Which of the following pairs of substances cannot exist together in a solution?

$A$ fixed volume of iron is drawn into a wire of length $L$. The extension $x$ produced in this wire by a constant force $F$ is proportional to

$A$ bullet is fired from a gun. The force on the bullet is given by $F = 600 - 2 \times 10^5 t$,where $F$ is in newton and $t$ is in second. The force on the bullet becomes zero as soon as it leaves the barrel. What is the impulse imparted to the bullet (in $N-s$)?

The phalangeal formula for the human hand is:

$A$ man fires a bullet standing between two cliffs. The first echo is heard after $3 \ s$ and the second echo is heard after $5 \ s$. If the velocity of sound is $330 \ m/s$,then the distance between the cliffs is .... $m$.

Which of the following pairs of ions are colourless?