Properties of Water What is the symbol for water? H ₂O

A few things we need to know about water. Unlike MOST substances, water expands when frozen. This makes frozen water much less dense than most frozen compounds. It floats!

Polar water molecule Because of the water molecule’s shape, one side tends to be positive and one side is negative. This property is called polarity

Hydrogen Bonds and Water Because of the hydrogen bonds in water, and water’s ability to form many hydrogen bonds, we say water is cohesive. Cohesion – attractions of molecules of the same substance

Because of cohesion, many insects and spiders can walk on the surface of water.

Surface tension - contractive tendency of the surface of a liquid that allows it to resist an external force. Ex, some objects floating on the surface of water, even though they are denser than water, and the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.liquidwater striderscohesion molecules

Adhesion – the attraction of molecules of different substances. Meniscus – the dip when you read a graduated cylinder

Capillary Action- Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draw water out of the roots of a plant and up into its stems and leaves. Cohesion holds the column of water together as it rises.

Mixtures Mixtures – a combinations of two or more elements or compounds that are mixed together, but not chemically combined.

Solutions and Suspensions Solution – when all particles are evenly distributed. Solute – the substance that is dissolved. Solvent – The substance in which the solute dissolves.

In salt water solution, what is the solvent and what is the solute? Other examples?

Suspensions Suspensions – a mixture of water and non-dissolved matter. The particles can settle over time. Blood is a great example of a suspension.

Acids, bases and pH pH – refers to measurement of H+ (hydronium ions) or OH¯ (hydroxide ions) If the compounds, if H+ and OH¯ ions are equal, we say the material is neutral. If the compounds have more H+, the material is acidic. If the compounds have more OH¯. The material is basic.

pH Scale

What do you think most foods we eat are, acidic, basic, or neutral?