Question

State whether each of the following is a physical or a chemical property: (a) Electricity decomposes water. (b) Ice floats on water. (c) Potassium metal fizzes in water. (d) Acetone dissolves in water.

Short answer

(a) Chemical property, (b) Physical property, (c) Chemical property, (d) Physical property.

Step-by-step solution

Understand Physical and Chemical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing its composition, such as color, density, and melting point. Chemical properties describe a substance's ability to undergo a specific chemical change, which alters its molecular structure, such as flammability or reactivity with acid.

Analyze Statement (a)

Statement (a) is "Electricity decomposes water." Decomposition of water into hydrogen and oxygen is a chemical change because it involves breaking down the water molecules into different substances. Therefore, this is a chemical property.

Analyze Statement (b)

Statement (b) is "Ice floats on water." This behavior is due to the difference in density between ice and liquid water. Since density is a physical property and ice floating does not change the chemical composition of water, this is a physical property.

Analyze Statement (c)

Statement (c) is "Potassium metal fizzes in water." When potassium reacts with water, it forms potassium hydroxide and hydrogen gas, which causes fizzing. This is a chemical property because it involves a change in composition.

Analyze Statement (d)

Statement (d) is "Acetone dissolves in water." Dissolving substance is a physical process, as the acetone-water mixture can be separated again without any change in chemical composition. Thus, it is a physical property.

Key concepts

Physical Properties

Physical properties are aspects of matter that can be measured or observed without changing the chemical identity of the substance. Physical properties include:

• Color
• Density
• Melting and boiling points
• Solubility
• Volume and mass

These properties are intrinsic to the substance and do not alter its chemical structure.

For example, when ice floats on water, it's due to its lower density compared to liquid water. Despite this difference, both ice and water maintain the chemical identity of H₂O. Understanding these properties is essential for recognizing how substances behave in different situations, such as ice melting into water without undergoing a chemical transformation.

Chemical Change

A chemical change involves a process where substances transform into new products with different chemical properties. This change fundamentally modifies the molecular structure of the original substances, forming new compounds or elements.

Indicators of a chemical change can include:

• Formation of a gas (bubbles or fizzing)
• Change in color
• Temperature change
• Emission of light or heat

For instance, potassium metal fizzing in water indicates a chemical reaction. This process results in the formation of potassium hydroxide and hydrogen gas, demonstrating a transformation from the original components into new substances. Understanding chemical changes is vital for identifying reactivity and potential new chemical properties.

Decomposition Reaction

Decomposition reactions are a type of chemical change where a single compound breaks down into two or more new substances. These reactions often require an external trigger, like heat or electricity, to occur.

In the example of water decomposition, electricity is used to separate water (\(\text{H}_2\text{O}\)) into hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)) – simpler substances. This is significant because it involves breaking the chemical bonds within the water molecule, resulting in the formation of entirely different substances.

This type of reaction is crucial in various industrial and chemical processes, showcasing how energy can facilitate the transformation of compounds into essential elements or simpler compounds.