Question: Has the chemical process reached its endpoint when readiness tests have been completed?
Answer: No, the chemical reaction does not finish completely.
The following short clip will further clarify the statement: The chemical reaction is not completed with the readiness tests.
Explanation
Each chemical reaction continues until it has reached its endpoint. This endpoint has only bean reached when no more water can come into contact with carbide. Water and carbide have to touch each other in order to react. The transformation to acetylene, the actual reaction, takes place extremely quickly (only a few microseconds are needed).
However, the time to reach the actual endpoint may vary as it depends on how intensely the two present reactants come into contact with each other.
When using the CM method, water is the mobile reactant. The slower water can move and the longer the distance is that it has to cover, the longer it takes until the reaction can end. If you shake the bottle, the carbide also turns into a mobile reactant.
If the pathway between water and carbide is blocked or either one of the reactants get held up (as water and carbide don’t stand in direct contact with each other: water droplets on the inside wall of an upright standing bottle). Therefor it can take up to days instead mere seconds until all of the water has been able to react.
There can also be a different kind of hindrance, being that the water isn’t located on the outer surface of the sample but is still stuck in the pore system of a sample, on the inner surface. The water inside such a system can only move extremely slowly (through diffusion and effusion), once again slowing down the reaction process.
Screeds, especially cementary systems, exhibit an extremely large inner surface area. You can imagine the pores as a system of canals, similar to a maze or the water ways of Venice.
If you leave a pressure bottle standing, unshaken, it will take a couple of weeks until the reaction is completed. If the bottle gets shaken constantly, the reaction will be able to reach its endpoint after only two days.
For Chemistry Fans
Generally, a chemical reaction runs until it has reached its chemical balance/equilibrium. The chemical equilibrium has only been reached, when the same amount of products are created in the forward reaction as starting products are created in the back reaction. Simply stated: Every reaction has a back reaction, which provides starting products.
Where the equilibrium lies, depends on the position of the equilibrium constant (Law of mass action). This lies, in the case of the calcium carbide and water reaction, clearly on the side of the product, the gaseous acetylene. The rest water content surviving the calcium carbide can be calculated and lies at a partial pressure of about 2×10-10 mbar
A little comparison: The vapor pressure of water is at 20°C and about 23 mbar, 100 billion times higher.
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