Olympiad Test: Physical And Chemical Changes - 2 - Class 7 MCQ

 C) Iron, ferrous sulphate

When a shaving blade is placed in a copper sulphate solution, the colour changes from blue to green due to the formation of iron sulphate, a new substance. Copper, another new substance, is the brown deposit on the iron nail.

The reaction can be written as:

Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

A firework explosion is a chemical change. Such an explosion produces heat, light, sound and unpleasant gases, which are characteristics of a chemical change.

Food contains fats which, when exposed to air, get oxidised and result in the formation of products which have bad taste and smell. This oxidation of food is known as rancidity.

Substance X is a base since the litmus paper turns blue. Phenolphthalein is an indicator that is used to test for alkalinity because it is colorless if used with an acidic or neutral substance and it is pink if it is used with a basic substance.

The chemical name of slaked lime is calcium hydroxide and calcium oxide is the chemical name of quick lime. 

Since, no new substances are formed in this change, it is a physical change. 

To determine the formula of rust, we need to understand the composition of rust and the elements involved. Rust is a common name for a very common compound, iron(III) oxide (Fe2O3).
Here is a detailed explanation of why the answer is C: Fe2O3:
1. Rust Composition:
- Rust is a type of corrosion that occurs when iron or steel reacts with oxygen and moisture in the presence of air.
- The main component of rust is iron oxide, which is a compound formed between iron and oxygen.
2. Iron Oxide:
- Iron oxide is a chemical compound made up of iron (Fe) and oxygen (O).
- It exists in different forms, depending on the ratio of iron to oxygen atoms.
3. Iron Oxide Formulas:
- Iron(II) oxide (FeO) is a compound where iron has a +2 oxidation state.
- Iron(III) oxide (Fe2O3) is a compound where iron has a +3 oxidation state.
- Iron(II,III) oxide (Fe3O4) is a compound where iron has both +2 and +3 oxidation states.
4. Rust Formula:
- Rust is commonly known as iron(III) oxide or ferric oxide.
- The formula for rust is Fe2O3, which means it contains two iron atoms (Fe) and three oxygen atoms (O).
Therefore, the correct answer is C: Fe2O3.

The process of separating a pure substance in the form of crystals from the hot saturated solution by cooling is called crystallization.
Crystallization is a process widely used in various industries, including pharmaceuticals, chemicals, and food processing. Here is a detailed explanation of the process:
1. Hot Saturated The process starts with a hot saturated solution. A saturated solution is a solution that contains the maximum amount of solute that can be dissolved at a given temperature. The solution is heated to dissolve as much solute as possible.
2. Cooling: The hot saturated solution is then allowed to cool slowly. As the temperature decreases, the solubility of the solute decreases. At a certain temperature known as the saturation point, the solute starts to come out of the solution in the form of crystals.
3. Crystal Formation: The solute molecules arrange themselves in an ordered, repeating pattern to form crystals. The size and shape of the crystals depend on various factors such as the rate of cooling, the concentration of the solute, and the presence of impurities.
4. Separation: Once the crystals have formed, they can be separated from the remaining liquid by filtration or centrifugation. The liquid portion, known as the mother liquor, contains the impurities and can be further processed or discarded.
5. Purification: The obtained crystals may still contain impurities. To obtain a pure substance, the crystals can be washed with a suitable solvent or undergo additional purification techniques such as recrystallization.
In summary, the process of crystallization involves cooling a hot saturated solution to induce the formation of crystals, which can then be separated and purified to obtain a pure substance.

Explanation:
When carbon dioxide gas (CO2) is passed through limewater (a solution of calcium hydroxide, Ca(OH)2), a chemical reaction takes place, resulting in a change in the limewater. This change is a chemical change rather than a physical change.
Reasoning:
1. A physical change involves a change in physical properties such as size, shape, or state of matter, without the formation of a new substance.
2. A chemical change involves the formation of new substances with different chemical properties.
Explanation of the reaction:
1. Limewater is a colorless solution of calcium hydroxide in water.
2. When carbon dioxide gas is passed through limewater, it reacts with the calcium hydroxide, forming a white precipitate of calcium carbonate (CaCO3).
3. The reaction can be represented by the following chemical equation:
Ca(OH)2 + CO2 → CaCO3 + H2O
4. The formation of calcium carbonate is a chemical change, as it involves the rearrangement of atoms and the formation of a new substance with different chemical properties.
5. The milky white precipitate formed indicates the presence of calcium carbonate and confirms the occurrence of a chemical change.
Conclusion:
The change of limewater in the presence of carbon dioxide is a chemical change, as it involves the formation of a new substance (calcium carbonate) with different chemical properties.

The Crystals Obtained After Crystallization

Crystallization is a process that involves the formation of solid crystals from a solution. The crystals obtained after crystallization can vary in purity depending on the specific conditions and techniques used. However, in general, the purity of the crystals is determined by the amount of impurities present in the crystal lattice structure.

Options:


A: 10% pure

This option suggests that the crystals obtained after crystallization are only 10% pure. This means that 90% of the crystal is composed of impurities.

B: between 50% to 90% pure

This option suggests that the crystals obtained after crystallization can range in purity from 50% to 90%. This means that the crystals may contain a significant amount of impurities, but they are still relatively pure.

C: between 20% to 50% pure

This option suggests that the crystals obtained after crystallization can range in purity from 20% to 50%. This means that the crystals have a moderate amount of impurities present.

D: more than 90% pure

This option suggests that the crystals obtained after crystallization are more than 90% pure. This means that the crystals have a high level of purity, with minimal impurities present.

Conclusion:

The correct answer is option D: more than 90% pure. The crystals obtained after crystallization are typically highly pure, with minimal impurities present in their crystal lattice structure.