Answer:
1. efflorescent substances are solids that can undergo spontaneous loss of water from hydrated salts WHILE hygroscopic substances are solids that can absorb or adsorb water from it's surroundings.
2. efflorescent substances are crystals WHILE hygroscopic substances are called humectants.
3. efflorescent substances have no considerably affinity for water WHILE hygroscopic substances have a less affinity for water .
4. efflorescent substances do not absorb water vapour WHILE hygroscopic substances can either absorb or adsorb water vapour.
5. efflorescent substances do not form a solution WHILE hygroscopic substances do not form a solution but can absorb water vapour.