Q1. Define a mixture. Explain homogeneous and heterogeneous mixtures with examples and ways to separate them.
A mixture is a combination of substances that retain their properties.
A homogeneous mixture looks the same throughout. Example: sugar water.
A heterogeneous mixture has visible parts. Example: salad.
In mixtures, composition is variable. It can change.
We can separate mixtures by physical methods like filtration, evaporation, and decantation.
Example: Sugar water can be separated by evaporation. Salad parts can be picked out.
Q2. Why do components of a mixture retain their properties? Use given examples to explain.
In a mixture, substances are only physically combined.
They do not form new substances. So they keep their own properties.
In air, oxygen still helps in breathing, and carbon dioxide is still present as a gas.
In a salad, tomatoes are still juicy, and cucumbers stay crisp.
In sand and salt, the saltiness and the grainy sand texture both remain.
Because of this, mixtures can be separated physically without any chemical change.
Q3. What is a compound? Explain its fixed composition, changed properties, and separation.
A compound is a pure substance formed by chemical combination.
It has fixed proportions. Example: Water (H₂O) always has 2 parts hydrogen and 1 part oxygen.
Its properties are different from its elements.
Example: Sodium (reactive) and Chlorine (poisonous) make table salt (safe).
Compounds have a chemical formula like H₂O, NaCl, CO₂.
They cannot be separated by physical means. They need chemical reactions to split.
Q4. State four key differences between mixtures and compounds with examples.
In a mixture, composition is variable. In a compound, it is fixed.
Mixtures retain properties of their parts. Compounds have new properties.
Mixtures use physical separation. Compounds need chemical changes.
Examples of mixtures: air, salad, sand and salt.
Examples of compounds: water, table salt, carbon dioxide.
So, mixtures are physical combinations, while compounds are chemical combinations.
Q5. Describe three physical separation methods for mixtures and when each is used.
Filtration separates insoluble solids from liquids. Example: sand from water.
Evaporation removes a liquid to get a dissolved solid. Example: salt from saltwater.
Decantation separates a heavier solid or liquid by pouring off the top layer.
These methods work because components retain their properties in mixtures.
No new substances form during these steps.
They are simple and need no chemical reactions.
High Complexity (Analysis & Scenario-Based)
Q6. You are given a mixture of sand and salt. Design a complete method to separate them and explain why it works.
First, add water. Salt dissolves. Sand does not.
Stir and let the sand settle.
Use filtration to separate the sand on the filter paper.
Heat the salt solution to evaporate water. You get salt back.
This works because a mixture allows physical separation using solubility, filtration, and evaporation.
No new substances are formed. Properties are retained.
Q7. A student mixes sugar in water and gets a clear solution. Identify the type of mixture and suggest a way to separate sugar from it. Explain your reasoning.
The solution is a homogeneous mixture. It looks uniform.
Sugar retains its sweetness even though we cannot see it.
The composition is variable. You can add more or less sugar.
To separate, use evaporation. Heat the solution to remove water.
Sugar stays behind as water changes to vapor.
This is a physical method and suits mixtures.
Q8. A friend says, “Air is a compound.” Do you agree? Justify your answer using key features from the lesson.
I do not agree. Air is a mixture of many gases.
Its composition is variable. It changes from place to place.
Each gas in air retains its own properties.
Air can be separated by physical means because it is a mixture.
It is not in a fixed proportion like a compound.
So, air fits the mixture definition, not the compound one.
Q9. “Baking a cake turns a mixture into a compound.” Discuss this idea using properties and separation.
Cake batter is a mixture of ingredients.
On baking, chemical changes occur. New substances form with new properties.
The final cake cannot be separated by physical methods like filtration.
It no longer shows the individual properties of raw ingredients.
So, after baking, it behaves more like a chemically changed product than a simple mixture.
The key idea: mixing is physical, baking involves chemical changes and new properties.
Q10. You receive an unknown sample. How would you decide if it is a mixture or a compound using simple tests and observations?
Check the appearance. Is it uniform or has visible parts?
Try physical separation: filtration, decantation, or evaporation.
If parts separate physically, it is likely a mixture.
Observe properties. Do the parts retain their own properties?
Check composition. If it varies, it is a mixture. If it is fixed and shows a formula, it is a compound.
If separation needs a chemical reaction, it is a compound.
