Two students are working togteher to build a birdhouse. Student 1 applies a force of 10N to a wooden board in order to slide it across the tabel to student 2. If the force of friction resisting the student's push is 4N, what is the net force acting on the board?

Answer is: impurities lower melting point (melting point depression) and increase boiling point (boiling point elevation).
For example pure water has lower boiling point than solution of water and some salt, because partial pressure is increased. Impurities in solids lower the melting point because they disrupt crystal structure of substance.