Q#7 Which of the two air is heavier? Winners Announced

November18, 2020
by Aneesh Bangia
7 min read

Dear school leaders,
For MTLT#7, we asked you this question:

Which of the two air is heavier?
(Assume same temperature and pressure.)

(A) Humid Air
(B) Dry Air

Correct answer: (B) Dry Air


Most people find it hard to believe that dry air is heavier than humid air. To know why is that, we need to first understand Avogadro’s law.

According to Avogadro’s law, equal volumes of all gases, at same temperature and pressure, have the same number of molecules.

Now let us apply this law to air. Imagine one cubic meter of perfectly dry air which contains 78% of  nitrogen molecules and 21% of oxygen molecules and remaining 1% of other gases. 

Here the molecules are free to move in and out of our one cubic meter of air. If we add water vapor  molecules to this, some of the nitrogen and oxygen molecules will have to be displaced, as number of molecules should remain same for the same volume of air. 

Since, molecular weight of one water vapour molecule is 18 units which is less than that of nitrogen  molecule (28 units) and oxygen molecule (32 units), the total weight of our one cubic meter of air, containing water vapor becomes less than that of perfectly dry air. Thus, for the same one cubic meter   of volume, humid air weighs less than dry air.

Therefore, dry air is heavier than humid air.

How School Leaders responded?

Lucky Draw Winners:

1. Sheela Menon, Ambassador School, Dubai 👏
2. N.S.RAO, Bhavya Talent school, Hyderabad 👏
3. Vinaya Mary, Matha Public School, Pathanamthitta, Kerala 👏

School Leaders who submitted the correct answer:

1. Dr. Pampa Chaudhuri, PICT Model School Pune
2. Mayur Thourani, MM School, Raipur
3. Priya Sangwan, SRBPS
4. Bindiya Kishore, The Samhita Academy
5. Nita Arora, Sri Venkateshwar International School, New Delhi
6. Roshan Menezes, Carmel High school Bangalore
8. Shankar Singh Adhikari, The Rajkumar College Rajkot
9. Rita Taneja, Delhi Public School Jaipur
10. Sheela Menon, Ambassador School, Dubai
11. Lata Nakra, JSS international school, Dubai
12. Prashant Sharma, Ambuja Vidya Niketan, Ambujanagar
13. Kanchan Joshi, Cygnus World School
14. Asha Sharma, Indian Learners Own Academy, Kuwait

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