Which is hotter 50 mls of 75 dgrees celcius water or 200 mls of 70 degree celcius water and why?
By "hotter," I am assuming you are asking the question "which contains more energy." Well, let's think about it this way: while the 50 ml has a higher temperature, the 200 ml of water has a larger capacity. It's similar to asking the question "which is worth more? 2 one hundred dollar bills or 5 fifty dollar bills." Although the one hundred dollar bills are certainly more impressive, in reality, the stash of fifty dollar bills is definitely more useful.
To explain this scientifically, we think of "hotness" as "how much energy a molecule stores." The unit for the measurement of energy is Joules. In order to raise the temperature of one gram of water by one degree Celsius, 4.184 Joules are required. So essentially, since 1 milliliter of water is equal to 1 gram of water, the 50 ml collection of water has absorbed 50 grams x 75 Celsius x 4.184 Joules per gram per Celsius = 15,690 Joules. On the other hand, the 200 ml collection of water has absorbed 200 grams x 70 Celsius x 4.184 Joules per gram per Celsius = 58,576 Joules. This shows that the 50 ml collection of water contains less energy than the 200 ml collection of water, despite having a higher temperature.