Science M.C.Q. [1 M]

2. Milk of magnesia.

Explanation:

Milk of magnesia is a suspension of magnesium hydroxide in water. It has a milk-like texture. The particles are suspended in the solvent. Milk is a colloid, whereas salt solution and vinegar are solutions.

3. Sand and sulphur.

Explanation:

Mixture of sand and sulphur cannot be separated by using water as a solvent because both are insoluble in water.

3. Corrosion and it is a chemical change.

Explanation:

Rusting of iron takes place in the presence of air (O₂) and water vapour. Action of air and moisture on iron converts it to its oxide. This process is called corrosion and it leads to the formation of a new compound. Therefore, it is a chemical change.

2. Nitrogen, argon, oxygen.

Explanation:

Boiling point of liquid nitrogen, liquid argon and liquid oxygen are -196°C, -186°C and -183°C respectively. So, nitrogen will boil off first followed by argon and oxygen.

2. Simple distillation.

Explanation:

In the mixture of iodine and alcohol, both are miscible. So, distillation process is used to recover both the components.

3. Carbon disulphide.

Explanation:

Sulphur is soluble in carbon disulphide but carbon is not.

4. Crystallization.

Explanation:

Crystallization is the process of cooling a hot concentrated solution of a substance to obtain its crystals in pure form.

3. Silica.

Explanation:

Silica is a compound made up of Si(silicon) and O(oxygen) and chemically called Silicon dioxide.

3. Hydrogen.

Explanation:

Hydrogen cannot be separated from the mixture of air as it is not present in free state. The three major components of air are nitrogen, oxygen and argon. These are separated using fractional distillation.

3. (i), (iii) and (iv)

Explanation:

Is a chemical change because rusting causes formation of  a new compound. Other statements are examples of physical changes where no new compounds are formed. In the process mentioned in other statements, original form of iron can be obtained back from moulded forms.

3. Sublimation of naphthalene.

Explanation:

Sublimation of naphthalene is not a chemical change because it only allows naphthalene to change its state from solid to vapour. No new compound is formed and the vapour can again be condensed to solid form.

3. (i) and (iv)

Explanation:

Ice and air are homogeneous in nature as their compositions remain same throughout.

3. (i) and (iii)

Explanation:

CO and NO are compounds. While CO is made up of carbon and oxygen, NO is made up of nitrogen and oxygen.

3. Silica.

Explanation:

Silica does not sublime on heating.

1. (i) and (ii)

Explanation:

The processes in statements (i) and (ii) are chemical changes because decaying and burning causes formation of new compounds; thus they are chemical reactions. The processes in statements (iii) and (iv) are physical changes where no new compounds are formed.

4. Bromine and mercury.

Explanation:

Mercury is a metal that exists in liquid state and bromine is a non-metal that exists in liquid state.

2. (i) and (iii)

Explanation:

A pure substance is made up of only one kind of particles like elements. The composition of pure compounds is also same throughout.

4.  Homogeneous and does not show Tyndall effect.

Explanation:

Sulphur dissolves completely in carbon disulphide. So, it forms a homogeneous solution. As the particles of solute are too small to scatter light, they will not cause tyndall effect. 

3. Copper metal is ductile and has low electrical resistance.

Explanation:

Copper is a good conductor of electricity. It means that it offers very low electric resistance. Also, it is highly ductile due to which it can be drawn into very thin wires.

2. An impure substance.

Explanation:

Milk is considered to be an impure substance as it is an emulsion. Emulsion is a colloid in which minute droplets of one liquid are dispersed in another liquid, which is not miscible with it.

2. A metal.

Explanation:

Metals are malleable and ductile as they can be hammered into thin sheets and can be stretched into wires.

3. Sugar solution.

Explanation:

Sugar solution does not show Tyndall effect because sugar solution is a true solution and not a colloid.

1. A 15% alcohol solution means 15ml alcohol and 85ml water.

2. First absorbed and then released.

Explanation:

When a mixture of iron and sulphur is heated strongly, they first absorb the energy to get excited. This is followed by the formation of a new compound iron sulphide (FeS) with the release of huge amount of energy in the form of heat and light.

3. Chalk powder and water mixture.

Explanation:

Chalk in water will most likely scatter the beam of light when passed through it because it forms a suspension.

3. Separating funnel.

Explanation:

Milk and groundnut oil are two immiscible liquids. So, they can be separated by using a separating funnel.

2. Gernmanium.

Explanation:

Germanium is metalloid. It is neither a good conductor of electricity nor an insulator. It is a semiconductor.

1. By sublimation.

Explanation:

Naphthalene sublimes on heating, whereas sand does not. So, when the mixture is heated, solid naphthalene turns into vapours, which can be condensed. Sand remains in the container.

4. A suspension.

Explanation:

Milk of magnesia is a suspension of magnesium hydroxide in water. It has milk like texture.

3. Chromium.

Explanation:

Chromium does not have magnetic property. Therefore, it will not be attracted by a magnet.

3. Gallium.

Explanation:

Boron, silicon and germanium are metalloids. Gallium is a metal.

3. Air.

Explanation:

Air is a homogeneous mixture of different gases. Hence, it does not have a fixed melting or boiling point.

4. Chromatography.

Explanation:

Chromatography is the technique used to separate the contents of a mixture, where solute particles are in extremely small quantity.

4. (i), (iii) and (iv)

Explanation:

Statements (i), (iii), and (iv) are correct. When two elements combine in a chemical reaction to give a product, the product is a compound. Composition of a compound has a fixed ratio and its properties are different from the elements participating in the reaction.

4. Shaving cream.

Explanation:

Shaving cream is not an emulsion but foam.

2. Bread.

Explanation:

Solid foam is a colloid in which a gas is dispersed in a solid medium. Bread is an example of solid foam.

3. Alcohol.

Explanation:

Air, kerosene oil and petrol are mixtures of various compounds of carbon and hydrogen (called hydrocarbons). Alcohol is a compound and not a mixture.

2. Hydrogen.

Explanation:

Washing soda (soda ash), Na₂CO₃ contains Na, C and O, whereas baking soda, NaHCO₃, is made up of Na, C, H and O. So, the element, which is not common in the two, is hydrogen.

4. Iodine in alcohol.

Explanation:

Iodine dissolved in alcohol is called tincture of iodine.

1. A 5 percent sugar solution means that 5g of sugar is dissolved in 95g of water.

3. Chlorine.

Explanation:

Chlorine is added to water to purify it from the impurities such as bacteria.

3. -186°C

Explanation:

The boiling point of argon is -186°C, therefore it is extracted after nitrogen.

2. Centrifugation.

Explanation:

Centrifugation is a process, in which suspended particles are separated from the solvent. In this process, the solution is rotated at a high rate in a centrifuge thereby separating the solute particles and the solvent.

4. Crytallisation.

Explanation:

Crystallisation is a better technique to recover sugar from sugar solution because sugar decomposes or gets charred on heating. Also, the soluble impurities get removed by crystallisation.

2. 204g

Explanation:

Solubility of a substance in water is defined as the maximum amount of solute that can be dissolved in 100g of water at a given temperature. The solubility of sugar in water at 20°C is 204g.

2. Brass.

Explanation:

Brass is an example of solution of solid in solid. It is an alloy of copper and zinc.

1. Brine.

Explanation:

Brine is salt solution. When it is heated, water will evaporate and salt will be left as residue.