The sense that the equal sign “=” means that the two sides are balanced is the most key concept in Maths. The golden rule in manipulating maths is:
” What you do to one side you to do the other”. I find this concept is poorly understood even by middle ability y11 students; this leads me to believe that the concept has not been introduced strongly enough at primary or KS3.
I think that one of the key pieces of equipment is therefore is balancing arm. I like this simple balancing arm from Hanson http://www.handson.co.uk/primary-resources-maths/mathematical-balance.html which is only £9.99. Four of these for a class is a good investment.