgun: Uiharu SS
On August 1st, 1500 kilograms of hydrazine was to be used as a fuel for the rocket that was to take an additional module for Hikoboshi II into space. However, the vehicle that carried the satellite and its fuel was taken over by a terrorist, who threatened that they would take over the vehicle's automatic evasion system to cause an accident. This forced the driver of the vehicle, Sangaku Youko, to follow the terrorist's instructions. The terrorist's target was a conference between the 12 members of the Board of Directors that ran Academy City,and 7 heads of state from other countries in the International Convention Center in District 3. However, the board anticipated this, and deemed that it would be far better for the vehicle to crash and incur civilian casualties in order to protect the guests. They hired Sunazara Chimitsu to destroy an overpass in order for the vehicle not to come close.

With combined efforts of Uiharu Kazari, Yomikawa Aiho, Sangaku Youko, and Anti-Skill, the disaster was prevented. Misaka Mikoto and Shirai Kuroko would later go on to capture the perpetrator.

Toaru Majutsu no Index: Road to Endymion
After deducing that Ureapaddy Exica will be using the rockets that are used for the Venus Probe Contest, Touma narrowed down the possible ways she could sabotage it, and concluded that since Hydrazine-filled container for the auxiliary boosters were loaded in District 10 and then transferred to  District 23, she would need to intercept one of the vehicles carrying it for her to sabotage it.

Touma determined the quickest route and discovered her there. She used the Brahma Astra to pierce the fuel tank and escape. Index then told Stiyl Magnus to use his flame sword to blow the fuel away, instructing him how to modify it for it to be able to remove the fuel's toxicity. Stiyl complied and ignited the fuel as Ureapaddy escaped.