We all know that the FP of alcohol is - 1150C and BP is780C. But the FP of mercury is - 350C and BP is 3570C. So, mercury has a normal physical state at 1000C and can measure temperature. But, alcohol starts boiling if the temperature raised to 780C. So, it changes to a gas state at 1000C and cannot measure temperature. Hence, we can measure the temperature of 1000C with a mercury thermometer but not with an alcohol thermometer.