Test: Class 9 General Science NCERT Based - 1

Brass is an alloy made primarily of copper and zinc. The proportions of the copper and zinc are varied to yield many different kinds of brass. Basic modern brass is 67% copper and 33% zinc.

However, the amount of copper may range from 55% to 95% by weight, with the amount of zinc varying from 5% to 40%.2 Lead is commonly added to brass at a concentration of around 2%.

The lead addition improves the machinability of brass. However, significant lead leaching often occurs, even in brass that contains a relatively low overall concentration of lead. Uses of brass include musical instruments, firearm cartridge casing, radiators, architectural trim, pipes and tubing, screws, and decorative items.

Barometer, device used to measure atmospheric pressure. Because atmospheric pressure changes with distance above or below sea level, a barometer can also be used to measure altitude.

There are two main types of barometers: mercury and aneroid. In the mercury barometer, atmospheric pressure balances a column of mercury, the height of which can be precisely measured.

Sublimation is the transition from the solid phase to the gas phase without passing through an intermediate liquid phase.

This endothermic phase transition occurs at temperatures and pressures below the triple point. The term "sublimation" only applies to physical changes of state and not to the transformation of a solid into a gas during a chemical reaction.

Graphite does not leave any residue when burnt in air because :- Graphite is completely made up of carbon, and when we burn it in ample amounts of oxygen it does not leave any residue behind.

Graphite is non flammable but can be burnt only in high temperatures. Graphite needs around 400 degrees to get burnt.It reduces when combined with air. it acts as a reducing agent.

A nanometer (also "nanometre") is a unit of measurement used to measure length. One nanometer is one billionth of a meter, so nanometers are certainly not used to measure long distances.

Instead, they serve to measure extremely small objects, such as atomic structures or transistors found in modern CPUs. A single nanometer is one million times smaller than a millimeter. If you take one thousandth of a millimeter, you have one micrometer, or a single micron.

If you divide that micron by 1,000, you have a nanometer. Needless to say, a nanometer is extremely small.

Since integrated circuits, such as computer processors, contain microscopic components, nanometers are useful for measuring their size. In fact, different eras of processors are defined in nanometers, in which the number defines the distance between transistors and other components within the CPU.

The smaller the number, the more transistors that can be placed within the same area, allowing for faster, more efficient processor designs.

Atoms are not able to exist independently. All the elements on the far right side of the periodic table (inert or noble gas) atoms can exist independently because they all fulfil the octet rule. They do not need bond to become stable.

An atom is a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms.

An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud.

The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too.