Image: Airgas.

After big gains to start the week, the stock market treaded water on Tuesday, with most major benchmarks finishing close to the unchanged level. Yet for Airgas (NYSE: ARG), Nuance Communications (NASDAQ:NUAN), and Syngenta (NYSE: SYT), the day brought substantial gains for shareholders, putting them among the best-performing stocks for the session.

Airgas soared almost 30% after receiving an offer from French peer Air Liquide to purchase the industrial gas company. The French acquirer agreed to pay $143 per share in cash for Airgas, and with the buyer assuming Airgas debt, the total enterprise value of the deal will amount to roughly $13.4 billion. The two companies said that the combination should bring together complementary businesses that will allow it to extend their joint geographical reach and compete more effectively against their rivals. With the post-merger company on pace to become the largest player in the global industrial gas industry, Air Liquide could be the big winner in the deal even though Airgas investors will get a roughly 50% premium to where the stock traded on average over the past month.

Nuance Communications climbed 19% after the company reported favorable financials in its fiscal fourth-quarter report. Despite a decline in sales and a GAAP loss for the quarter, Nuance continued to make progress on its transformative efforts. The voice-recognition software provider gave favorable views for the coming fiscal year, with strength in Nuance's Enterprise and Diagnostics divisions building on favorable performance from the company's Dragon Medical software suite and its Clintegrity platform. With a combination of stock repurchases and expense reductions, Nuance is working hard to recover some of its former glory, and the shares reflected that enthusiasm by pushing to levels not seen since early 2013.

Finally, agricultural chemical and seedmaker Syngenta (NYSE: SYT) jumped 9% on renewed speculation that rival Monsanto (NYSE:MON) might seek to make a major acquisition in the near future. Syngenta has been the target of merger advances from Monsanto in the past, with the U.S. company having dropped a hostile $46 billion bid for Syngenta just this past summer. Analysts point out that buying Syngenta would have the dual advantage of giving Monsanto a more complete portfolio of crop-protection products for its agricultural customers as well as providing a basis under which Monsanto could execute a tax inversion to move its headquarters to Syngenta's tax home of Switzerland. Syngenta has argued that Monsanto's past offers were insufficient, so if Monsanto decides to pursue its takeover plans further, it will probably need to make an even sweeter deal in order to persuade its Swiss rival to move forward more productively this time around.