Why is the word けしゴム is half written in hiragana and half in katakana?
It just has to do with the words that it is made up of:
消し｛けし｝ comes from 消す｛けす｝which means to erase.
ゴム is a foreign word (usually katakana) meaning gum or rubber.
So you put the two together and you get the amalgam 消し｛けし｝ゴム.