The best way to distinguish between Na2CO3and NaHCO2 is by.

Analyze the question: You are asked for the best way to tell sodium carbonate (Na₂CO₃) from sodium hydrogencarbonate (NaHCO₃). The "NaHCO2" in the question is almost certainly a typing error for NaHCO₃, since that is the compound normally compared with Na₂CO₃.

Concept: Both compounds are sodium salts of carbonic acid, so they behave similarly in many tests. The key difference is thermal stability.

• Adding acid (A): Both fizz and give off CO₂, so this doesn't separate them clearly.
• Adding alkali (B): Neither gives an obvious reaction with a base like NaOH, so this is not a useful test.
• Dissolving in water (C): Both dissolve and give basic solutions. Na₂CO₃ is more alkaline, but the difference is not a clear visual test.
• Exposing to air (D): Neither gives a clear distinguishing change. Na₂CO₃·10H₂O effloresces, but that depends on the hydrate and is not reliable.
• Heating (E): NaHCO₃ decomposes readily, while Na₂CO₃ is stable to heat.

Solution:

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating)

Na₂CO₃ → no change at ordinary Bunsen temperatures

When you heat NaHCO₃ in a test tube:

• CO₂ is released, and it turns limewater milky.
• Water droplets form on the cooler part of the tube.

When you heat Na₂CO₃, none of this happens.

Answer: E, heating.

Exam tip: Remember that "hydrogencarbonates decompose on heating, but carbonates of Na and K do not." Whenever you see a question on distinguishing a carbonate from a hydrogencarbonate, think heat, then limewater.