It is a mixture. take salt water for example if you boil the water the salt will be left behind. salt water on its own is not a pure substance a pure substance would be just the water by its self. hope that helps.
The endosperm is formed through double fertilization where one male gamete nucleus with polar nuclei to form a triploid nucleus called the primary endosperm. The endosperm therefore will contain 15 chromosomes since the polar nuclei is diploid and the male gamete is haploid forming a triploid nucleus.
