የኬሚስትሪ መግቢያ ፈተና፣ 2014 ዓ.ም. (2022)

Pure aluminum oxide (alumina, Al2O3) melts at about 2045 degrees Celsius. Keeping the cell that hot would use far too much energy. Cryolite (Na3AlF6) dissolves the alumina and brings the working temperature down to about 1000 degrees Celsius. The molten mixture also conducts electricity well, which the electrolysis needs.

Cryolite is not a catalyst and it does not purify the metal, so options B and C are wrong. Option D says the opposite of what cryolite does.

Textbook: Chemistry Grade 10, Unit 5 Metals and Nonmetals, section 5.2.3 Production of Aluminum, Iron and Copper (page 207).

A chemical property describes how a substance changes into a new substance during a reaction. When aluminum reacts with dilute HCl or H2SO4, it forms a salt and gives off hydrogen gas. A new substance forms, so this is a chemical property.

Color (A), density (C) and conduction of heat and electricity (D) can all be observed without changing the aluminum into anything new. They are physical properties.

Textbook: Chemistry Grade 10, Unit 5 Metals and Nonmetals, section 5.2.1 Properties and Extraction of Metals (page 203).

Tanning turns the protein of a raw hide into stable, durable leather. In mineral tanning, the most common modern method, the skin is placed in solutions of chromium salts such as chromium sulphate. Chrome tanning is fast and gives stretchable leather.

Option A describes liming, a preparatory step that loosens hair before tanning. Soaking in water (B) only cleans the hide, and dilute acid (C) is used for deliming. None of these is the tanning step itself.

Textbook: Chemistry Grade 12, Unit 3 Industrial Chemistry, section 3.4.6 Tannery (page 187).

An industry that burns coal releases large amounts of carbon dioxide. CO2 is a greenhouse gas: it traps heat in the atmosphere, so the global temperature rises. This is the main environmental effect of coal use.

Coal contains almost no lead, so option D is wrong. Coal burning can release some nitrogen and sulphur oxides, but the rise in global temperature from CO2 is the effect the question points to.

Textbook: Chemistry Grade 12, Unit 5 Introduction to Environmental Chemistry, section 5.3 Global Warming and Climate Change (page 265).

The SI system has seven base (fundamental) quantities: length, mass, time, electric current, temperature, amount of substance and luminous intensity. Length, measured in metres, is one of them.

Force, speed and pressure are derived quantities. Their units are built from the base units: speed is m/s, force is kg m/s squared (the newton), and pressure is force per area (the pascal).

Textbook: Chemistry Grade 9, Unit 2 Measurements and Scientific Methods, section 2.1 Measurements and Units in Chemistry (page 20).

The enthalpy change, \(\Delta H\), is the heat given out or absorbed by a reaction at constant pressure. Most reactions in the laboratory happen in open containers, where the pressure stays at atmospheric pressure, so the heat exchanged equals the enthalpy change.

Options B and C each cover only one direction of heat flow, but \(\Delta H\) can be negative (heat released) or positive (heat absorbed). Option D adds constant volume, which is wrong: heat at constant volume is the internal energy change, not the enthalpy change.