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Physical versus Chemical Changes

In Section 1.3 we divided the variety of observable properties of matter into two classes: physical and chemical. Physical properties are those properties whose observation does not involve a change in the composition of the sample. A sample of water does not change its composition if we pour it from a tall pitcher into a flat bowl. It can be frozen to a solid or vaporized to steam. Yet it remains water, with the formula H2O. Through all these physical changes, the composition of water in unchanged. Another physical change, the crushing of limestone, is illustrated in Figure 8.1. Even though the limestone is crushed to smaller particles, the composition of the limestone does not change.

The observation of a chemical property involves a change in the composition of the sample - that is, a chemical change. When an electric current is passed through water that contains a few drops of sulfuric acid, the water decomposes to hydrogen and oxygen. Water molecules are no longer present; instead, we have two new substances, hydrogen and oxygen. A chemical change is also illustrated in Figure 8.1. When heated, limestone (known chemically as calcium carbonate) is converted into two new substances, lime (calcium oxide) and carbon dioxide, that have very different properties from those of limestone.


FIGURE 8.1 The crushing of limestone is a physical change; it does not alter the chemical composition of the limestone. The heating of limestone is a chemical change; the limestone decomposes into two other substances, lime and carbon dioxide.

As shown in Section 3.4, a chemical reaction can be described with an equation that shows the formula of each reactant and each product. An equation must be balanced to show that no mass is lost or gained during the reaction. An equation may also show the physical state of each component and the energy change associated with the reaction. For example, a complete equation for the burning of propane, C3H8, is:

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) H = -2220 kJ

The symbols after each formula show the physical state of that substance. The equation can be read as: One molecule of gaseous propane reacts with five molecules of gaseous oxygen to yield three molecules of gaseous carbon dioxide and four molecules of liquid water.

An equation can also be read in terms of moles. This equation states: One mole of gaseous propane reacts with five moles of gaseous oxygen to yield three moles of gaseous carbon dioxide and four moles of liquid water.

The H term following the equation describes the enthalpy change H that accompanies the reaction. For this equation, the enthalpy term states that the combustion of one mole of gaseous propane to gaseous carbon dioxide and liquid water is accompanied by the release of 2220 kJ of energy. We will discuss enthalpy changes accompanying a reaction more thoroughly in Section 8.5.

The equation given for the burning of propane is a complete equation. When a discussion centers on the reactants and products of a reaction, rather than on the associated energy change, the physical states and the enthalpy change (H) are often omitted.



State the complete meaning of the equation:

Solution The equation shows that, when an electric current is passed through water, two molecules (or two moles) of liquid water will decompose to yield two molecules (or two moles) of gaseous hydrogen and one molecule (or one mole) of gaseous oxygen. Energy of 591 KJ is absorbed for every two moles of water decomposed. The equation also shows that a molecule of water contains two atoms of hydrogen and one atom of oxygen and that hydrogen and oxygen are datomic gases.


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