Question

Which of the following is a physical change and which is a chemical change? (a) Boiling water (b) Decomposing water by passing an electric current through it (c) Exploding of potassium metal when placed in water (d) Breaking of glass

Short answer

(a) Physical change; (b) Chemical change; (c) Chemical change; (d) Physical change.

Step-by-step solution

Understand Physical and Chemical Changes

A physical change affects the form of a substance, but not its chemical composition. Examples include changes in state, shape, and size. A chemical change, on the other hand, results in the formation of new chemical substances.

Analyze Boiling Water

Boiling water is a physical change. The water changes from a liquid to a gas, but its molecular structure (H₂O) remains the same, without the formation of new substances.

Examine Decomposing Water

Decomposing water by passing an electric current through it results in a chemical change. The process of electrolysis splits water ( H₂O) into hydrogen (H₂) and oxygen (O₂), creating new substances.

Evaluate Exploding Potassium Metal

When potassium metal explodes in water, it undergoes a chemical change. The potassium reacts with water to form potassium hydroxide (KOH) and hydrogen gas ( H₂), which are different chemical substances than the reactants.

Consider Breaking of Glass

Breaking glass is a physical change. The glass changes shape and size, but the material itself is not chemically altered and no new substances are formed.

Key concepts

physical change

A physical change is when a substance changes its physical form or appearance but its chemical structure remains the same. A good example of this is when water boils. While it changes from a liquid state to a gas, its molecules remain as H₂O. No new substances are created or destroyed in this process. Similarly, breaking glass is also a physical change. The shape and size of the glass pieces change, but they remain chemically unchanged. Let's summarize some key characteristics of physical changes:

• No new substances are formed
• Chemical composition remains the same
• Commonly involve changes in state, shape, or size

chemical change

Chemical changes involve a transformation at the molecular level, resulting in the formation of new substances. When potassium metal explodes in water, it is a vivid example of a chemical change. The reaction forms potassium hydroxide (KOH) and hydrogen gas (H₂) from the original potassium and water. This demonstrates the creation of substances that were not present before the reaction. Here are some key points about chemical changes:

• New substances are formed
• Initial substances' chemical composition changes
• Often involve energy change (light, heat, etc.)
• Usually irreversible under normal conditions

electrolysis

Electrolysis is a process in which electrical current is used to drive a chemical reaction. A classic example of electrolysis is decomposing water into hydrogen and oxygen gases. Here, water (H₂O) breaks down to form H₂ and O₂ upon the application of electrical energy. This is a chemical change as it results in the formation of new substances. Electrolysis is often used in industrial processes and has several applications, such as the extraction of metals and electroplating. Understanding the electrolysis process can be simplified by remembering these points:

• Uses electrical current to induce chemical changes
• Involves breaking down compounds into simpler substances
• Results in new products different from the reactants