Gas does not explode, gas expands. It might seem like it's exploding if you heat air in a closed container, and the pressure builds and builds until the thing explodes, but this is just a big release of pressure because you were holding it in. It's like if you were all curled up in a ball inside a small cardboard box and you suddenly decided to use all your strength to uncurl and stretch out, it would seem like the box is exploding. It's not really exploding. It's just breaking because you are putting pressure on it from inside. Same with the kind of gas explosion you are talking about. A real explosion, with dynamite or a bomb is caused by chemicals reacting together and creating *new* gas and force that blows out in all directions. This is different than the expansion of gas from heating.
The earth is not enclosed in any kind of can or box. The only thing around it is empty space--a vacuum. If the earth gets hotter and hotter, the gas of the atmosphere will just expand out into space, and that is it. Nothing is holding it in except gravity. It would be like if you were curled up in a ball but *not* inside a box. Uncurling fast would not explode anything. Gravity keeps the air here. Heating might make it a tiny tiny bit thicker, but it would stay on earth. The hottest molecules of gas would sure fly off into space, but few would escape the earth's gravity. Now if it was heated tremendously, then yes, all the atmosphere might be blasted off into space. But there would be no explosion. And you'd really need a lot of heat.
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