Olympiad Test: Changes Around Us -1 - Class 6 MCQ

• Melting of ice is a reversible change.
• As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change.
• Melting is an example of a physical change.
• Physical changes can further be classified as reversible or irreversible.
• The melted ice cube may be refrozen, so melting is a reversible physical change.

In a chemical change, the molecules in the reactants interact to form new substances. In a physical change, like a state change or dissolving, no new substance is formed. Explain that another way to say that no atoms are created or destroyed in a chemical reaction is to say, “Mass is conserved.”

Boiling water is an example of a physical change and not a chemical change because the water vapor still has the same molecular structure as liquid water (H₂O). If the bubbles were caused by the decomposition of a molecule into a gas (such as H₂O →H₂ and O₂), then boiling would be a chemical change.

In a chemical change, energy can either be absorbed or released, depending on the nature of the reaction:

• Exothermic reactions: These release energy, often in the form of heat or light (e.g., combustion).
• Endothermic reactions: These absorb energy from the surroundings (e.g., photosynthesis).

Therefore, the correct answer is A) Energy is either absorbed or given out.

The other options are incorrect because energy in a chemical change can either be absorbed or released, not just absorbed or always given out.

The process described is an example of thermal expansion and contraction:

• When making a wooden wheel with an iron rim, the iron rim is made smaller than the wooden wheel.
• The iron rim is heated, causing it to expand (increase in size).
• While the iron rim is still hot and expanded, the wooden wheel is placed inside it.
• As the iron rim cools down, it contracts (shrinks) and fits tightly around the wooden wheel.

This process ensures a strong, secure fit between the two materials.

Thus, the correct answer is (A).

Metal wires, like electric or telephone wires, expand when heated and contract when cooled.
Here's how it works:

• In winter, the temperature is lower, so the wires cool down and contract, becoming tighter.
• In summer, the temperature increases, causing the metal wires to expand and become looser or sag a little.

Thus, metals expand on heating and contract on cooling, making the correct answer A) Expands on heating.

• A: Salt dissolved in water is a physical change, where salt dissolves in water but retains its properties. This is an example of mixing two substances.
• B: Mixing sand and water is also a physical change, where the two substances combine but their individual properties remain unchanged.
• D: Sugar dissolved in water is another physical change, where sugar dissolves in water without forming any new substance.
• C, Burning of a matchstick, is a chemical change because it involves a chemical reaction that produces new substances (such as carbon dioxide and ash), and it cannot be reversed.

Thus, the correct answer is C) Burning of a matchstick.

Undesirable Changes
Examples of Undesirable Changes:
- Eruption of volcano: Volcanic eruptions can cause widespread destruction, including the release of toxic gases, ashfall, and lava flows, which can lead to loss of life and property damage.
- Rusting of iron: Rusting of iron occurs when iron reacts with oxygen and moisture in the air, resulting in the formation of iron oxide. This can weaken structures made of iron and lead to corrosion and deterioration over time.
- A and B: Both the eruption of a volcano and rusting of iron are examples of undesirable changes as they can cause harm and have negative impacts.
Overall, undesirable changes refer to those changes that are not beneficial to us and may cause harm or negative consequences. These changes can have adverse effects on the environment, human health, and societal well-being. It is important to identify and understand such changes in order to mitigate their impacts and promote sustainable practices.

A: Cooking rice is a physical change:

• False. cooking rice is a chemical change because during this process one substance reacts with another to undergo a change in chemical composition 

B: Rotation of a fan is a Circular motion:

• True. The rotation of a fan is a repetitive and continuous motion, not a change.and it is circular motion .

C: Heat is absorbed or liberated during a change involving energy:

• True. Heat can be absorbed or released during various changes involving energy. For example, in a chemical reaction, heat can be absorbed (endothermic) or released (exothermic).

D: B and C are true:

• True. Both statement B and C are true. The rotation of a fan is a circular motion, and heat can be absorbed or released during changes involving energy.

Therefore, the correct answer is D: B and C are true.

Observation of the experiment:
Introduction:
The experiment involves adding washing soda to lemon juice in flask A and adding salt to lemon juice in flask B. The objective is to observe the reaction and determine whether a chemical or physical change takes place.
Bubbles formation:
- In flask A, after adding washing soda to lemon juice, lots of bubbles will be formed. This is because washing soda (sodium carbonate) reacts with the citric acid present in the lemon juice to produce carbon dioxide gas. The release of carbon dioxide gas is responsible for the formation of bubbles.
No bubbles formation:
- In flask B, after adding salt to lemon juice, no significant bubbles will be observed. This is because salt (sodium chloride) does not react with lemon juice to produce any gas. Therefore, there will be no bubbling effect.
Chemical change:
- The formation of bubbles in flask A indicates a chemical change. The reaction between washing soda and citric acid is a chemical reaction that results in the formation of a new substance (carbon dioxide gas).
Physical change:
- The absence of bubbles in flask B indicates a physical change. The addition of salt to lemon juice does not lead to the formation of any new substance or chemical reaction.
Conclusion:
Based on the observations, it can be concluded that the formation of bubbles in flask A indicates a chemical change, while the absence of bubbles in flask B indicates a physical change.

• 1: Zinc oxide changes color with heat (yellow when heated, white when cooled). This is a physical change because it is reversible; the color change is due to a change in the physical state of the zinc oxide, not a chemical reaction.
• 2: Magnetization of iron is a physical change. When a piece of iron is stroked with a magnet, it becomes magnetized. However, if the magnetized iron is hammered, it loses its magnetism. This is a reversible physical change.
• 3: Wax melting and solidifying is a classic example of a physical change. When wax is heated, it melts into a liquid. When cooled, it solidifies again. This change is reversible.

Since all the listed examples involve physical changes that can be reversed, the correct answer is C) 1, 2, and 3.

To analyze the given experiment and draw a conclusion, we need to consider the observations and their implications. Let's break it down step by step:
Observations:
1. A small amount of lead nitrate was taken in a dry test tube.
2. The test tube was heated over a flame.
3. A brown gas with a pungent smell was produced.
4. The gas turned blue litmus red.
5. A yellow solid residue remained in the test tube.
Analysis:
1. Heating lead nitrate (Pb(NO3)2) can cause it to decompose and release various gases.
2. The brown gas with a pungent smell is likely to be nitrogen dioxide (NO2), which is a common product of the decomposition of lead nitrate.
3. Nitrogen dioxide is acidic in nature, which is supported by the observation that it turns blue litmus red.
4. The yellow solid residue is most likely lead oxide (PbO), which is a product of the decomposition of lead nitrate.
Conclusion:
Based on the observations and analysis, the correct conclusion of the experiment is:
The brown gas evolved was nitrogen dioxide (NO2), which is acidic in nature. The residue was the yellow solid of lead oxide (PbO).
Therefore, the answer is A: The brown gas evolved was nitrogen dioxide which is acidic in nature. The residue was the yellow solid of lead oxide.

All the given examples demonstrate how energy keeps moving from one source to another, and that energy is not static:

• (A) Glowing of a light bulb: When electrical energy flows through the filament of the bulb, it is converted into light and heat energy. The energy is transferred from the electrical source to the bulb and then converted into different forms of energy (light and heat), showing the movement of energy.
• (B) Burning of a candle: In a candle, chemical energy stored in the wax is released as heat and light energy when the candle burns. The energy shifts from chemical form to thermal and luminous energy.
• (C) Baking of a cake: In baking, heat energy from the oven is transferred to the cake. This heat energy causes chemical reactions in the cake batter, transforming it into a fully baked cake.

In all of these cases, energy is not static but is transferred and transformed from one form to another.

Thus, the correct answer is D) All of them.

During a physical change, the arrangement of particles may change but the mass, number of atoms and number of molecules will stay the same. During a chemical change, the mass and number of atoms is conserved, but the number of molecules is not always the same. Chemical reactions involve large changes in energy.

However, one thing that does not indicate a chemical change is the change of shape. When the new form occurs from these two substances are mixed mean that it could be something else like a physical change. There are different types of chemical changes. One is the chemical change, and the other is organic change.

The common feature among the given phenomena is that they all involve a chemical change. Let's look at each phenomenon individually:

1. A slice of apple, if kept in the open, develops a brown color: This is due to a chemical reaction called oxidation, where the oxygen in the air reacts with the compounds in the apple, leading to the formation of new substances.
2. Burning of magnesium ribbon: This is a chemical reaction known as combustion. The magnesium combines with oxygen in the air to form magnesium oxide, which is a new substance.
3. A new substance is formed during this change: This statement is true for both the browning of the apple slice and the burning of magnesium ribbon. In both cases, the original substances are transformed into new substances with different properties.

Based on these explanations, we can conclude that all the given phenomena are examples of chemical changes, as new substances are formed during the processes. Therefore, the correct answer is option C: All are examples of chemical changes.

A. Hardening of cement is an irreversible change as we cannot reverse back to the original cement powder form once the cement hardens.
B. Freezing of ice-cream is reversible change as it can be reversed back by heating and it will convert to original form.
C. Opening of a door is reversible change as it can be reversed simply by closing the door.
D. Melting of chocolate is reversible change as it can be reversed by cooling the chocolate to its original form.
Hence, option A is the correct answer.

The incorrect statement is C: When the temperature of a solid is increased, the thermal energy of its particles increases.

• The statement is incorrect because when the temperature of a solid is increased, the thermal energy of its particles remains the same.
• Thermal energy is the total kinetic energy of all the particles in a substance, including both kinetic energy due to motion and potential energy due to intermolecular forces.
• When the temperature of a solid is increased, the average kinetic energy of its particles increases, but the potential energy remains the same.
• The increase in temperature causes the particles to vibrate faster and move more vigorously, resulting in an increase in kinetic energy.
• However, the total thermal energy of the particles, which is the sum of kinetic and potential energy, remains constant unless there is a change in the substance (e.g., a phase change).

Therefore, statement C is not correct.

A chemical change is permanent change and new substance is formed in it. Forms of energy, such as heat, light or electricity, may be emitted or absorbed during a chemical reaction. A chemical change is generally irreversible. Hence option B is correct.

Mixing salt in water is a change that can be reversed by evaporation. It is a type of reversible change in which the physical state of salt changes, from solid to aqueous, other than that its chemical composition remains the same. Hence the water will get converted into its gaseous form(steam), leaving behind solid salt.