Science crossword

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Across
  1. 2. Gas humans breathe in (6)
  2. 4. Opposite of acid (6)
  3. 6. Living things interacting with their environment (9)
  4. 8. Centre of an atom (6)
Down
  1. 1. Smallest part of an element (4)
  2. 3. Field pulling objects towards Earth (7
  3. 5. Process plants use to make food (14)
  4. 6. Change from liquid to gas (11)
  5. 7. Unit of force (6)