Heating Copper Sulphate: Unveiling Water of Crystallization
A Colorful Transformation and the Role of Water
Many crystalline salts appear dry, but they actually contain water molecules as part of their crystal structure. This "water of crystallization" gives them their characteristic shape and color. In this activity, we will explore what happens when we heat a common hydrated salt, copper sulphate, and observe its fascinating transformation.
Aim of the Activity
- To observe the changes in color and physical state of copper sulphate crystals upon heating.
- To identify the presence and source of water droplets during heating.
- To investigate if the original color of copper sulphate can be restored by adding water.
Materials You'll Need
- Copper sulphate crystals ($CuSO_4 \cdot 5H_2O$)
- Dry boiling tube
- Test tube holder or tongs
- Bunsen burner or spirit lamp
- Dropper
- Distilled water
- Safety goggles (Mandatory)
Procedure
- Prepare for heating: Take a few crystals of copper sulphate in a clean and dry boiling tube.
- Heat the crystals: Using a test tube holder or tongs, gently heat the boiling tube over a Bunsen burner flame. Ensure even heating by moving the tube slightly.
- Observe changes during heating: Pay close attention to the color of the copper sulphate crystals. Also, observe if any droplets of liquid appear on the cooler, upper parts of the boiling tube.
- Cool the sample: Once the color change is complete (or no further changes are observed), stop heating and allow the boiling tube and its contents to cool down completely.
- Rehydration test: After cooling, add 2-3 drops of distilled water onto the sample of copper sulphate obtained after heating.
- Observe rehydration: Note any changes in color and temperature.
Observation
- Initially, copper sulphate crystals are blue in color.
- Upon heating, the blue copper sulphate crystals gradually turn white or off-white.
- Small water droplets are observed condensing on the cooler, upper inner walls of the boiling tube.
- After cooling and adding 2-3 drops of water to the white powder, the sample's color is restored to blue. The test tube might also feel slightly warm.
Explanation
Copper sulphate crystals ($CuSO_4 \cdot 5H_2O$) are hydrated, meaning they contain five molecules of water of crystallization chemically bound within their crystal structure. This water is responsible for their characteristic blue color.
- Upon Heating: When heated, the hydrated copper sulphate loses its water of crystallization. The water molecules evaporate and condense as droplets on the cooler parts of the boiling tube. The copper sulphate becomes anhydrous (without water), and its color changes from blue to white. This is a reversible physical change.
$CuSO_4 \cdot 5H_2O(s) \xrightarrow{\text{heat}} CuSO_4(s) + 5H_2O(g)$
(Blue Hydrated Copper Sulphate) → (White Anhydrous Copper Sulphate) + (Water Vapor)
- Upon Rehydration: When water is added back to the anhydrous (white) copper sulphate, it rehydrates, forming the hydrated copper sulphate crystals again. This process is exothermic, releasing heat, which is why the test tube might feel warm. The blue color is restored because the water molecules become re-incorporated into the crystal structure.
$CuSO_4(s) + 5H_2O(l) \rightarrow CuSO_4 \cdot 5H_2O(s) + \text{Heat}$
(White Anhydrous Copper Sulphate) + (Water) → (Blue Hydrated Copper Sulphate) + (Heat)
Conclusion
This activity clearly demonstrates the concept of water of crystallization. Hydrated salts contain water molecules that are essential for their crystalline structure and color. Heating removes this water, leading to a change in color and properties, which can often be reversed by reintroducing water.
Multiple Choice Questions
1. What is the color of hydrated copper sulphate crystals?
- a) White
- b) Blue
- c) Green
- d) Red
2. What color does copper sulphate turn into after heating?
- a) White
- b) Yellow
- c) Green
- d) Black
3. The water droplets observed in the boiling tube during heating come from:
- a) Condensation of atmospheric moisture
- b) Water of crystallization from copper sulphate
- c) Water vapor from the flame
- d) Impurities in the copper sulphate
4. Is the rehydration of anhydrous copper sulphate an exothermic or endothermic process?
- a) Exothermic
- b) Endothermic
- c) Neutral
- d) Neither