• chemical change

ans: Milk is converted into curd or yogurt by the process of fermentation. Milk consists of globular proteins called casein. Here curd forms because of the chemical reaction between the lactic acid bacteria and casein. During fermentation, the bacteria use enzymes to produce energy (ATP) from lactose.

• Bursting of a cracker- Chemical change
• The Occurence of an earthquake- Natural change
• Revolution of the earth around the sun- Periodic change
• The stretching of a spring can be classified as a physical change as there is no change in the chemical composition of the spring. Also, it's a reversible change because when the applied force is removed, the spring comes back to its original shape.

hop its help u

(a) Conversion of milk into yoghurt:

• This is a chemical change because it involves a change in the chemical composition of the milk.
• This chemical reaction results in the formation of yoghurt, which has a different chemical composition and physical properties than the original milk.

(b) Bursting of a cracker:

• This is a chemical change because it involves a change in the chemical composition of the cracker.
• When a cracker bursts, the energy stored in the chemical bonds of the cracker's ingredients is released, resulting in a chemical reaction that produces gases, heat, and light.

(c) Occurrence of an earthquake:

• This is a physical change because it involves a change in the physical state or appearance of the earth, but not in its chemical composition.
• An earthquake is caused by the movement of tectonic plates, which can result in the shaking and displacement of the earth's surface.
• This physical change does not involve a chemical reaction, and the earth's chemical composition remains unchanged.

(d) Revolution of the earth around the sun:

• This is a physical change because it involves a change in the position and movement of the earth, but not in its chemical composition.
• The earth revolves around the sun as a result of the gravitational forces acting upon it. 3
• This physical change does not involve a chemical reaction, and the earth's chemical composition remains unchanged.

(e) Stretching of a spring:

• This is a physical change because it involves a change in the shape and size of the spring, but not in its chemical composition.
• When a spring is stretched, it becomes longer and may also become thinner, but its chemical composition remains unchanged.
• This physical change does not involve a chemical reaction

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