Olympiad Test: Changes Around Us -2 - Class 6 MCQ

When a solid substance, like wax, melts, it undergoes a change of state from solid to liquid. This process is a physical change, and the mass of the substance remains the same before and after the change of state. Therefore, 10 gm of solid wax will form 10 gm of molten wax after melting, as no mass is lost during the melting process.

A periodic change refers to a change that occurs at regular intervals or in a repeated pattern.

• The heartbeat is a perfect example of a periodic change because it occurs at regular intervals, repeating in a predictable cycle.
• A landslide, car accident, and volcanic eruption are non-periodic events as they do not happen at regular intervals and are often unpredictable.

When cement gets wet and then hardens as it dries, it undergoes a chemical change. The water causes a chemical reaction with the cement, resulting in the formation of solid cement (hardening). Once this chemical reaction has occurred, it cannot be easily reversed, which means the change is irreversible.

• B: While it's true that the change is irreversible, option A provides a more accurate explanation by mentioning the chemical change involved.
• C: The change cannot be reversed because it's a chemical change, not a physical one.
• D: Cement hardening is a chemical change, not a physical change, so it is not easily reversible.

Thus, A is the most accurate option.

When a few drops of petrol are poured on the palm, the following changes occur:

1. Evaporation: The drops of petrol start to evaporate, which means they transition from a liquid state to a gaseous state.
2. Heat Absorption: During the evaporation process, the petrol molecules gain energy from the surroundings, including the palm. This absorption of energy leads to cooling of the palm and gives a cool sensation.

Based on these observations, we can conclude that the change is an endothermic change because it involves the absorption of heat from the surroundings.
Therefore, the correct answer is D: Endothermic change.

The examples given in the question are:

1. The motion of planets around the sun
2. The motion of fan blades
3. The blinking of a traffic light

These examples are all related to periodic changes.
Periodic changes are changes that occur in a regular, repeating pattern. In these examples:

1. The motion of planets around the sun: Planets move in predictable orbits around the sun, completing their revolutions in fixed periods of time. This motion is periodic.
2. The motion of fan blades: When a fan is turned on, its blades rotate in a continuous, repetitive motion. Each rotation is a complete cycle, and the motion repeats itself. This motion is also periodic.
3. The blinking of a traffic light: Traffic lights have a specific pattern of blinking to indicate different signals, such as green for "go," yellow for "caution," and red for "stop." This blinking pattern repeats in a cycle, making it a periodic change.

Therefore, the correct answer is B: Periodic changes.

In an endothermic reaction, energy is absorbed from the surroundings, typically in the form of heat. This causes the surroundings to feel cooler. The reaction requires an input of energy to proceed, and this energy is used to break the bonds in the reactants. Examples of endothermic reactions include photosynthesis and the melting of ice.

• A: Chemical reactions slow down in a cool environment – Lower temperatures reduce the rate of chemical reactions, which helps preserve the potency and shelf life of medicines and food by slowing down degradation or spoilage reactions.
• C: Microorganisms cannot survive in a cool environment – Many microorganisms, including bacteria and fungi, grow more slowly or cannot survive in cooler temperatures. This helps prevent food from spoiling or medicines from losing effectiveness due to microbial contamination.

Thus, both slowing down chemical reactions and inhibiting microbial growth are reasons for storing in a cool and dry place.

The product formed by dissolving a substance into another is called a solution.
A solution is a homogeneous mixture formed when a solute is dissolved in a solvent. The solute is the substance that is being dissolved, while the solvent is the substance that dissolves the solute.
Here is a detailed explanation:
Definition of a:

• A solution is a homogeneous mixture composed of two or more substances.
• The solute is the substance that is being dissolved, and the solvent is the substance that dissolves the solute.
• When a solute is dissolved in a solvent, the resulting mixture is called a solution.

Characteristics of a:

• A solution is a homogeneous mixture, meaning that the composition is uniform throughout.
• It can be in the form of a liquid, gas, or solid.
• The solute particles are evenly dispersed and cannot be easily separated from the solvent.
• A solution can be separated into its components by physical means, such as evaporation or filtration.

Examples of Solutions:

• A sugar solution, where sugar (solute) is dissolved in water (solvent).
• Saltwater, where salt (solute) is dissolved in water (solvent).
• Carbonated drinks, where carbon dioxide gas (solute) is dissolved in a liquid (solvent).

In summary, a solution is the product formed when a substance (solute) is dissolved in another substance (solvent). It is a homogeneous mixture with uniform composition throughout. The solute particles are evenly dispersed and cannot be easily separated from the solvent.

Atoms combine through the interaction of electrons.

• The process of atom combination is known as chemical bonding.
• Chemical bonding occurs due to the interaction of electrons between atoms.
• Electrons are negatively charged particles that orbit around the nucleus of an atom.
• Atoms have a tendency to gain, lose, or share electrons to achieve a stable electron configuration.
• The interaction of electrons allows atoms to form chemical bonds and create compounds.
• There are different types of chemical bonds, including ionic bonds, covalent bonds, and metallic bonds.
• In ionic bonding, electrons are transferred from one atom to another, resulting in the formation of ions.
• In covalent bonding, electrons are shared between atoms to complete their outer electron shells.
• In metallic bonding, electrons are delocalized and shared among a lattice of metal atoms.

In summary, atoms combine through the interaction of electrons, which leads to the formation of chemical bonds and the creation of compounds.

Substances react with each other and form chemical bonds primarily to become stable. In a chemical reaction, atoms or molecules rearrange to form new compounds. This typically occurs because the resulting compounds are more stable than the individual reactants.

The concept of stability is often related to the energy of the system. Atoms tend to form bonds to lower their overall energy, achieving a more stable, lower-energy state. This is why the formation of chemical bonds is associated with the release of energy in exothermic reactions or the absorption of energy in endothermic reactions.

• Forming newer compounds (Option A) is a result of the chemical reaction, but the driving force is the goal of stability.
• Increasing energy (Option B) is not typically the goal of bond formation. Instead, energy is often released as a result of bond formation, making the products more stable.
• Decreasing energy (Option C) is a more accurate reflection of what happens during chemical reactions as the system tends to lower its energy for stability.

Therefore, the main goal is to become stable by forming new, more stable compounds.