Short Answer

CO₂ is nonpolar because it has a linear shape, and the polar C=O bond dipoles cancel each other out. Even though each C=O bond is polar, the molecule becomes nonpolar due to perfect symmetry.

H₂O is polar because it has a bent shape caused by two lone pairs on oxygen. The O–H bond dipoles do not cancel but add together, creating a net dipole moment. Therefore, CO₂ is nonpolar, while H₂O is polar.

Detailed Explanation :

Why CO₂ Is Nonpolar but H₂O Is Polar

Although both CO₂ and H₂O contain polar covalent bonds, one molecule is nonpolar (CO₂) while the other is polar (H₂O). The key reason for this difference lies in their molecular shapes and arrangement of electron pairs. Bond polarity alone does not determine molecular polarity—geometry decides whether dipoles cancel or combine to form a net dipole moment.

CO₂ is symmetrical, causing dipole cancellation, while H₂O is bent, causing dipoles to add up. Understanding this difference helps explain many physical properties such as solubility, boiling point, and intermolecular forces.

1. Polarity Depends on Shape and Dipole Arrangement

A molecule’s polarity is determined by:

1. The presence of polar bonds
2. The shape of the molecule
3. Whether bond dipoles cancel (symmetry) or add up (asymmetry)

Both CO₂ and H₂O have polar bonds, but their shapes make them behave differently.

2. Polarity in CO₂

CO₂ has:

• Two C=O polar bonds
• Oxygen is more electronegative → each O pulls electron density
• Carbon becomes partially positive

However, CO₂ is nonpolar because of its shape.

Shape of CO₂: Linear

CO₂ has a linear shape with a bond angle of 180°:

Because of this:

• The dipole of each C=O bond points outward in opposite directions
• They have equal magnitude
• They cancel each other completely

Thus:

Despite strong C=O bond polarity, the molecule remains nonpolar due to perfect symmetry.

3. Polarity in H₂O

H₂O has:

• Two O–H polar bonds
• Oxygen is strongly electronegative
• Hydrogen becomes partially positive

Unlike CO₂, H₂O is polar because of its bent shape.

Shape of H₂O: Bent (V-shaped)

The molecule has:

• Two bonding pairs
• Two lone pairs on oxygen

Lone pairs repel bonding pairs and push the hydrogen atoms downward, creating a bent geometry with an angle of about 104.5°.

Because of this shape:

• The dipole moments of the two O–H bonds do not oppose each other
• They add together
• This creates a strong overall dipole moment

Thus:

The bent shape prevents cancellation of dipoles, making water a polar molecule.

4. Role of Lone Pairs in Water Polarity

The two lone pairs on oxygen:

• Occupy more space
• Increase repulsion
• Bend the molecule
• Cause electron density to shift upward

This makes the oxygen side δ– (negative) and hydrogen side δ+ (positive), further strengthening the dipole moment.

Thus, lone pairs are a major reason why water is polar.

5. Comparison Summary

Feature	CO₂	H₂O
Bond type	C=O (polar)	O–H (polar)
Shape	Linear	Bent
Dipole cancellation	Yes	No
Result	Nonpolar	Polar

Even though CO₂ and H₂O both contain polar bonds, shape decides their final polarity.

6. How Polarity Affects Their Properties

Because CO₂ is nonpolar:

• It dissolves in nonpolar solvents
• It has weak intermolecular forces
• It exists as a gas at room temperature

Because H₂O is polar:

• It dissolves polar substances (“like dissolves like”)
• It forms hydrogen bonds
• It has a high boiling point
• It is an excellent solvent for ionic compounds

Thus, molecular polarity plays a major role in determining chemical behaviour.

Conclusion

CO₂ is nonpolar because its linear and symmetrical shape causes the two polar C=O bond dipoles to cancel completely. H₂O is polar because its bent shape, caused by lone pairs on oxygen, prevents dipole cancellation and creates a strong net dipole moment. Therefore, shape—not just bond polarity—is the key factor that decides molecular polarity.