A mixture is a material system made up of two or more different substances which are mixed but are not combined chemically. A homogeneous mixture is a type of mixture in which the composition is uniform and every part of the solution has the same properties. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures.

Air is a homogeneous mixture of a number of gases. It's two main constituents are oxygen and nitrogen. Other gases are present in small quantities.

The solution in which the concentration of solute is the same in every part of the solution is known as a homogeneous solution, sugar is completely soluble in water and forms homogeneous solution.

Air is a homogeneous mixture and can be separated into its components by fractional distillation.

Aqueous solution of low molecular mass are true solutions. A solution in which size of the solute particles is about $10^{10}$m is called a true solution.

A true solution has solute completely dissolved in the solvent. In case of chalk powder, it does not completely dissolve in water. Hence chalk powder in water is a heterogeneous mixture & is not a true solution.

Soluble substances are those that easily dissolve in a solvent, such as water, and include sugar, salt, alcohol and some dish-washing detergents. In chemistry, solubility of a substance is a quantitative term that refers to the amount of substance that can dissolve in a given volume of a solvent.

Amalgam used for dental filling in Ag - Sn amalgam.

Option A is the correct answer. 

A handful of soil is heterogeneous mixture of solids. It is not uniform in composition. 

Gun powder is a mixture of Sulphur, Charcoal and Potassium nitrate.

Sugar solution is a homogeneous mixture because any substance dissolve in water is homogeneous mixture.

The air that we breathe is a homogeneous mixture of oxygen, nitrogen, argon, and carbon dioxide, along with other elements in smaller amounts. Because each layer of the Earths atmosphere has a different density, each layer of air is its own homogeneous mixture.

In chemistry, a mixture is a material system made up of two or more different substances which are mixed but are not combined chemically. A mixture refers to the physical combination of two or more substances on which the identities are retained and are mixed in the form of solutions, suspensions, and colloids.

Note that a mixture:

• consists of two or more different elements and/or compounds physically intermingled,
• can be separated into its components by physical means, and
• often retains many of the properties of its components.

Apple juice is a fruit juice made by the maceration and pressing of apples. 
so its a mixture.

True Solution is a homogeneous mixture of two or more substances in which substance dissolved (solute) in solvent has the particle size of less than 10-9 m or 1 nm. Simple solution of sugar in water is an example of true solution. Particles of true solution cannot be filtered through filter paper and are not visible to naked eye.
Colloidal Solution is a heterogeneous mixture in which particle size of substance is intermediate of true solution and suspension i.e. between 1-1000 nm. Smoke from a fire is example of colloidal system in which tiny particles of solid float in air. Just like true solutions, Colloidal particles are small enough and cannot be seen through naked eye.They easily pass through filter paper. But colloidal particles are big enough to be blocked by parchment paper or animal membrane.
blood, starch, ink : Colloidal Soln.
sugar sol., salt sol : true soln.

In chemistry, a mixture is a material system made up of two or more different substances which are mixed but are not combined chemically. A mixture refers to the physical combination of two or more substances on which the identities are retained and are mixed in the form of solutions, suspensions, and colloids.

Note that a mixture:

• consists of two or more different elements and/or compounds physically intermingled,
• can be separated into its components by physical means, and
• often retains many of the properties of its components.

A mixture is a material system made up of two or more different substances which are mixed but are not combined chemically. Seawater is a complex mixture of 96.5 percent water, 2.5 percent salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases. 

A heterogeneous mixture is composed of components that are not uniform and have different properties. Milk, soil, smoke are the heterogeneous mixture. 

Soil is a heterogeneous mixture of minerals, organic matter, gases, liquids and the countless organisms that together support life on earth.

Brass is an alloy made up of copper and zinc and the proportions of copper and zinc can be varied to create a range of brasses with varying properties.So it is a mixture.

(a) Air is a mixture of several gases that are invisible and odorless. It consists of about 78% nitrogen, 21% oxygen, and less than 1% of argon, carbon dioxide, and other gases.

Stainless steel is a metal alloy made up of steel mixed with elements such as chromium, nickel, molybdenum, silicon, aluminium, and carbon. The content of these elements is not uniform throughout the solid solution which is a necessary condition to be a homogenous mixture. Thus stainless steel is a heterogeneous mixture.

Brass is an alloy made up of uniform composition of copper and zinc. It is a homogeneous mixture in which zinc and copper can be varied to create a range of brasses with varying properties.

Particles of a true solution will settle with gravity. Take an example when mud is dissolved in water and stirred vigorously, particles of mud are distributed evenly in water. After some time, the particles of this solution settle underwater due to the influence of gravity.

Vinegar is a solution of acetic acid in water.It is a homogenous liquid in  liquid mixture.

Brass consists of two elements: Copper and Zinc. The ratios of these can vary greatly; however, they are a solid mixture and are homogeneous. 

Nitrogen (N₂) form ammonia (NH₃) by a reaction with hydrogen (H₂) using a metal catalyst under high temperatures and pressures. It is a chemical combination reaction. Hence the mixture of nitrogen and hydrogen is difficult to separate because a new product is formed.

When two or more substances are mixed in any ratio, a mixture is formed.

Alloys are homologous mixtures of metals and cannot be separated into its constituents by physical methods. It is still considered a mixture because it shows the property of its constituents and can have variable composition.

The some particles in a true solution do not settle down a true solution is clear and transparent. It do not scatter light.

Kerosene is an example of homogeneous mixture of many hydrocarbons. Properties and composition are uniform throughout.

${ CO } _{ 2 }$ and ${ NH } _{ 3 }$ are a compound as it is made up of different elements.${ Fe(s) } _{  }$ is a pure substance as it is made up of a single element.

NaCl is ionic in nature and soluble in polar solvent and insoluble in non-polar solvent like hexane because polar water molecules form ion-dipole forces with the ions of NaCl, which is not sceen in case of hexane.

A heterogeneous mixture show variations in composition on a particular and macro scope level. A heterogeneous mixture do not have uniform composition. it show variation at any level whether microscope or macroscopic.

A glass of iced tea is a heterogeneous mixture as it contains both solid and liquid phases simultaneously.

The main reason for adding helium to the breathing mixture is to reduce the proportions of nitrogen and oxygen below those of air, to allow the gas mixture to be breathed safely on deep dives. A lower proportion of nitrogen is required to reduce nitrogen narcosis and other physiological effects of the gas at depth.

Homogeneous mixture- it is a mixture of solids, liquids and gases that has the same proportions of its components in any sample,

Air is a mixture of gas in gas.