I was the prefect in charge of running our school libraries in 1970/71 and I was entrusted with researching titles suitable for the junior library. The 'go to' series for science books (as well as for books on many other subjects) was definitely the Dorling Kindersley series. They were lovely, glossy tomes, packed with illustrations and they were incredibly popular with young readers. I suspect it was one of those books that you were reading.
The problem with researching the
exact title though is that even now, 50 years later, Dorling Kindersley (or 'DK' as they're now known) is still publishing fantastic science books. (If I'm looking for a non-fiction book for a youngster, I always put DK books at the top of my list). The modern equivalent title would probably be among these
https://www.dk.com/uk/category/science-and-nature/
but, because the books have evolved over half a century, it could be very hard to find the exact volume which you remember reading back in the 1970s.
The only other series of books which might fit the bill would be Time Life Science Library ones, which were also very popular in libraries around that time:
https://en.wikipedia.org/wiki/Life_Science_Library