With visual observation the ice cube can be seen getting smaller in size until it disappears. With a thermometer it is possible to see the temperature of the original mixture of ice and water decreasing.
The temperature of the ice cube is
The temperature of the water is greater than
The temperature of the mixture will equalize over time. The ice water molecules will gain energy from kinetic collisions with the liquid water molecules. The liquid water molecules will lose kinetic energy to the solid water molecules in the ice.
When the kinetic energy the ice molecules gain in calories ( or joules) is great enough the intermolecular bonds will break apart and the molecules will become liquid. This can be visually observed, as the ice cube becomes smaller and finally disappears.
The temperature of the mixture will be lower because of the kinetic energy used to break the intermolecular bonds between the ice molecules. This drop in temperature can be observed by using a thermometer. The smaller the volume of water and the larger the ice cube, the larger the observable drop in temperature will be.