There may be some poetic descriptions of Soma conferring immortality - but are the Gods explicitly declared to be immortal in Rig Veda?
Yes there are many such verses addressed to different gods. Since you have mentioned it let us see a verse about Soma conferring immortality before we move on to the others. Rig Veda 8.48 mentions:
- WISELY have I enjoyed the savoury viand, religious-thoughted, best to find out treasure, The food to which all Deities and mortals, calling it meath, gather themselves together. 2 Tlou shalt be Aditi as thou hast entered within, appeaser of celestial anger. Indu, enjoying Indra's friendship, bring us—as a swift steed the car—forward to riches. 3 We have drunk Soma and become immortal; we have attained the light, the Gods discovered. Now what may foeman's malice do to harm us? What, O Immortal, mortal man's deception?
Leaving aside what may be interpreted as poetic license while talking about Soma let us see some other verses that explicitly state the immortality of gods. The verse RV 10.72 mentions:
- LET US with tuneful skill proclaim these generations of the Gods, That one may see them when these hymns are chanted in a future age. 2 These Brahmaṇaspati produced with blast and smelting, like a Smith, Existence, in an earlier age of Gods, from Non-existence sprang. 3 Existence, in the earliest age of Gods, from Non-existence sprang. Thereafter were the regions born. This sprang from the Productive Power. 4 Earth sprang from the Productive Power the regions from the earth were born. Dakṣa was born of Aditi, and Aditi was Dakṣa's Child. 5 For Aditi, O Dakṣa, she who is thy Daughter, was brought forth. After her were the blessed Gods born sharers of immortal life.
The famous Purush Sukta - RV 10.90 says this about the Adi Purush:
- A THOUSAND heads hath Puruṣa, a thousand eyes, a thousand feet. On every side pervading earth he fills a space ten fingers wide. 2 This Puruṣa is all that yet hath been and all that is to be; The Lord of Immortality which waxes greater still by food.
Similar verses are dedicated to Ka or Brahma. For example, Rig Veda Verse 10.121 Hiranyagarbha Sukta mentions:
- IN the beginning rose Hiranyagarbha, born Only Lord of all created beings. He fixed and holdeth up this earth and heaven. What God shall we adore with our oblation? 2 Giver of vital breath, of power and vigour, he whose commandments all the Gods acknowledge -. The Lord of death, whose shade is life immortal. What God shall we adore with our oblation?
The verse RV 8.19 calls Agni the immortal god:
- SING praise to hiin, the Lord of Light. The Gods have made the God to be their messenger, And sent oblation to Gods. 2 Agni, the Bounteous Giver, bright with varied flames, laud thou, O singer Sobhari- Him who controls this sacred food with Soma blent, who hath first claim to sacrifice. 3 Thee have we chosen skilftillest in sacrifice, Immortal Priest among the Gods, Wise finisher of this holy rite.......... 24 The God, the Friend of man, who bears our gifts to heaven, the God with his sweet-smelling mouth, Distributes, skilled in sacrifice, his precious things, Invoking Priest, Immortal God.
Again Rig Veda Verse 10.91 states:
11 When mortal man presents to thee Immortal God, Agni, his fuel or his sacrificial gift, Then thou art his Adhvaryu, Hotar, messenger, callest the Gods and orderest the sacrifice.
The Verse RV 10.17 mentions this about Saranyu, the daughter of Vishwakarma who was married to Surya:
- TVASTAR prepares the bridal of his Daughter: all the world hears the tidings and assembles. But Yama's Mother, Spouse of great Vivasvān, vanished as she was carried to her dwelling. 2 From mortal men they hid the Immortal Lady, made one like her and gave her to Vivasvān. Saranyu brought to him the Aśvin brothers, and then deserted both twinned pairs of children.
I hope these references shall help to answer your query.