Question

Determine whether each change is physical or chemical. (Chapter 3) a. Water boils. b. A match burns. c. Sugar dissolves in water. d. Sodium reacts with water. e. Ice cream melts

Short answer

a. Physical change b. Chemical change c. Physical change d. Chemical change e. Physical change

Step-by-step solution

a. Water boils.

Boiling water is a change in the physical state of water from liquid to gas without altering its chemical composition. The water molecules are still H2O during and after boiling. Therefore, this change is a physical change.

b. A match burns.

When a match burns, a chemical reaction occurs in which the match's chemicals (phosphorus, sulfur, and other components) react with oxygen in the air to produce heat, light, and new products such as smoke and ash. This change in chemical composition makes this change a chemical change.

c. Sugar dissolves in water.

When sugar dissolves in water, the sugar molecules are still the same both before and after dissolving (C12H22O11). The dissolving process involves the sugar molecules being separated by water molecules without any chemical reaction happening. Therefore, this change is a physical change.

d. Sodium reacts with water.

When sodium reacts with water, a chemical reaction occurs in which sodium (Na) and water (H2O) form new products like sodium hydroxide (NaOH) and hydrogen gas (H2). This change involves a change in the chemical composition of the substances and is, therefore, a chemical change.

e. Ice cream melts.

Melting ice cream is a change in the physical state of the ice cream from solid to liquid without changing its chemical composition. The molecules in the ice cream are still the same before and after melting. Therefore, this change is a physical change.

Key concepts

Boiling Water

Boiling water is a fascinating example of a physical change. When water boils, it undergoes a transformation from a liquid state to a gas, specifically steam or water vapor. This change occurs because heat energy is introduced, causing the water molecules to move more vigorously until they have enough energy to break free from the liquid and disperse as gas.
It's important to note that during this entire process, the chemical composition of the water remains unchanged as it is composed of the same H2O molecules. This is what classifies boiling as a physical change - no new substances are created, and the original substance retains its molecular structure.

• No change in chemical composition.
• Transition from liquid to gas.
• H2O molecules remain the same.

Boiling is a reversible process, meaning the water vapor can condense back to liquid water without any chemical alteration.

Chemical Reactions

A chemical reaction involves transforming substances where original molecules convert into new substances. One of the well-known examples is the burning of a match. When you ignite a match, the compounds in the match head, including sulfur and phosphorus, react with oxygen in the air, causing a combustion process.
This reaction results in new products like smoke and ash, making it a prime example of a chemical change. Unlike physical changes, chemical changes are usually irreversible and involve energy exchange, often in the form of heat and light.

• Original substances change into new products.
• Usually irreversible.
• Involves energy changes.

Chemical reactions are fundamental to various natural and industrial processes, shaping the world around us.

Dissolving Sugar

Dissolving sugar in water is a classic example of a physical change, although it can initially seem like a chemical process due to the apparent change. In reality, the sugar molecules merely disperse among the water molecules, forming a homogeneous mixture called a solution.
The chemical structure of sugar (C12H22O11) remains unaltered throughout this process. If the water is evaporated, the sugar will recrystallize, proving no chemical change has happened.

• Sugar molecules remain intact.
• Formation of a solution.
• Reversible by evaporation.

This experiment often helps demonstrate the difference between a physical and a chemical change in educational settings.

Melting Ice Cream

Melting ice cream is a delightful and delicious example of a physical change. When ice cream melts, it transitions from a solid to a liquid state. This change is purely physical as the chemical composition of the ice cream remains unchanged.
This physical transformation is due to the increase in temperature, which provides energy for the molecules to move more freely and transition into a liquid form.

• Change from solid to liquid.
• Chemical structure remains unchanged.
• Reversible through freezing.

The process is reversible if you return the melted ice cream to a colder environment, where it can refreeze.

Sodium and Water Reaction

When sodium comes into contact with water, a rapid and energetic chemical reaction occurs. Sodium (Na) reacts with water (H2O) to produce sodium hydroxide (NaOH) and hydrogen gas (H2).
This reaction produces significant heat and results in a completely different substance from either of the reactants, showcasing a chemical change. Often, this reaction can be seen with a fizzing or bubbling as hydrogen gas is released.

• Formation of new substances (NaOH and H2).
• Exothermic (releases heat).
• Naturally occurs with reactive metals like sodium.

This demonstration is a dramatic illustration of how reactive certain elements can be when introduced to water.