Physical and Chemical Changes Video Lecture - Science Class 9

Ans. A physical change is a type of change in which the substance involved undergoes a transformation that does not change its chemical composition. It only affects the physical properties of the substance, such as its shape, size, color, or state of matter. Examples of physical changes include melting ice, boiling water, crushing a can, or dissolving sugar in water.

Ans. A chemical change, also known as a chemical reaction, is a process that results in the formation of new substances with different chemical compositions and properties. It involves the breaking and forming of chemical bonds between atoms. Chemical changes often produce noticeable changes in color, odor, temperature, or the formation of a precipitate. Examples of chemical changes include burning wood, rusting of iron, digestion of food, or the fermentation of grapes to make wine.

Ans. The following characteristics can help distinguish between physical and chemical changes: - In a physical change, the substance retains its original chemical composition, whereas in a chemical change, new substances with different chemical compositions are formed. - Physical changes only affect the physical properties of a substance, such as its shape or state of matter, while chemical changes involve a rearrangement of atoms and result in the formation of new substances. - Physical changes are usually reversible, while chemical changes are often irreversible. - Physical changes do not involve the breaking or forming of chemical bonds, whereas chemical changes do. - Physical changes do not usually produce energy changes, while chemical changes often release or absorb energy.

Ans. Some examples of physical changes include: - Melting ice: The solid ice changes into liquid water without any change in its chemical composition. - Boiling water: The liquid water changes into water vapor without any change in its chemical composition. - Stretching a rubber band: The rubber band changes its shape but remains chemically the same. - Crushing a can: The can is physically deformed but does not undergo any chemical change. - Dissolving salt in water: The salt crystals break apart and mix with the water, but the chemical composition of salt remains unchanged.

Ans. Some examples of chemical changes include: - Burning wood: The wood undergoes combustion and transforms into ash, smoke, and gases, resulting in the release of heat and light. - Rusting of iron: Iron reacts with oxygen in the presence of moisture to form iron oxide, resulting in the characteristic reddish-brown rust. - Digestion of food: The complex molecules in food are broken down into simpler substances through chemical reactions in the digestive system. - Cooking an egg: The heat causes the proteins in the egg to denature and form new chemical compounds, resulting in a solidified and cooked egg. - Souring of milk: Bacteria ferment the lactose in milk, converting it into lactic acid, which changes the taste and texture of the milk.