Properties Of Water

 71% of the Earth is covered by water, and 97% of this water is in the oceans.  Water is made up of two atoms of hydrogen, and one atom of oxygen.  The chemical equation for water is: 2H2 + O2  2H2O

 The picture below of a water molecule illustrates two hydrogen atoms bonded to one oxygen atom.

 Because of water's electronic structure, the oxygen atom has a slight negative charge on it and the hydrogen atoms are slightly positive.  When water molecules are close together, their positive and negative regions are attracted.

 These attractive forces are known as hydrogen bonds.  Hydrogen bonds are the reason for water's very special properties which make life on Earth possible. The structure of water showing the slight negative charge on the oxygen atom and the slight positive charges on the hydrogen atoms. The charged nature of the water molecule enables it to form hydrogen bonds with other water molecules.

 Water is the only natural substance that is found as a gas (water vapor), a liquid and a solid (ice) on Earth.

 When hydrogen and oxygen atoms join together to form water, oxygen shares one of its electrons with each of the hydrogen atoms.  It also has electrons it doesn’t share. This makes the oxygen part of a water molecule act like the negative (–) pole of a magnet and the hydrogen parts act like the positive (+) pole of a magnet.  Just as the + end of one magnet attracts the – end of another, the Hs from one water molecule attract the Os of others. This attraction, is called a hydrogen bond.

Properties of Water Water is the solvent of Life! Solute – substance dissolved in a solvent to form a solution Solvent – fluid that dissolves solutes Example: Ice Tea – water is the solvent and tea and sugar the solutes Universal Solvent

 Hydrogen bonds make water a sticky liquid.  Water molecules stick to each other – a force called cohesion.  They also stick to other things – a force called adhesion.  Examples: cohesion: water forming a dome adhesion: water clinging to a paper towels

 Cohesion and adhesion help water soak into things.  They help water soak into the ground so plants and animals can use it.  When a water molecule turns from liquid to gas, it rises into the air

 Capillary action, or capillarity, is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials.  This effect can cause liquids to flow against the force of gravity.  It makes it possible for water to travel from the roots of plants to the tips of their leaves—even to the tops of towering trees!

 When water molecules evaporate from a plant’s leaves (top), they draw other water molecules up through the stem (center). More water molecules enter the plant through its roots (bottom).

Properties of Water Capillary Action Continued Because water has both adhesive and cohesive properties, capillary action is present. Capillary Action = water’s adhesive property is the cause of capillary action. Water is attracted to some other material and then through cohesion, other water molecules move too as a result of the original adhesion. Ex: Think water in a straw Ex: Water moves through trees this way

 The stickiness of the hydrogen bonds also causes liquid water to form a kind of “skin”— called surface tension—where it meets the air.  This skin allows insects called water striders to walk across the surface of a pond.

 Density is a measure of how compact a substance is.  It is defined as the mass of a substance divided by its volume.  Water is less dense as a solid! This is because the hydrogen bonds are stable in ice – each molecule of water is bound to four of its neighbors. Solid – water molecules are bonded together – space between fixed Liquid – water molecules are constantly bonding and rebonding – space is always changing

 For most substances, the solid form is denser than the liquid form. This is not the case with water.  As water molecules move together to form ice, the pushing and pulling of – and + ends cause them to line up in a lacy pattern with lots of empty spaces in between.  As a result, water expands when it freezes.

 Adding salt to water increases its density. It also prevents the formation of hydrogen bonds.  This means seawater, unlike pure water, doesn't have its maximum density at 4 o C, but when it freezes into ice.  It also means that seawater freezes below 0 o C (this is why we put salt onto roads on cold nights to lower the risk of ice being formed).

 Empty space trapped in ice makes ice less dense than water.  But because solid water is less dense than liquid water, ice stays on top of the water.

 The fact that water expands when it freezes also accounts for many of the changes in the world around us.  When water freezes in cracks in rocks, the ice helps to break the rock apart.  This is part of the process by which soil forms.

 Temperature is a measure of how much molecules are moving around.  When you heat a liquid, the molecules move around more and the temperature goes up.  But hydrogen bonds make it harder for molecules to move. As a result you can add a lot of heat to water, but the temperature only goes up a little bit.  We say water has a high heat capacity.

Properties of Water In order to raise the temperature of water, the average molecular speed has to increase. It takes much more energy to raise the temperature of water compared to other solvents because hydrogen bonds hold the water molecules together! Water has a high heat capacity. “The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.” High Heat Capacity

 Every kind of molecule requires energy to change from a solid to a liquid and from a liquid to a gas.  Because it takes extra energy to break hydrogen bonds, water takes more energy than most substances to change from one form to another.  That means water soaks up a lot of heat energy as it turns from ice to liquid or from liquid to gas

 Water’s ability to absorb heat helps protect Earth from wild temperature swings from night to day and summer to winter.  It keeps ocean temperatures fairly constant, so creatures that live there have a relatively stable environment. WATER GAINS HEAT from the sun during daytime and releases it after dark. This helps keep air temperatures from changing wildly from night to day.

 Water’s ability to soak up heat as it changes from one form to another helps keep you cool.  When your sweat evaporates, it draws heat from your body.  Dogs, rabbits, birds, and other animals cool off by holding their mouths open and letting the water inside evaporate.  Turkey vultures pee on their legs when they are hot. As the water evaporates, it cools them off.

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