What is meant by controlled variable in chemistry

1) this is the structure in the cell nucleus that houses a cell's genetic information

Margaret wants to make an orange flavored drink by stirring powdered drink mix into a glass of water. she doesn't like drinks that have small clumps of powdered solid in them, so she wants the drink to be a perfect solution. what factors should margaret not consider when deciding how much powder to add to her glass of water?

What is a metal replacement reaction