Physical and Chemical Changes

One of the first chemical reactions that students learn about is the reaction of baking soda and vinegar. If you have seen an erupting volcano at a science fair or a homemade bottle rocket, then you may have seen this reaction in action. Baking soda is sodium bicarbonate (NaHCO3), a white solid that usually comes in powder form. Vinegar is a liquid mixture. The active ingredient in vinegar, the substance that reacts with baking soda and gives vinegar its bite, is acetic acid (CH3COOH). Most vinegars are 4-8% acetic acid by volume.

baking soda & vinegar reaction

When baking soda and vinegar are mixed together, the sodium bicarbonate and acetic acid react to produce sodium acetate (CH3COONa), water (H2O), and carbon dioxide (CO2). Because carbon dioxide is a gas, the mixture will foam as carbon dioxide is produced and the gas expands in volume. This is what gives a baking soda and vinegar bottle rocket its thrust.

If you take a bottle of soda water, shake it vigorously, and then loosen the cap… the soda water will also foam as carbon dioxide expands into a gas. However, producing foaming carbon dioxide gas by shaking a bottle of soda water is a physical change, while producing foaming carbon dioxide gas by combining baking soda and vinegar is a chemical change.

CO2 (aq) + H2O (l) → CO2 (g) + H2O (l)

The key difference is that carbon dioxide molecules already existed in the soda water before you shook it. Soda water is carbonated water, which means that it is water with carbon dioxide dissolved in it. If you could examine the individual molecules in soda water, you would find a mixture of both CO2 and H2O molecules. All shaking the bottle does is release the CO2 molecules as a gas. [“CO2 (aq)” stands for carbon dioxide dissolved in aqueous solution and “CO2 (g)” stands for carbon dioxide in a gas state.] Because no chemical bonds are broken and no new molecules are formed, this is a physical change in the system.

In contrast, there are no carbon dioxide molecules in the baking soda or the vinegar. There are carbon and oxygen atoms in both, but those atoms are chemically bonded in other molecules (pure substances). To create CO2 molecules, the chemical bonds in sodium bicarbonate and acetic acid must be broken in a chemical reaction. Since pure substances (reactants) are transformed into other pure substances (products), this is a chemical change in the system.

So how can you tell if a change is physical or chemical? The surest way to do so is to identify the types of molecules in the system before and after the change. If the types of molecules are the same, then the change is physical. If some molecules are transformed into other molecules, then the change is chemical. To identify a molecule or substance, you would need to analyze its physical and chemical properties.

VIdentifying Reactants and Products
Some everyday chemical changes

There are many chemical changes that occur around us every day. One of them is the oxidation of iron metal to form rust. Iron metal is iron (Fe) atoms held together by metallic bonds. Rust consists of a mix of iron (III) oxides and iron (III) oxide-hydroxides. Rust is formed when iron reacts with the oxygen in the air and water. The reaction mechanism for this chemical change involves many steps, but the overall reaction is:

4Fe + 3O2 + 6H2O → 4Fe(OH)3 ⇌ 4FeO(OH) + 4H2O ⇌ 2Fe2O3·3H2O

When you turn on a gas stove or a gas grill, combustion (burning) occurs. We have already looked at the combustion of hydrogen (H2) gas to form water (H2O). The primary component of natural gas used in gas stoves is methane (CH4). Propane (C3H8) gas is used in gas grills. The reaction mechanisms for combustion of methane and propane involve dozens of steps but the overall reactions are:

CH4 + 2O2 → CO2 + 2H2O + energy
C3H8 + 5O2 → 3CO2 + 4H2O + energy

The combustion of methane releases 1.5 × 10-18 J of energy per molecule and the combustion of propane releases 3.4 × 10-18 J of energy per molecule. The burning of wood, or gasoline in an internal combustion engine, is far more complicated because those substances are mixtures of many different hydrocarbons and organic molecules.

The carbon and nitrogen cycles

In a physical change, the number and types of molecules remain the same. An example of a physical change is the melting of an ice cube. The water molecules transition from a solid state to a liquid state, but you still have the same water molecules after the change that you had before the change. In a chemical change, the types of molecules change, but the number and types of atoms remain the same. An example of a chemical change is the rusting of iron. Iron metal (Fe), water (H2O), and oxygen (O2) molecules are taken apart and used to create “rust” molecules (Fe(OH)3, FeO(OH), and Fe2O3·3H2O), but you still have the same iron (Fe), hydrogen (H), and oxygen (O) atoms after the change that you had before the change.

The only times that atoms are created or destroyed are when nuclear reactions occur. And because nuclear reactions do not occur all that often on Earth, the number of iron or oxygen or carbon or nitrogen atoms on Earth is relatively constant. This means that those atoms must be recycled whenever new molecules are created with those atoms.

Two important elements in biochemistry are carbon and nitrogen. Carbon and nitrogen atoms are both essential building blocks for proteins and amino acids (DNA), and carbon atoms are the backbone of carbohydrates. Living organisms need to constantly build new molecules with carbon and nitrogen atoms in order to survive. So the recycling of carbon and nitrogen atoms are critical processes in the Earth’s system.

An in-depth study of the carbon and nitrogen cycles is beyond the scope of this unit. However, it is helpful to be aware of how and where carbon and nitrogen atoms tend to get stored. Most of the carbon in the Earth’s atmosphere is stored in carbon dioxide. Carbon dioxide (CO2) is an extremely stable molecule, and it usually takes quite a bit of energy to transform CO2 into other substances. Living organisms breathe out CO2 during respiration. CO2 is also released through the decay of animal and plant matter, and by fires and the combustion of fossil fuels.

CO2 in the atmosphere dissolves in the ocean and forms carbonic acid (H2CO3). The carbonic acid then reacts with calcium ions in the water to form calcium carbonate (CaCO3), which is stored in the shells of microscopic organisms and in sediments on the ocean floor as limestone. Over millions of years, these sediments can be converted into fossil fuels, such as natural gas (CH4), coal (C), and petroleum (e.g., C8H18 or C12H26).

CO2 in the atmosphere is also absorbed through the leaves of plants and used to create glucose (C6H12O6) molecules. This process is called photosynthesis. Plants use glucose for energy and to build new cells. Glucose gets stored in plant cells as carbohydrates. Animals obtain glucose by eating plants and other animals. When plants and animals die, the carbon gets stored in the soil… primarily as cellulose [(C6H10O5)n], starch [(C6H12O6)n], and lignin (e.g., C9H10O2)… until microorganisms can break it down and release it as CO2 and CH4.

Most of the nitrogen in the Earth’s atmosphere is stored as nitrogen gas, a major component of air. Nitrogen (N2) gas is released when microorganisms break down decaying animal and plant matter. Unfortunately, N2 is unusable by almost all living organisms. The only organisms that can pull N2 molecules apart and use the nitrogen atoms to build other molecules are nitrogen-fixing bacteria that live in the soil or the root nodules of legumes (e.g., peas and beans). These bacteria convert N2 into ammonium (NH4+), which can then be converted by other bacteria into nitrites (NO2-) and nitrates (NO3-). Nitrates are then absorbed through the roots of plants and used to create other molecules in plant cells. Animals obtain nitrogen by eating plants and other animals.