Sodium bicarbonate is the chemical name for baking powder. its chemical formula is nahco3. what does the subscript 3 mean?

 subscript 3 mean 3 oxygen atoms

Explanation:  NaHCO3 is an ionic compound which when dissolved in water gets dissociated into its constituent ions that are Na+ and HCO3- .

Thus the subscript 3 explains the number of oxygen that are attached to the carbon in the hydrogen carbonate ion.

The structure of the following is given below -

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.