Question

Table 6.1 lists some clues that a chemical reaction has occurred. However, these events do not necessarily prove the existence of a chemical change. Give an example for each of the clues that is not a chemical reaction but a physical change.

Short answer

In summary, here are examples of physical changes that can be mistaken for chemical reactions based on the given clues: 1. Change in color: A chameleon changing its color to blend in with its surroundings. 2. Formation of a gas: Boiling water, which turns into water vapor. 3. Formation of a solid: Freezing of water, which turns into ice. 4. Change in temperature: Melting ice cream, which changes from solid to liquid. 5. Change in size or shape: Breaking a glass, which shatters into smaller pieces. These examples demonstrate physical changes, not chemical reactions, as they do not result in the formation of new substances.

Step-by-step solution

1. Change in color

An example of a physical change that involves a change in color is the rust forming on a metal surface when exposed to water and oxygen for an extended period. This is a chemical reaction between iron, water, and oxygen to form iron oxide (rust). To demonstrate a physical change that involves a change in color, consider a chameleon changing its color to blend in with its surroundings. Though the color change may appear to be the result of a chemical reaction, it is, in fact, a physical change caused by the rearrangement of pigments within the chameleon's skin.

2. Formation of a gas

An example of a physical change that involves the formation of a gas is boiling water. When water boils, it turns into water vapor, which is a gas. This may appear to be a chemical reaction, but it is actually a physical change. In this case, the water molecules are undergoing a change in state from liquid to gas rather than forming a new substance.

3. Formation of a solid

An example of a physical change that involves the formation of a solid is the freezing of water. When water freezes, it turns into ice, which is a solid. This may seem like a chemical reaction, but it is actually a physical change. In this case, the water molecules are undergoing a change in state from liquid to solid rather than forming a new substance.

4. Change in temperature

An example of a physical change that involves a change in temperature is melting ice cream. When ice cream melts, it changes from a solid to a liquid, and its temperature increases. This may seem like a chemical reaction, but it is actually a physical change. In this case, the ice cream molecules are undergoing a change in state from solid to liquid, and there is no formation of a new substance.

5. Change in size or shape

An example of a physical change that involves a change in size or shape is breaking a glass. When a glass breaks, it shatters into smaller pieces, which may seem like a chemical reaction. However, it is actually a physical change as the glass's chemical composition remains the same, and only its size and shape have changed. In conclusion, while these examples may appear to be chemical reactions based on specific clues, they are all instances of physical changes that do not result in the formation of new substances.

Key concepts

Physical Changes

Physical changes involve alterations in the physical properties of a substance without altering its chemical composition. This means the molecules stay the same but change in form or appearance. Common examples of physical changes include

• cutting paper,
• melting ice,
• or stretching rubber bands.

These changes may involve

• a shift in state,
• a change in texture,
• or a modification of shape.

However, they don't create a new substance. For instance, when water boils, it becomes steam, but remains molecularly identical.

Chemical Change Indicators

Recognizing a chemical reaction can be determined by observing specific indicators. However, these indicators aren't foolproof signs of chemical change as they can also result from physical changes. Key indicators include

• color change,
• gas formation,
• solid formation (precipitation),
• temperature change,
• and change in odor.

For example, rusting seems like a chemical reaction manifesting through a color change. However, a butterfly changing color is a physical change as no new substance forms. It is crucial to apply critical analysis to discern the type of change.

States of Matter

Matter primarily exists in four states: solid, liquid, gas, and plasma. Each state of matter is determined by the arrangement and energy of its particles. Allowing changes among these states, such as melting, freezing, or evaporating, usually signifies physical changes. For instance, water transitions between solid ice, liquid water, and gaseous steam without becoming a different substance. This illustrates how state changes do not equals chemical changes, but merely physical ones. The states of matter are fundamental to understanding both physical and chemical changes.

Physical vs Chemical Change

Understanding the difference between physical and chemical changes is essential for grasping the nature of matter. Physical changes are all about changing the form. Chemical changes involve forming one or more new substances. In a physical change, like breaking a glass, the particles are rearranged, but their identities remain unchanged. Conversely, a chemical change is irreversible under normal conditions, such as burning wood to form ash and smoke. Recognizing this difference can help one identify the types of change happening around them.

Matter Properties

Matter has specific properties that help distinguish it and determine its state or kind of change it can undergo. These properties are divided into two categories: physical and chemical.

• **Physical properties** include characteristics like color, mass, and volume, observed without changing the substance. They are essential in identifying the substance or predicting how it behaves during physical change.
• **Chemical properties** describe a substance’s ability to undergo a chemical change; this includes reactivity with substances like oxygen or acid.

Understanding these properties aids in predicting and interpreting how a substance will react or change under certain conditions.