Science 2 Marks Questions

The formation of soil from rocks involves both physical and chemical changes. Natural factors like wind, rain and temperature break down rocks into smaller pieces (physical change), while chemical processes, like weathering, also change the minerals in the rocks (chemical change). Both types of changes work together to form soil.

Both, the change of water to ice and water to steam are physical changes. In both processes, the chemical composition of water remains the same; only its state changes from liquid to solid (ice) or from liquid to gas (steam). No new substance is formed which is a characteristic of physical changes.

Melting of ice into water is a reversible physical change because water can be frozen back into ice.
Inflating and deflating a balloon is also reversible because no new substance is formed.

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and usually light.
The prerequisite conditions for combustion to occur are the presence of a combustible substance, sufficient supply of oxygen, and a suitable ignition temperature.

Melting of wax is a physical change and burning of wax is a chemical change when a candle burns.
Another example is burning of LPG gas, where gas burning is a chemical change and melting of solid impurities is a physical change.

The burning of crackers is a chemical change because new substances are formed along with heat, light, and sound, and the change is irreversible.

Setting of curd from milk is a chemical change because new substances are formed and milk cannot be obtained again from curd.

(a) Photosynthesis is a chemical change because new substances like glucose and oxygen are formed.
(b) Dissolving sugar in water is a physical change because no new substance is formed and sugar can be recovered.
(c) Burning of coal is a chemical change because new substances like carbon dioxide and ash are produced.
(d) Melting of wax is a physical change because only the state of wax changes and no new substance is formed.
(e) Beating aluminium to make aluminium foil is a physical change because only the shape of aluminium changes.
(f) Digestion of food is a chemical change because food is converted into new substances.

(i) Folding of paper to make a boat is a physical and reversible change because only the shape of the paper changes and it can be unfolded back.
(ii) Cooking chapati from wheat flour is a chemical and irreversible change because new substances are formed and the original flour cannot be obtained again.

Burning of wood is a chemical change because new substances like ash and gases are formed and it is irreversible.
Cutting wood into small pieces is a physical change because only the size changes and no new substance is formed.

Bioluminescence is the ability of living organisms to produce light by a chemical reaction within their bodies.
An example of bioluminescence is the glow produced by fireflies.

The burning of a candle involves both physical and chemical changes because the melting of wax is a physical change, while the burning of wax produces new substances like carbon dioxide and water vapour, which is a chemical change.

It is a chemical change because carbon dioxide gas is produced and new substances are formed.

Ignition temperature is the minimum temperature at which a substance starts burning in air.

Lime water turns milky when carbon dioxide gas is passed through it because carbon dioxide reacts with lime water to form insoluble calcium carbonate, which makes the solution appear milky.

Making popcorns from corns is a chemical and irreversible change because new substances are formed and it cannot be reversed.

Two examples of physical changes are melting of ice and dissolving sugar in water.

Rust is a reddish-brown substance formed when iron reacts with oxygen and moisture in the air.

Characteristics of a chemical change:
1. Mostly chemical reactions are irreversible.
2. They cause release or absorption of energy.
3. New substances with new properties are formed.
4. The properties of reactants are different from the products.

(a) On passing carbon dioxide gas through lime water, the lime water turns milky due to the formation of a white coloured insoluble calcium carbonate.
(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate due to the formation of carbon dioxide gas.

Get a small and thin strip or ribbon of magnesium. Clean its tip with sand paper. Bring the tip near a candle flame. It burns with a brilliant white light. After combustion white powdery ash is left, it is called magnesium oxide.

The changes in which a substance undergoes a change in its physical properties are called physical changes.
Physical changes are generally reversible. In such changes, no new substances are formed. For example, melting of ice, formation of steam, etc.

Conclusion: Burning of a candle involves both physical and chemical change. Melting of wax, evaporation of wax to wax vapour and solidification of melted wax to solid wax are all physical changes. While burning of wax to produce new substance, carbon dioxide is a chemical change.

Observation and conclusion: In figure (a), candle keeps on burning as it is not covered and supply of air (oxygen) is available. But in figure (b), as candle is covered with a glass tumbler, does not get a continuous supply of air, so, it gets extinguished. This concludes oxygen is required for combustion.

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Generally, the chemical changes that occur during cooking are difficult to reverse.
Example: when you bake a cake, the ingredients undergo chemical reactions to form new substances with different properties. It is not possible to return the cake back to its original raw ingredients.

When a burning candle is covered with a glass, the flame will eventually extinguish due to the consumption of oxygen.
Combustion requires oxygen to sustain the burning process. As the candle burns inside the closed glass, it uses up the available oxygen. Once the oxygen level drops below a critical point, the flame can no longer be supported, and the candle goes out. This demonstrates that oxygen is essential for combustion.

Changes in the environment, such as temperature fluctuations and the presence of water or chemicals, can significantly impact natural processes like erosion and weathering. Temperature changes can cause rocks to break apart, while water and chemicals can react with rocks, leading to their decomposition. These processes contribute to the formation of soil and the shaping of landscapes over time.

Yes, physical changes can have harmful effects.
Example: The increased consumption of fuels in cars and airplanes leads to a rise in carbon dioxide levels in the atmosphere, contributing to climate change. Similarly, the drying of paint on walls and furniture releases substances through evaporation, causing atmospheric pollution.

To confirm if a change is chemical, check for indicators like gas evolution, color change, heat production, or precipitate formation.
If mixing substances A and B results in a new substance with significantly different properties (like a new color, gas, or solid), and the change is not easily reversible, it indicates a chemical reaction. These signs suggest that new chemical bonds have formed.

→ Mixing vinegar and baking soda leads to a chemical change that produces new substances.
→ The reaction releases carbon dioxide gas, which you see as bubbles. Additionally, other new substances are formed, indicating that the original compounds have been transformed into something new.

The making of curd from milk exemplifies a chemical change because it involves the transformation of milk into a new substance with different properties. During curdling, bacteria convert lactose (milk sugar) into lactic acid, resulting in a change in taste, texture, and acidity. This conversion indicates a chemical reaction, making curd formation a chemical change.

Burning a candle involves both physical and chemical changes.
→ Physically, the wax melts (state change) due to the heat, and this melted wax can solidify again if cooled.
→ Chemically, the wax undergoes combustion, reacting with oxygen to produce carbon dioxide and water vapor (new substances), releasing heat and light. This combustion is irreversible.

Rusting of iron is a chemical change because it involves the formation of a new substance (rust) with different properties than the original iron.

A physical change alters the form or appearance of a substance but doesn't change it into a new substance.
Example:
(i) Melting ice
(ii) Chopping vegetables.