Question

You want to cook some raw noodles to make a pasta dish for supper. So, you turn on the gas stovetop (which uses a flame), place a large pot of water over the flame, wait for the water to boil, place the noodles in the boiling water for 11 minutes, and then drain the water from the noodles. Identify one example of a physical change and one example of a chemical change in this process. Explain your answer for each.

Short answer

The physical change in the process of cooking noodles is the boiling of water, which is a change in state from liquid to gas without altering its chemical composition (H2O). This change is reversible, and the water can return to its liquid state if cooled. The chemical change in the process is the combustion of the gas in the gas stovetop, where natural gas (mostly methane, CH4) reacts with oxygen (O2) to produce carbon dioxide (CO2) and water vapor (H2O). This reaction is not reversible and results in the formation of new substances with different properties.

Step-by-step solution

Identify the physical change in the process

The physical change in this process is the boiling of water. Boiling water is a change in its state from liquid to gas (steam) without changing its chemical composition (H2O). This change is reversible, meaning that once the water cools, it will return back to its liquid state.

Identify the chemical change in the process

The chemical change in this process is the combustion of the gas to produce the flame for the gas stovetop. When natural gas (mostly methane, CH4) is ignited in the presence of oxygen (O2), it undergoes a chemical reaction producing carbon dioxide (CO2) and water vapor (H2O). This reaction is not reversible, which confirms that it is a chemical change.

Explain the physical change

As mentioned in Step 1, boiling water is a physical change because it only involves a change of state (liquid to gas) without altering the chemical composition of the water molecules (H2O). This change is temporary and can be reversed easily by cooling the water back down.

Explain the chemical change

As mentioned in Step 2, the combustion of natural gas in the presence of oxygen is a chemical change because it leads to the formation of new substances: carbon dioxide (CO2) and water vapor (H2O). These products have different properties compared to the reactants, and the reaction is not reversible, which confirms that it is a chemical change.

Key concepts

Boiling Water

Boiling water is a common kitchen procedure, often as simple as turning on a stove and heating a pot of water. During this process, water undergoes a physical change, transitioning from a liquid to a gas, or steam. Interestingly, the chemical composition of the water remains unchanged: it is still made up of hydrogen and oxygen atoms in the molecular formula of H2O.In the context of cooking pasta, boiling is merely the first step. As waters reaches the boiling point (100 degrees Celsius or 212 degrees Fahrenheit), it begins to vaporize. Despite this dramatic transformation, once removed from heat, steam will condense and revert to water, perfectly illustrating the reversible nature of a physical change. This ability to reverse the change without altering the substance at a molecular level is what makes boiling water an ideal example of a physical change.

Physical Change

A physical change refers to a transformation that occurs in a substance without altering its inherent chemical identity. It involves changes in physical properties, such as shape, state, and size, but not its chemical composition. Common examples include ice melting, paper being cut, and, as demonstrated in our cooking scenario, water boiling.

Characteristics of Physical Changes

• They are usually reversible.
• No new chemical species are formed.
• Involve changes in physical properties such as state, texture, and color.

To differentiate from chemical changes, if you can envision the process being reversed simply by altering the physical conditions (e.g., temperature or pressure), it is likely a physical change.

Combustion Reaction

Moving from boiling water to a combustion reaction elevates the discussion from a simple physical alteration to a complex chemical change. Combustion occurs when a substance, typically a fuel, reacts energetically with oxygen, producing heat and light. In a cooking scenario, turning on the gas stove involves igniting a mixture of methane gas (CH4) with oxygen (O2), producing carbon dioxide (CO2), water vapor (H2O), and energy in the form of heat.

Key Aspects of Combustion

• Produces energy, often visible as flames.
• Results in the formation of new chemical products.
• Is an irreversible chemical process.
• Requires a source of fuel and oxidant (typically oxygen).

Combustion is integral to various applications, from household cooking to driving automotive engines and is a powerful demonstration of a chemical change in everyday life.

Chemical Reaction

Chemical reactions are processes where reactants undergo a transformation to become entirely different products. These reactions are central to chemistry and take place all around us, from the simple rusting of iron to the complex biochemical processes within our bodies.

Signs of a Chemical Reaction

• The emission of gas, often observed as bubbling or fizzing.
• The formation of a precipitate, a solid that emerges from a liquid solution.
• Change in color, indicating a chemical change at the molecular level.
• Change in energy, either through the release (exothermic) or absorption (endothermic) of heat.
• Irreversibility, which is not absolute but indicates a significant energy barrier to reverse the process.

In our example of pasta cooking, the combustion reaction on the stovetop is such a chemical reaction, where the fuel burns in the presence of oxygen to create new products, namely CO2 and H2O. Unlike the physical change of boiling water, these chemical processes result in new substances that were not present before the reaction occurred.