How would you confirm that a colourless liquid given to you is pure water

General
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how would you confirm that a colourless liquid given to you is pure water

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How would you confirm that a colourless liquid given to you is pure water?

Answer:
To confirm that a colourless liquid given to you is pure water, you can employ several scientific methods. Here are some key approaches:

  1. Physical Properties:

    • Boiling Point:

      • Pure water has a boiling point of exactly 100°C (212°F) at 1 atmosphere (atm) of pressure. Measure the boiling point of the liquid and compare it to this standard value.
      • Measurement:
        \text{Boiling Point} = 100°C \, \text{at 1 atm}

    • Freezing Point:

      • Pure water has a freezing point of exactly 0°C (32°F) at 1 atm. Measure the freezing point of the liquid.
      • Measurement:
        \text{Freezing Point} = 0°C \, \text{at 1 atm}

  2. Density Testing:

    • Pure water has a density of approximately 1 g/cm³ at 4°C. Measure the mass of a known volume of the liquid and calculate its density.

    • Density Formula:

      \text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}

    • Measurement:

      • If the density calculated is 1 g/cm³ at 4°C, the liquid is likely pure water.

  3. Conductivity Test:

    • Pure water is a poor conductor of electricity due to the very low concentration of ions. Use a conductivity meter to measure the electrical conductivity of the liquid.
    • Expected Result:
      • Pure water should have a very low conductivity (typically around 0.055 \, \mu S/cm at 25°C).

  4. pH Test:

    • Pure water has a neutral pH of about 7. Use pH paper or a pH meter to test the pH level of the liquid.
    • Expected Result:
      \text{pH} \approx 7

  5. Chemical Tests:

    • Distillation:

      • Distilling the liquid and collecting the distillate is another way to confirm purity. If the liquid is pure water, only water will be collected as distillate after boiling.

    • Evaporation Test:

      • Evaporate a small amount of the liquid on a clean surface. If the liquid is pure water, it will leave no residue.

    • Reactions with Anhydrous Copper Sulfate:

      • Anhydrous copper sulfate is white. When exposed to water, it turns blue due to the formation of hydrous copper sulfate.
      • Reaction:
        \text{CuSO}_4 (\text{white}) + 5\text{H}_2\text{O} \rightarrow \text{CuSO}_4 \cdot 5\text{H}_2\text{O} (\text{blue})

  6. Spectroscopic Analysis:

    • Infrared (IR) Spectroscopy:
      • An IR spectrum can show the characteristic absorption peaks for water, particularly the broad O-H stretching vibration around 3200-3550 cm⁻¹.
    • Nuclear Magnetic Resonance (NMR):
      • H-NMR can confirm the presence of water by detecting the hydrogen peaks at a specific chemical shift.

Final Answer:
To confirm that a colourless liquid is pure water, you can measure physical properties like boiling and freezing points, density, and electrical conductivity; test its pH; perform distillation, evaporation, and chemical reactions with anhydrous copper sulfate; and use advanced techniques like IR or NMR spectroscopy. Consistent results with the known standards for pure water indicate the liquid is indeed pure water.