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Showing 6 questions in Chemistry for Class 9. Tap a card to reveal the answer.
ChemistryMatter in Our SurroundingsmediumWhy does evaporation cause cooling?
Reveal answer ↓
What it is
When the fastest particles escape a liquid's surface, the particles left behind are cooler.
Answer
During evaporation the fastest-moving (most energetic) particles at the surface of a liquid escape into the air. This lowers the average kinetic energy of the remaining particles, so the temperature of the liquid falls. The escaping particles also absorb latent heat from the surroundings, which is why evaporation produces a cooling effect - for example, sweating cools our body.
- •Fast (energetic) particles leave the surface
- •Remaining particles have lower average energy
- •Latent heat absorbed from surroundings
- •Example: sweating cools the body
Why learn this
It's why sweating, earthen pots (matka) and desert coolers keep things cold without a fridge.
💡 Memory trick
The 'hot-heads' leave first, so what's left behind is cooler.
ChemistryIs Matter Around Us PuremediumGive two differences between a mixture and a compound.
Reveal answer ↓
What it is
A mixture is a physical blend you can separate; a compound is a fixed chemical union with new properties.
Answer
In a mixture the components are just mixed physically in any ratio, keep their own properties and can be separated by physical methods like filtration. In a compound the elements are chemically combined in a fixed ratio to form a new substance with new properties, and they can be separated only by chemical methods. For example, a mixture of iron and sulphur versus the compound iron sulphide.
- •Mixture: physical, any ratio, keeps properties, separated physically
- •Compound: chemical, fixed ratio, new properties, separated chemically
- •Example: iron+sulphur mix vs iron sulphide
Why learn this
It's how chemists tell air (mixture) from water (compound) and separate ores and medicines.
💡 Memory trick
Mixture = Mixed (any ratio, easy split); Compound = Chemically bonded (fixed, hard to split).
ChemistryAtoms and MoleculesmediumState the law of conservation of mass with an example.
Reveal answer ↓
What it is
In any chemical reaction the total mass stays the same - atoms are only rearranged, never lost.
Answer
The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction; the total mass of the reactants equals the total mass of the products. For example, when 12 g of carbon burns completely in 32 g of oxygen, exactly 44 g of carbon dioxide is formed (12 + 32 = 44).
mass of reactants = mass of products
- •Mass is neither created nor destroyed
- •Total mass of reactants = total mass of products
- •Example: 12 g C + 32 g O2 -> 44 g CO2
Why learn this
It's why every chemical equation must be BALANCED - the foundation of all of chemistry.
💡 Memory trick
Atoms in = atoms out. Nothing vanishes, it just rearranges.
ChemistryStructure of the AtomeasyDefine the atomic number and the mass number of an atom.
Reveal answer ↓
What it is
Atomic number = number of protons (the element's identity); mass number = protons + neutrons.
Answer
The atomic number (Z) is the number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral atom; it decides the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. So the number of neutrons = A - Z.
A = Z + number of neutrons
- •Atomic number Z = number of protons (= electrons if neutral)
- •Mass number A = protons + neutrons
- •Neutrons = A - Z
- •Z identifies the element
Why learn this
These two numbers decide every element in the periodic table and its isotopes.
💡 Memory trick
Z = protons (Zee identity); A = All heavy bits (protons + neutrons). Neutrons = A - Z.
ChemistryIs Matter Around Us PuremediumWhat is the Tyndall effect, and what does it tell us about a mixture?
Reveal answer ↓
What it is
The Tyndall effect is light scattering off colloid particles, revealing the beam's path.
Answer
The Tyndall effect is the scattering of a beam of light by the tiny particles of a colloid, which makes the path of the light visible. It shows that the mixture is a colloid (like milk or fog), because a true solution's particles are too small to scatter light while a colloid's particles are just the right size.
- •Scattering of light by colloidal particles makes the beam visible
- •Seen in colloids (milk, fog), not in true solutions
- •Shows the particle size is in the colloidal range
Why learn this
It's why you see sunbeams through fog or a projector beam in a dusty room.
💡 Memory trick
If you can SEE the light beam, it's a colloid (Tyndall). Crystal-clear = true solution.
ChemistryStructure of the AtommediumWhat are isotopes? Give one example.
Reveal answer ↓
What it is
Isotopes are atoms of the same element with the same protons but different numbers of neutrons.
Answer
Isotopes are atoms of the same element that have the same atomic number (the same number of protons) but different mass numbers (different numbers of neutrons). Because they have the same number of protons and electrons, they show the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon.
Same Z, different A
- •Same element, same protons (same atomic number)
- •Different neutrons (different mass number)
- •Same chemical properties
- •Example: carbon-12 and carbon-14
Why learn this
Isotopes are used in cancer treatment, carbon dating and nuclear energy.
💡 Memory trick
Same element (same protons), different mass (different neutrons). ISO = same place in the table.