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To investigate the affects of heavy metals such as Lead Chloride, on the length of both the stem and roots of cress seeds.

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Affects of Heavy Metal on Stem & Root Length Aim: To investigate the affects of heavy metals such as Lead Chloride, on the length of both the stem and roots of cress seeds. Background Investigation: Lead is a soft metal having little tensile strength, and it is the densest of the common metals excepting gold and mercury. It has a metallic luster when freshly cut but quickly acquires a dull colour when exposed to moist air. Heavy metals such as lead are harmful to both humans and plants. Lead is used in the manufacture of batteries, metal products, paints, and ceramic glazes. In humans; exposure to lead can occur from breathing contaminated workplace air or house dust or eating lead-based paint chips or contaminated dirt. ...read more.


In general, inorganic lead compounds are of lower toxicity to plants than organic lead compounds such as trialkylead and tetralkylead compounds. Tetralkylead becomes toxic by decomposition into the ionic trialkylead. The toxicity of inorganic lead salts is strongly dependent on environmental conditions such as water hardness, pH, and salinity. When solid Lead Chloride is placed in water the following happens (in equilibrium): PbCl2 (s) < = > Pb+2 (aq) + 2Cl- (aq) The solubility of PbCl2 increases very rapidly as the temperature of the solution rises. However, PbCl2 precipitates very slowly, particularly when other ions that form insoluble chlorides are not present. The precipitation can be speeded up by vigorously rubbing the inside of the test tube with a stirring rod. ...read more.


As a result high lead concentrations in the soil are needed to cause visible toxic effects on photosynthesis and growth. Thus, lead is only likely to affect plants high concentrations of lead. There are degrees of tolerance to heavy metals in plants. Tolerance is inherited, i.e.tolerant parents transmit tolerance to their offspring. Within a plant species, there are tolerant and sensitive populations. Tolerance, therefore, develops by selection, rather than by adaptation of individuals. Two possible mechanisms for tolerance, to metals in general, are "external" mechanism prevents metal entering the plant, while an "internal" mechanisms allows entry but prevents the metal from coming into contact with sensitive processes within the organism. Prediction: We will perform the investigation seven times - each time using a different concentration of Lead Chloride solution. Hence we should be able to distinguish, clearly, the relationship between lead concentration and stem/root development. Ali Mahdi ...read more.

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