dissacharides

Disaccharides are less  soluble in water,less sweet compared to monosaccharides.By dehydration of 2 monosaccharides they create one dissacharide and water.The bond that holds two monosaccharides are called Glycoside Bonds.All Disaccharides have same formulas.It is C12 H22 O11  

Glucose+Glucose=Maltose+Water
Glucose+Fructose=Sucrose+water
Glucose+Galactose=Lactose+water

You can reverse the process by hydrolysis