One litre of water at 30°C is mixed with one litre of water at 50°C. What will be the temperature of the mixture? 

between 30°C and 50°C

The correct answer is option 4. between 30°C and 50°C.

When two substances at different temperatures are mixed, they tend to exchange heat until they reach thermal equilibrium. The principle involved is based on the conservation of energy, and heat transfer occurs until the temperatures of the substances equalize. In the given scenario:

1. Water at 30°C is mixed with water at 50°C.

2. Heat will flow from the water at 50°C to the water at 30°C.

3. The water at 30°C will absorb heat and increase in temperature.

4. The water at 50°C will lose heat and decrease in temperature.

5. The final temperature of the mixture will be somewhere between 30°C and 50°C.

The exact final temperature depends on the specific heat capacities and masses of the two water samples. However, it is certain that the final temperature will be within the range of the initial temperatures of the two substances.