Chapter 2 – Physical and Chemical Changes

A set of flashcards covering definitions, classifications, characteristics, examples, and energy aspects of physical and chemical changes as presented in Chapter 2.

How is a physical change defined?

A change in which no new substance is formed; it is usually temporary and reversible, and the chemical composition of the substance remains the same.

How is a chemical change defined?

A permanent, irreversible change in which entirely new substance(s) with different properties are formed, usually accompanied by energy changes such as heat, light, or sound.

What distinguishes a slow change from a fast change?

Slow changes occur over a long period (hours, days, months, years), whereas fast changes happen in seconds or milliseconds.

Give two examples of slow changes.

Examples include germination of seeds and rusting of iron nails.

Give two examples of fast changes.

Examples include bursting of a balloon and burning of a matchstick.

What is a reversible change?

A change that can be undone by reversing the conditions, restoring the original state of the substance.

State two examples of reversible changes.

Melting and refreezing ice; stretching and releasing a rubber band.

What is an irreversible change?

A change that cannot be undone; the original substance cannot be recovered by simple means.

State two examples of irreversible changes.

Burning of paper and curdling of milk.

Define a periodic change.

A change that repeats itself at regular, fixed intervals of time.

Give two examples of periodic changes.

Swinging of a clock pendulum and phases of the Moon.

Define a non-periodic change.

A change that does not repeat at regular intervals and can occur unpredictably.

Give two examples of non-periodic changes.

Earthquakes and car accidents.

What are desirable changes?

Changes that benefit us in some way, such as drying clothes or ripening fruits.

What are undesirable changes?

Changes that cause harm or destruction, such as floods or food spoilage.

Differentiate between endothermic and exothermic changes.

Endothermic changes absorb heat (e.g., melting ice), whereas exothermic changes release heat (e.g., burning a matchstick).

List four conditions that affect the rate of evaporation.

Nature of the liquid, temperature, surface area, and humidity of the surrounding air.

Why does rubbing alcohol feel cool on the skin?

Because its evaporation is an endothermic physical change that absorbs heat from the skin.

Explain why dissolving sugar in water is a physical change.

The sugar molecules remain present; evaporation of the water recovers the same sugar without new substances forming.

Why is rusting of iron considered a chemical change?

Iron reacts with oxygen and moisture to form a new, non-magnetic substance (rust) with different properties and greater mass.

What is pasteurisation?

A method of heating (e.g., milk at 63 °C for 30 min) followed by rapid cooling to kill most bacteria and prolong freshness.

Define fermentation.

A chemical change in which microorganisms convert sugars into alcohols, acids, or gases, altering taste, smell, and composition.

State two characteristics of physical changes.

(1) Usually reversible, (2) no new substance forms and mass remains unchanged.

State two characteristics of chemical changes.

(1) Produce new substances with new properties, (2) generally irreversible and involve energy changes.

How can rusting of iron be prevented?

By applying grease, oil, or paint to create a barrier against oxygen and moisture.

What is meant by ‘interaction’ in the context of changes?

When two or more substances act on each other, causing changes in their properties (e.g., axe blade dulling while cutting wood).

How are energy changes involved in a physical change of state?

Heat is absorbed during melting/evaporation and released during freezing/condensation.

Give an example where light energy drives a chemical change.

Photosynthesis, where green plants use sunlight to convert carbon dioxide and water into glucose and oxygen.

Why is burning of candle wax both a physical and a chemical change?

Wax melts (physical), while combustion produces new substances like CO₂, water vapour, and soot (chemical).

What type of change occurs when the leaflets of a Mimosa plant close on touch?

A reversible physical change.

Classify magnetisation of an iron rod.

A physical, reversible change; the iron can lose magnetism on heating.

Name two controllable changes.

Rusting of iron (can be slowed) and fermentation (can be regulated by temperature).

Name two uncontrollable changes.

Change of day and night, and phases of the Moon.

What criterion distinguishes physical from chemical changes regarding mass?

Physical changes show no overall mass change, whereas chemical changes may involve mass change (e.g., rusting increases mass).

Why is cooking of food classified as a chemical change?

It produces new substances with different taste, smell, and composition that cannot revert to raw ingredients.

What is an example of energy absorption during a chemical change?

Photosynthesis absorbs light energy to form glucose.

Describe one example of exothermic dissolution.

Dissolving quicklime (CaO) in water forms slaked lime (Ca(OH)₂) while releasing large amounts of heat.

Which property of metals allows them to be hammered into sheets, and what type of change is involved?

Malleability; the hammering is a physical change that alters shape without forming new substances.

What happens to total energy during any change?

Energy is conserved; it is either absorbed from or released to the surroundings but not destroyed.

State two examples of physical changes that are irreversible.

Breaking a glass tumbler and tearing a sheet of paper (cannot restore the exact original form).

Why is the formation of curd from milk a chemical change?

It produces new substances (lactic acid, curd proteins) with different taste and cannot be converted back into milk.

What type of change is melting butter and why?

A physical change because only the state and shape change; composition remains butter.

Explain tidal energy in terms of periodic change.

Tides are periodic rises and falls of sea levels; their predictable motion can be harnessed to drive turbines for electricity.

Give two examples of desirable chemical changes in daily life.

Cooking of food and fermentation in bread making.

Which factor among temperature, surface area, humidity, and liquid nature decreases evaporation rate?

High humidity slows down evaporation.

How does humidity influence the drying of clothes?

Lower humidity increases evaporation, making clothes dry faster; higher humidity slows the process.

Why is beating heart classified as a periodic change?

Because it repeats at regular intervals (heartbeat cycles) throughout life.

What is meant by ‘chemical reaction’?

A process in which chemical changes occur, producing new substances with different chemical identities.