STD 7science Chapter 3

Explanation
The capacity of air to hold moisture depends on the temperature of the air. Warmer air can hold more moisture, while colder air has a lower capacity to hold moisture. This is because as air temperature increases, the water molecules in the air move faster and spread out, allowing more water vapor to be present. Conversely, as air temperature decreases, the water molecules move slower and become more condensed, resulting in a lower capacity for holding moisture.

Explanation
Water does not have a specific shape because it takes the shape of its container. However, it does have a definite volume and mass, meaning it occupies a specific amount of space and has a certain amount of matter.

Explanation
When water freezes, its density increases. This is because the molecules in water slow down and come closer together as they lose energy and transition from a liquid to a solid state. As a result, the volume of the water decreases while the mass remains the same. Therefore, the correct answer is density.

Explanation
The given statement "Soil has pH 7" indicates that the soil has a pH level of 7, which is considered neutral. A pH level of 7 means that the soil is neither acidic nor alkaline. Therefore, the correct answer is "Neutral."

Explanation
The sun is the correct answer because it is the primary source of energy for the Earth. Through a process called nuclear fusion, the sun emits vast amounts of energy in the form of sunlight. This sunlight reaches the Earth and provides heat and light, which are essential for supporting life on our planet. Without the sun's energy, the Earth would be a cold and dark place.

Explanation
Sandy soil has low capacity for holding water because it has larger particles compared to other soil types. These larger particles create larger gaps or pores between them, which allows water to drain quickly through the soil. As a result, sandy soil cannot retain water effectively, leading to low water-holding capacity.

Explanation
Sea water is not a bad conductor of electricity. In fact, it is a good conductor due to the presence of dissolved salts and minerals. The high concentration of ions in sea water allows electric current to flow easily through it. This is why it is dangerous to swim during a thunderstorm as the water can conduct electricity and pose a risk of electrocution.

Explanation
A solvent is a substance in which a solute dissolves. When a solute is added to a solvent, it disperses and forms a solution. The solute particles become evenly distributed throughout the solvent, resulting in a homogeneous mixture. Solvents can be liquids, gases, or even solids, depending on the nature of the solute being dissolved. Therefore, it is correct to say that the substance in which a solute dissolves is called a solvent.

Explanation
The pressure exerted by air on objects at the Earth's surface is known as atmospheric pressure. This pressure is caused by the weight of the air above the object. It is a result of the Earth's gravitational pull on the air molecules. Atmospheric pressure decreases with increasing altitude, as there is less air above. Therefore, the statement that the pressure exerted by air is called atmospheric pressure is true.

Explanation
Soil cannot be given any required shape. It is a loose material composed of minerals, organic matter, water, and air, and its shape is determined by the forces acting upon it, such as gravity and the surrounding environment. While soil can be manipulated to some extent, it cannot be molded into any desired shape like clay or other materials. Therefore, the correct answer is False.

Explanation
Water is considered a universal solvent because it has the ability to dissolve a wide variety of substances. This is due to its unique molecular structure, which allows it to form hydrogen bonds with other molecules. These hydrogen bonds break apart the solute particles and surround them, dispersing them evenly throughout the water. As a result, water is able to dissolve many different types of solutes, making it an effective solvent for a wide range of chemical reactions and biological processes.

Explanation
Roots get sufficient supply of oxygen because they require it for cellular respiration. Oxygen is necessary for the breakdown of glucose molecules in the roots, which releases energy for various metabolic processes. Furthermore, oxygen is also needed for the roots to carry out aerobic respiration, which is essential for the production of ATP (adenosine triphosphate), the energy currency of cells. Therefore, a sufficient supply of oxygen is crucial for the healthy growth and functioning of roots.

Explanation
Colour is an important property of soil because it provides valuable information about the soil's composition, fertility, and health. The colour of soil can indicate the presence of organic matter, minerals, and nutrients. For example, dark brown or black soil often indicates high organic matter content, which is beneficial for plant growth. Additionally, certain colours may suggest the presence of specific minerals or contaminants. Therefore, understanding the colour of soil can help farmers, gardeners, and scientists make informed decisions about soil management and crop selection.

Explanation
Multani refers to Multani Mitti, also known as Fuller's Earth, which is a type of clay soil. It is commonly used in cosmetics due to its absorbent and cleansing properties. Multani Mitti is known for its ability to remove excess oil and impurities from the skin, making it a popular ingredient in face masks, scrubs, and other skincare products. Therefore, the correct answer is Multani.

Explanation
China clay, also known as kaolin, is a type of industrial mineral. Kaolinite is the main mineral present in china clay, making it the correct answer. Kaolinite is a clay mineral that is known for its fine particle size and white color. It is widely used in various industries such as ceramics, paper, paint, rubber, and cosmetics due to its unique properties like high plasticity, low shrinkage, and good suspension. Therefore, the correct answer for the given question is Kaolinite.