What does below chemistry rebus means?

Out of the nine cats that you can see in the picture, there are only two that are completely identical. Can you find them out?

As easy as you can get into me, it is as hard to get out of me.

Do you know what I am?

Can you write down eight eights so that they add up to one thousand?

I start and end with 500 and carry a 5 in my heart. But I require the first letter and the first number to be complete. I am the name of a King.

Who am I ?

The more you take me, the more you leave me behind.
Who Am I?

I can be cracked, made, told, and played. What am I?

Can you make the number 24 by utilizing the numbers 1, 3, 4 and 6? You must use one number only one time and you can use mathematical operation symbols anytime anywhere.

Can you find the odd one out from the below words ?

First Second Third Forth Fifth Sixth Seventh, Eighth Nine Ten Eleven Twelve.

You measure my life in hours
I serve you by expiring
I vanish faster when I am thin
and slower when I am thick
The wind is my energy
What am I?