|Why does steam make ironing cotton pants so much easier? — AB, Virginia
Water "plasticizes" the cotton. A plasticizer is a chemical that dissolves into a plastic and lubricates its molecules so that they can move across one another more easily. Cotton is almost pure cellulose, a polymer consisting of sugar molecules linked together in long chains. Since sugar dissolves easily in water, water dissolves easily in cellulose. Even though cellulose scorches before it melts, it can be softened by heat and water. When you iron cotton pants, the steam dissolves into the cellulose molecules and allows the fabric to smooth out beautifully.