| A | B |
|---|
| absolute zero | the lowest possible temperature, about -273°C |
| acid | the substance that has a pH of less than 7 |
| acid rain | rain made from acid by absorbing sulfur and nitrogen oxides from polluted air |
| adrenalin | a hormone of the adrenal gland, secreted in times of emergency |
| air mass | large body of air characterized by certain values of temperature and humidity |
| algae | simple green organisms with cell walls but without the complex structure of plants |
| alkali | a strongly basic compound |
| alloy | a substance composed of two or more metals |
| alternating current(AC) | an electronic current that reverses direction periodically |
| amino acid | organic chemicals with an acid group at one end and an amino group at the other |
| anemia | substandard concentration of red blood cells |
| antibody | a blood protein that protects the body from foreign chemicals or microorganisms |
| aorta | the largest artery carrying blood out of the left ventricle |
| artery | a muscular-wall blood vessel that distributes blood to the body tissues |
| atmosphere | a layer of air surrounding the Earth |
| atom | smallest part of an element |
