CBSE Class 9 Chemistry Is Matter Around Us Pure MCQs Set 08

Answer: (a) a compound

Explanation:
1. Evolution of heat and entirely new properties show that a chemical combination has taken place.
2. The product is a compound.

Teacher's Note:
1. A mixture keeps the properties of its components.
2. Chemical combination gives new properties.

Answer: (d) Adding milk to water

Explanation:
1. Milk and water mix without a chemical reaction and keep their properties, so they form a mixture.
2. Crushing and breaking give only smaller pieces, and sodium reacts chemically with water.

Teacher's Note:
1. A mixture needs two or more substances.
2. Crushing one substance does not make a mixture.

Answer: (d) vary depending upon its components

Explanation:
1. A mixture has variable composition, so its properties such as colour, density and boiling point vary with the components and their proportions.

Teacher's Note:
1. A compound has fixed properties, a mixture does not.
2. Amount or volume does not decide intensive properties.

Answer: (b) rust powder, iron powder

Explanation:
1. Rust is hydrated iron oxide, which is a compound.
2. Iron powder is an element.

Teacher's Note:
1. Elements contain one kind of atom, compounds contain two or more.
2. Lime is calcium oxide, also a compound.

Answer: (d) during formation of a mixture there is a change in molecular composition

Explanation:
1. A mixture forms by simple mixing, so the molecules of the components are unchanged.
2. Statement (d) is therefore false.

Teacher's Note:
1. Mixtures: no chemical change.
2. Compounds: new substance with new properties.

Answer: (c) millions of molecules of water

Explanation:
1. A single drop is made of a very large number of water molecules.
2. Each water molecule is \( \ce{H2O} \).

Teacher's Note:
1. Even a tiny drop has about 10 to the power 21 molecules.
2. One molecule is far too small to see.

Answer: (d) metalloid

Explanation:
1. Germanium has properties between those of metals and non-metals, so it is a metalloid.

Teacher's Note:
1. Metalloids include boron, silicon, germanium, arsenic and antimony.
2. Silicon is the best known.

Answer: (c) Bronze, zinc

Explanation:
1. Bronze is an alloy of copper and tin, which is a mixture.
2. So it is not an element, while the other substances named are elements.

Teacher's Note:
1. Alloys are mixtures.
2. Zinc is an element.

Answer: (b) Water

Explanation:
1. Water is formed by chemical combination of hydrogen and oxygen in a fixed ratio.
2. Steel and brass are alloys (mixtures) and iodine is an element.

Teacher's Note:
1. Alloys are mixtures.
2. Elements have only one kind of atom.

Answer: (a) Glass

Explanation:
1. Glass has a uniform composition throughout.
2. Dirt and flower are heterogeneous.

Teacher's Note:
1. Homogeneous means the same composition everywhere.
2. A candle is a solid wax but with a wick, so it is heterogeneous.

Answer: (d) Both (b) and (c)

Explanation:
1. A salt solution is a mixture of salt and water.
2. Rock salt is common salt with other minerals.
3. Distilled water is a pure substance.

Teacher's Note:
1. Pure substances have only one kind of particle.
2. Solutions and natural rocks are mixtures.

Answer: (b) copper and zinc

Explanation:
1. Brass is an alloy of copper and zinc.
2. Bronze is copper and tin.

Teacher's Note:
1. An alloy is a homogeneous mixture of metals.
2. Brass is used for utensils and musical instruments.

Answer: (d) ethyl alcohol

Explanation:
1. Tincture means a solution in alcohol.
2. Tincture of iodine is iodine dissolved in ethanol.

Teacher's Note:
1. Used as an antiseptic.
2. Iodine is a solute and alcohol is the solvent.

Answer: (b) Molecule

Explanation:
1. A molecule is the smallest particle of a compound that shows its properties.
2. Atoms of the elements would show different properties.

Teacher's Note:
1. One molecule of water still behaves as water.
2. Its atoms do not.

Answer: (b) less than 1 nm in diameter

Explanation:
1. In a true solution the solute particles are smaller than 1 nm.
2. They do not scatter light and cannot be filtered.

Teacher's Note:
1. Solution under 1 nm, colloid 1 to 1000 nm, suspension over 1000 nm.
2. Use particle size to classify.

Answer: (b) melting point

Explanation:
1. A pure solid melts sharply at a fixed temperature.
2. Impurities lower and broaden the melting point.

Teacher's Note:
1. Melting point is the standard test for purity of solids.
2. Boiling point is used for liquids.

Answer: (d) air

Explanation:
1. Air has a uniform composition throughout, so it is a homogeneous mixture.
2. Milk is a colloid, muddy water is a suspension and smoke is a colloid.

Teacher's Note:
1. Solutions are homogeneous mixtures.
2. Air is a solution of gases.

Answer: (a) diffusion

Explanation:
1. Gas particles move randomly and spread into each other, mixing completely.
2. This process is diffusion.

Teacher's Note:
1. Different gases always mix uniformly.
2. Diffusion is fastest in gases.

Answer: (c) saturated solution

Explanation:
1. A solution holding the maximum amount of solute at a given temperature is saturated.

Teacher's Note:
1. A saturated solution cannot dissolve more solute at that temperature.
2. Raising the temperature usually increases solubility.

Answer: (c) can be a true solution or a colloidal solution

Explanation:
1. Soil contains dissolved salts, which form a true solution, and very fine clay particles, which can stay as a colloid.
2. Filter paper does not hold back either.
3. So the filtrate can be either.

Teacher's Note:
1. Filtration removes only suspended particles.
2. Colloidal particles pass through filter paper.

Answer: (a) amount of solute more than saturation level

Explanation:
1. A supersaturated solution holds more solute than normal saturation at that temperature.
2. It is unstable and the excess crystallises easily.

Teacher's Note:
1. It is made by cooling a hot saturated solution carefully.
2. A seed crystal makes the excess crystallise.

Answer: (a) visible to naked eye

Explanation:
1. Suspension particles are larger than 1000 nm and can often be seen with the naked eye.

Teacher's Note:
1. Suspensions settle on standing.
2. They scatter light and can be filtered.