CHEMISTRY [2 Mark Question Answer]

The dissolved impurities in water are salts and minerals.
Dissolved salts provide specific taste to water.
Salts and minerals are essential for growth and development.
They supply the essential minerals needed by our body.

Methods by which hydrous substances can be made anhydrous:

• By heating
• Exposure to dry air

In the laboratory, crystals may be obtained by the following methods:
By cooling a hot saturated solution gently
By cooling a fused mass
By sublimation
By slowly evaporating a saturated solution

Stir this mixture with the help of a glass rod and dissolve the copper sulphate crystals. Similarly, go on dissolving more copper sulphate (1 gram at a time) with constant and vigorous stirring. A stage is reached when no more copper sulphate dissolves. It is called a saturated solution at this temperature.

Three factors on which the solubility of a solid depend:

1. Temperature
2. Nature of the solid
3. Nature of the solvent

Table salt becomes sticky on exposure during the rainy season, because it generally contains a small percentage of magnesium chloride and calcium chloride as impurities. These impurities absorb moisture from the monsoon air due to their deliquescent nature, and thus, table salt become sticky.

The property of anomalous expansion of water enables marine life to exist in the colder regions of the world, because even when water freezes on the top, it is still liquid below the ice layer, as the density of water is greater than that of ice.

Water dissolves many substances forming an aqueous solution. It can dissolve solids, liquids and gases. When a solid dissolves in water, the solid is the solute, the water is the solvent and the resultant liquid is the solution. So, it is said that water is a universal solvent. In other words, water can dissolve nearly every substance.

Our body is almost 65% of water, and it has the property of specific heat. Due to high specific heat capacity, the presence of a large amount of water is able to modify the climate of the body and control the temperature of our body, which is warm in winter and cool in summer.

The temperature in Mumbai and Chennai do not fall as low as in Delhi because these cities are situated in the coastal areas. Due to high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas making them warmer in winter and cooler in summer. So, the temperature does not fall as low as it does in Delhi.

In the free state, water occurs in the solid, liquid and gaseous states.
Solid state: A large amount of fresh water is found in the form of snow or ice.
Liquid state: Most of the water present in oceans and found in streams, rivers, lakes, ponds and springs on land is water in the liquid state.
Gaseous state: Water vapour present in the air is in the gaseous state. Water vapour condenses in the sky to form clouds. Mist and fog are also examples of water in the gaseous form.

Large rivers do not freeze "through out" because, Water, ice, and snow are good insulators and poor conductors of heat. The portions of a lake or river that are exposed to the cold winter air will freeze into ice and this ice insulates the water below from further rapid freezing.

Burns caused by steam are more severe than burns caused by boiling water because of high specific latent heat of condensation. 2268 J/g of heat is released when 1 g of steam condenses to form 1 gm of water.
Due to the high specific latent heat of solidification of water, rivers and lakes do not freeze easily.

When a solid changes into a liquid, it absorbs heat equal to the latent heat of fusion. When a liquid changes into a solid, it loses heat equal to the latent heat of solidification.
When a liquid changes into a gas, it absorbs heat equal to the latent heat of vaporisation. When a gas condenses into a liquid, it loses heat equal to the latent heat of condensation.

Oxygen is more soluble in water than nitrogen. Air dissolved in water contains a higher percentage of oxygen (30-35%). Oxygen is only 21% in ordinary air. In this way, air dissolved in water is different from ordinary air.

1. Fish extracts the oxygen from the water and expels water through the gills. Fish can survive in water only through the dissolved oxygen present in water.
2. Aquatic plants make use of dissolved carbon dioxide for photosynthesis.
3. Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate.
4. Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.

Importance of dissolved salts in water:

• Dissolved salts provide specific taste to water.
• Dissolved salts act as micronutrients for the growth and development of living beings.

When tap water is boiled and evaporated:
Observations:

• A number of concentric rings of solid matter are seen on the watch glass after evaporation of tap water.

Conclusion:

• Tap water contains dissolved salts, minerals and impurities.