Science Question Answer (3 Marks)

Sodium chloride is soluble in water whereas sand is insoluble in water. The difference in their solubility is used to separate them. The mixture of sand and sodium chloride is dissolved in water, then it is filtered with the help of filter paper and remains as residue on the filter paper while sodium chloride solution is obtained as filtrate .The filtrate is then evaporated to get crystals of sodium chloride.

The mixture of cooking oil (groundnut oil) and water is put in separating funnel and allowed to stand for some time. The mixture separates into two layers according to the densities of water and groundnut oil. Water is heavier than groundnut oil. So, water forms lower layer while groundnut oil forms upper layer. On opening the stop clock of separating funnel, the lower layer of water comes out first and collected in beaker leaving behind groundnut oil.

Sulphur is soluble in carbon disulphide whereas iron filing is insoluble in carbon disulphide. The mixture of sulphur and iron filing is shaken with carbon-di-sulphide. Sulphur dissolves in carbon disulphide whereas iron filings remain un-dissolved. The solution is then filtered, iron filing is obtained as residue. On evaporating the filtrate, carbon disulphide solvent is eliminated and solid sulphur remains behind.

Centrifugation is used to separate cream from milk. Milk is a suspension of tiny droplets of oil in a watery liquid. It is put in closed container in a big centrifuge machine. When machine is switched on, milk is rotated at very high speed in its container. The cream being lighter floats over the skimmed milk and then can be removed.

Crystallisation is better method for recovering sugar from sugar solution than evaporation because:

1. Sugar decomposes or get charred on heating to dryness during evaporation. There is no such problem in crystallization.
2. The soluble impurities do not get removed in the process of evaporation. But such impurities get removed in crystallization.

1. Elements: An element is a substance which cannot be split up into two or more simpler substances by the usual chemical methods of applying heat, light or electricity.

Examples: Hydrogen, Oxygen.

2. Compounds: A compound is a substance made up of two or more elements chemically combined in a fixed proportion by mass.

Examples: Sodium chloride, calcium carbonate.

3. Mixtures: A mixture is a substance which consists of two or more elements or compounds not chemically combined together.

Sulphur is soluble in carbon disulphide whereas sodium chloride is insoluble in carbon disulphide. The mixture of sulphur and sodium chloride is shaken with carbon disulphide. Sulphur dissolves in carbon disulphide whereas sodium chloride remains un-dissolved. The solution is then filtered, sodium chloride is obtained as residue. On evaporating the filtrate, carbon disulphide solvent is eliminated and solid sulphur remains behind.

Air is a mixture because:

1. Air can be separated into its constituents like oxygen, nitrogen, etc. by physical process of fractional distillation.
2. Air shows the properties of all the gases present in it.
3. Liquid air does not have a fixed boiling point.

Water is compound because:

1. Water cannot be separated into its constituents, hydrogen and oxygen by physical methods.
2. Heat and light are given out when water is prepared by burning hydrogen in oxygen.
3. Water has standard b.p. of 100°C under standard atmospheric pressure.

Mixture of iron filings and powdered carbon can be separated by using magnet. A horse-shoe magnet is moved on the surface of the mixture. The iron filings are attracted by the magnet, they cling to the poles of the magnet and get separated. This process is repeated a number of times till complete separation of iron filings is done.

1. Cooking of food: Chemical change.
2. Boiling of water: Physical change.
3. Cutting of trees: Physical change.
4. Dissolving salt in water: Physical change.
5. Digestion of food: Chemical change.
6. Melting of ice: Physical change.

Mixture of iron filings and powdered carbon can be separated by using magnet. A horse-shoe magnet is moved on the surface of the mixture. The iron filings are attracted by the magnet, they cling to the poles of the magnet and get separated. This process is repeated a number of times till complete separation of iron filings is done.

Chromatography is a technique of separating two or more dissolved solids which are present in a solution in very small quantities. Its two applications are:

1. Chromatography is used to separate solutions of coloured substances (dyes and pigments).
2. Chromatography is used to separate small amounts of products of chemical reactions.

In factories, scrap iron is separated from the heap of waste materials by using big electromagnets fitted to a crane. When a crane fitted with a powerful electromagnet is lowered on to the heap of waste materials, then the scrap iron objects present in the heap cling to the electromagnet. The crane is then moved up and away to drop these scrap iron objects at a separate place.

A true solution does not scatter a beam of light passing through it but a colloidal solution scatters a beam of light passing through it and renders its path visible. A true solution is a homogeneous mixture of two or more substances whereas a colloidal solution is a kind of solution in which the size of solute particles is intermediate between those in true solutions and those in suspensions and is a heterogeneous mixture.

The mixture of mustard oil and water is put in separating funnel and allowed to stand for some time. The mixture separates into two layers according to the densities of water and mustard oil. Water is heavier than mustard oil. So, water forms lower layer while mustard oil forms upper layer. On opening the stop clock of separating funnel, the lower layer of water comes out first and collected in beaker leaving behind mustard oil.

Potash alum is soluble in water whereas sand is insoluble in water. The difference in their solubility is used to separate them. The mixture of sand and potash alum is dissolved in water, then it is filtered with the help of filter paper. Sand remains as residue on the filter paper while potash alum solution is obtained as filtrate. The filtrate is then evaporated to get crystals of potash alum.

1. METALS: A metal is an element that is malleable, ductile and conducts electricity.

Example: Iron, Copper.

2. NON-METALS: A non metal is an element that is neither malleable, nor ductile and does not conducts electricity.

Example: Carbon, Sulphur.

3. METALLOIDS: The elements which show some properties of metals and some other properties of non-metals are called metalloids.

Example: Boron, Silicon, Helium, Magnesium, Copper.

Mixture of iron pins and sand can be separated by using magnet. A horse-shoe magnet is moved on the surface of the mixture. The iron pins are attracted by the magnet, they cling to the poles of the magnet and get separated. This process is repeated a number of times till complete separation of iron pins is achieved.

Impure copper sulphate is dissolved in minimum amount of water in a china dish to make copper sulphate solution. It is then filtered to remove insoluble impurities. Now, copper sulphate solution is heated gently on a water bath to evaporate water and a saturated solution is obtained. Then heating is stopped and saturated solution of copper sulphate is allowed to cool slowly. Crystals of pure copper sulphate will be formed leaving behind impurities. These crystals are then separated and dried.