In "The Dragon and the Wolf" (Game of Thrones, S07E07), this happens:
it is shown that Viserion breaks the Wall with his breath.
I don't think he breathes ice. Does anyone know?
What does dead Viserion breathe if it's not ice?
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It is fire
The thing which the dragon was breathing radiates like flames, which only fire can do. Never seen any creature in Scifi breathing ice which radiates flames
Also, one can clearly see black and grey smoke coming out of the wall wherever the blue fire touches it. Ice on ice creates white smoke. But here, it is more greyish towards the upper side of the smoke, which again supports the view that it is fire.
Viserion is known to be smaller than his other two brothers, Drogon and Rhaegal. So, unless and until he magically grew very strong under the aegis of the Night King, there is no way he could have breathed ice so strong which can bring down another ice structure like the Ice Wall [which stood strong for 8000+ years]. So, it should be fire.
To be warned that all the answers here would be speculation. We shall only (probably) know in the next season.
This is confirmed in an interview by The Huffington Post with director of the episode "Dragon and the Wolf", Jeremy Podeswa, when asking the all important question "Was it Ice or Fire?". (Emphasis mine throughout)
“The way I looked at it was, when the sept burned down, that was green fire, and so then the dragon is going to have some kind of blueish fire,” he said. “It’s certainly still fire — it has the ability to burn the Wall and melt snow. But it’s going to have a different kind of magical quality to it, because it’s coming from an undead dragon.”
The Huffington Post in an interview with Jeremy Podeswa
This is re-confirmed in an interview with IndieWire where he discusses the humour of the acceptance of Green fire but not blue fire, although that 'chemically' the two would be different, coming from a dead being.
“It’s funny because people really accepted the green fire [of wildfire], which burned down the Sept [of Baelor] and the sea battle in the earlier season,” said Podeswa. “I think that fire can have many different manifestations. I think that the ice dragon fire is clearly still fire because it does melt The Wall and tear it down, but it had a particular quality because it’s coming from an undead dragon. So what the chemical nature of that is and the chemical nature of fire would actually be different coming from a non-sentient being. So there’s definitely some thought into that of what that should actually look like. But obviously, an ice dragon is going to have an icy kind of fire but it’s still fire.”
Jeremy Podeswa breaks down the final episode for IndieWire
Finally, when talking about the amount of practical vs visual effects used in the episode with The Hollywood Reporter, Jeremy Podeswa reveals that there are interactive elements with the scenes where the "flame" tears the wall down.
"... And there are many interactive elements also that people might not be aware of, like the dragon flame and the impact of that and what it does in terms of the effect it has on the Wall..."
The Hollywood Reporter in an interview with Jeremy Podeswa
And as the icing on top. Here's the clip of it again!