Ablock of ice with a total mass of of 25 grams is beginning to melt. how will this physical change affect the mass of the block ice.

Changes of state such as melting or boiling are physical changes and are generally easy to reverse though the end product may not always look exactly the same as the starting material. In physical changes no new materials are formed and the particles do not change apart from gaining or losing energy. Melted wax solidifies when cooled but unless it is shaped or moulded it will not be the same shape as at the start. On a microscopic level although the same particles are present they may be in different places within the solid.
Ice is made up of particles of water. When it melts the water which is formed is made up the same water particles and when it boils the steam is also made up of the same water particles. Particles stay the same unless there is a chemical change whether the matter is solid, liquid or gas. Only their arrangement, energy and movement changes. 
When substances change state there is no change in mass so if 100 g of ice is melted 100g of water are formed this will boil to form 100g of steam (this is called "conservation of mass"). If this steam could be collected, cooled and condensed it would form 100g of water which could be frozen to give 100g of ice. Children often think that solids get lighter when they melt as liquids are lighter than solids. 

Contained in chromosomes which are located in the nucleus of the cell.