NCERT MCQ Solutions for Class 8 Science Curiosity Chapter 8 Nature of Matter: Elements, Compounds and Mixtures

A mixture consists of two or more substances that retain their individual properties and do not chemically react with each other. Examples include poha and lemonade. These mixtures can be uniform or non-uniform depending on the visibility and distribution of their components.

Each substance that is part of a mixture is called a component. In mixtures, these components keep their original properties and do not chemically react. For example, chickpeas and tomatoes in a salad are components that can be seen and separated.

A compound is a substance formed when two or more elements combine chemically in a fixed ratio. Sodium chloride (NaCl) is a compound made of sodium and chlorine in a 1:1 ratio. Its properties differ completely from those of its elements.

In science, a pure substance has identical particles and cannot be separated into other substances by physical means. It can be either an element or a compound, such as pure gold or pure water, where all particles behave uniformly.

Elements are pure substances made of identical atoms. Aluminium is a metal element used in construction and daily items. It cannot be broken down into simpler substances and consists entirely of aluminium atoms.

Oxygen is an element. It is made up of identical O₂ molecules and cannot be broken down further. Unlike compounds, it does not consist of different types of atoms chemically bonded in a fixed ratio.

Air is a uniform mixture of gases like nitrogen, oxygen, and carbon dioxide. These gases are evenly distributed and cannot be seen separately, making air a uniform or homogeneous mixture.

Alloys are uniform mixtures of two or more metals. For example, brass (copper + zinc) and bronze (copper + tin) are alloys. They look like a single substance and have properties different from the individual metals that form them.

Water is formed by two atoms of hydrogen and one atom of oxygen combined chemically. This fixed ratio gives water new properties different from hydrogen and oxygen. It cannot be separated by physical methods, proving it is a compound.

Poha contains visible components like flattened rice, spices, and vegetables. These can be seen separately and are not evenly distributed throughout the dish, making poha a non-uniform or heterogeneous mixture.

In a physical mixture of iron and sulfur, iron retains its magnetic properties and can be separated using a magnet.
This confirms that mixtures retain the properties of their components and do not undergo chemical change unless heated.

When heated, iron and sulfur react chemically to form iron sulfide, a black compound with properties different from iron or sulfur.
Unlike a mixture, this compound cannot be separated by physical methods like using a magnet.

Sodium chloride is a compound formed by the chemical bonding of sodium (a metal) and chlorine (a non-metal) in a 1:1 ratio.
Its properties are different from those of its constituent elements, and it cannot be separated by physical means.

Water is made up of two hydrogen atoms and one oxygen atom chemically bonded together.
This fixed ratio of 2:1 is a key characteristic of water as a compound and determines its consistent chemical properties.

Evaporation is used to separate dissolved solids from a liquid. In a sugar-water mixture, heating the solution causes water to evaporate, leaving sugar crystals behind.
This method works because sugar does not evaporate with water.

Brass is a uniform mixture of copper and zinc, forming an alloy. It behaves like a single metal but retains a uniform composition of its components. Alloys like brass and bronze as uniform mixtures of metals with combined properties.

In a mixture, the individual substances retain their own properties. They are not chemically bonded, so no new substance is formed. This is seen in mixtures like salad, air or sand and salt, where each component behaves independently.

In a salt solution, salt dissolves completely in water and the particles are evenly distributed. The resulting liquid looks the same throughout. Such mixtures uniform or homogeneous, in contrast to non-uniform mixtures like fruit salad or sand and water.

Filtration is used to separate insoluble solids from liquids. For example, sand can be separated from a sand-water mixture using filter paper. 
It is use to demonstrate how physical properties like solubility and particle size help separate mixtures.

When elements combine to form a compound, it is a chemical change. New substances with entirely different properties are formed and the change is usually irreversible by simple physical methods.

Compounds have a fixed ratio of elements and properties different from their components. Mixtures have variable composition and retain the properties of individual substances. The compounds like water with mixtures like saltwater to explain this key difference.

Fog is a mixture of tiny water droplets (liquid) suspended in air (gas). It appears cloudy but is composed of small droplets evenly spread, making it a gas–liquid mixture. Fog as an example of a non-uniform mixture.

Table sugar is a compound made from carbon, hydrogen and oxygen in a fixed ratio. Its properties differ from those of its constituent elements. It cannot be separated by physical means, which classifies it as a chemical compound.

The law of conservation of mass applies during chemical changes. When elements react to form a compound, the total mass of the compound equals the sum of the masses of its elements. When discussing fixed composition and mass conservation.

Hydrogen is a gaseous element made up of only hydrogen atoms. It cannot be broken into simpler substances and exists naturally in gaseous form.
It among common elements found in nature.

In a saltwater mixture, salt is dissolved in water and cannot be removed by filtration. Heating the mixture allows water to evaporate, leaving the salt behind. This process is described in the chapter as a method to separate dissolved solids from liquids.

Mixtures can be made in any proportion depending on need. For example, lemonade can have more or less sugar.
This flexibility as an advantage of mixtures, unlike compounds which have a fixed composition.

Compounds are formed by chemical reactions and cannot be separated by physical processes. Compounds from mixtures by noting that mixtures can be physically separated, while compounds require chemical reactions for separation.

Elements are made of only one kind of atom and are pure substances. The simplest form of matter that cannot be broken into simpler substances by physical or chemical methods.

Lemonade appears uniform throughout. Even though it is made of lemon juice, sugar and water, its components are not easily distinguishable. Such mixtures as homogeneous or uniform, where all parts look the same to the eye.