Why is the accuracy of the experiment being performed here not affected by whether the container is covered with a lid or not?2 kg of ice at low...

You have mentioned that the accuracy of the experiment would not
be affected, but what is the objective of your experiment in the first place. Are you trying to
measure the heat required to melt ice, the specific heat of ice, the conductivity of the
polystyrene container, or something else.

If you are trying to
accomplish any of objectives that I have provided above, I think there will be a change in the
result when the container has a lid and when there is no lid on the
container.

You need to specify what you are trying to find through
the experiment in a more elaborate way so that a correct answer can be
provided.