Thermal decomposition of marble pieces, mainly calcium carbonate, CaCO3, at temperatures above 650 C:
CaCO3 (s) → CaO (s) + CO2 (g)

TUTORING Introductory and Advanced high school Chemistry CBSE, Edexcel, etc
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CaO formed is tested with water; reaction is very exothermic:
CaO (s) + H2O (l) → Ca(OH)2 (s)

On addition of more water Ca(OH) dissolves and dissociates into ions:
Ca(OH)2 (s) + aq → Ca(OH)2 (aq)
Ca(OH)2 → Ca2+ + 2OH-

Testing with phenolphthalein gives violet color, which means pH is more than 10, confirming the alkaline nature of the product.

Original marble pieces do not change color of phenolphthalein.

Chapters:
00:00 Heating up marble chips
00:31 Cooling down produced calcium oxide CaO
00:55 Reaction of CaO with water; formation of Ca(OH)2
01:27 Testing original marble pieces with phenolphthalein
01:42 Testing hydrated decomposed chips with phenolphthalein
01:58 Conclusion

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