When mixing powdered lemonade with water, the solvent is the water. A solvent is a substance that completely dissolves another substance in it. A solute is the substance that is being dissolved. In this case, the lemonade powder would be the solute.
Water is considered the universal solvent because it has the ability to dissolve a wide range of solutes. It is good at dissolving other substances because it has strong bonds; its bonds are covalent, meaning the bonds are formed because the atoms share electrons. It also is positively and negatively charged. These two attributes work in tandem. The charges help to attract other atoms. The covalent bonds hold the water together while separating the molecules of substances with weaker bonds, such as salt. This process dissolves the solute, making a solution.