If boiling water changes the shape of plastic, does it mean carcinogenic gas etc is released?

[Jennlpa]

Just panicking a bit as accidentally melted a bit of plastic cup and have a toddler

You 100% need help for your anxiety if this is stressing you out OP. Contact the GP today while it's fresh in your mind

No all that has happened is the plastic has softened and lost its shape in the same way chocolate or butter does when it gets to warm, once cool it will harden in its new shape

Agree that, if that is what you thought, you should probably be talking to someone about.
It's not a "standard" reaction to a melted cup

Now that really would be an overreaction.

No.

Some plastics melt every time they get above a specific temperature. It does not mean any gas is released much less carcinogenic gas.

@ErrolTheDragon contacting the GP about anxiety isn't an overreaction, it's a method that someone can get help for anxiety.

'Panicking a bit' about something she wasn't sure about doesn't sound like the sort of anxiety needing help. She's sensibly asked a question, got some straightforward helpful answers and hopefully that's an end of it.

I'm a PhD chemist, most people don't understand what may and may not pose a risk.

Not sure your PHD is relevant, you didn't even use it to answer the OPs question. The thing she panicked about is what suggests the anxiety. Neither of us will no for sure if she has anxiety or not though. The point still stands that seeking help for anxiety is not an over reaction.

I didn't need to answer the OPs question as Hoardasurass had already done so. You didn't, just diagnosed 'anxiety' on very little evidence. Just struck me as a singularly unhelpful first response to a pretty simple question.

The point of me mentioning my PhD is that I think its much more sensible of someone to be cautious and ask a question about this sort of thing if they're not sure than to be gung ho.

@ErrolTheDragon but anyone can claim to anything online. Also the op could have Googled the answer, I've never melted a plastic cup and thought 'cancer' it's a drinking vessel that's food safe, my first thought would be "bollocks that's annoying" not to come on Mumsnet

Also the op could have Googled the answer,

Bad idea for almost anything to do with 'chemicals' IMO, there's a lot of shit out there which is exactly why some people do worry.

And we all know Mumsnet is the font of knowledge

To answer your question OP, your plastic cup is a thermoplastic. When heated, the polymer chains it is made of can move as the forces holding them together weaken. When it cools, the intermolecular forces are strong enough for the plastic to hold it's shape. At the temperatures you are describing, the chemical structure of the plastic remains unchanged and nothing is added or taken away so it's totally safe for use.